MODULE 1

BASIC CIRCUIT COMPONENTS

DIODE

A diode is a two-terminal electronic component with an asymmetric transfer characteristic,

with low (ideally zero) resistance to current flow in one direction, and high (ideally infinite)

resistance in the other. A semiconductor diode, the most common type today, is a crystalline

piece of semiconductor material with a p-n junction connected to two electrical terminals.

The most common function of a diode is to allow an electric current to pass in one direction

(called the diode's forward direction), while blocking current in the opposite direction (the

reverse direction). Thus, the diode can be viewed as an electronic version of a check valve.

This unidirectional behavior is called rectification, and is used to convert alternating current

to direct current.

However, diodes can have more complicated behavior than this simple on–off action.

Semiconductor diodes begin conducting electricity only if a certain threshold voltage or cut-

in voltage is present in the forward direction (a state in which the diode is said to be forward-

biased). The voltage drop across a forward-biased diode varies only a little with the current,

and is a function of temperature; this effect can be used as a temperature sensor or voltage

reference.

A p–n junction diode is made of a crystal of semiconductor. Impurities are added to it to

create a region on one side that contains negative charge carriers (electrons), called n-type

semiconductor, and a region on the other side that contains positive charge carriers (holes),

called p-type semiconductor. When two materials i.e. n-type and p-type are attached together,

a momentary flow of electrons occur from n to p side resulting in a third region where no

charge carriers are present. It is called Depletion region due to the absence of charge carriers

(electrons and holes in this case). The diode's terminals are attached to each of these regions.

The boundary between these two regions, called a p–n junction, is where the action of the

diode takes place. The crystal allows electrons to flow from the N-type side (called the

cathode) to the P-type side (called the anode), but not in the opposite direction.

Two commonly used materials for diodes are germanium and silicon. While both germanium

diodes and silicon diodes perform similar functions, there are certain differences between the

two that must be taken into consideration before installing one or the other into an electronic

circuit.

Circuit symbol

Silicon diode : The construction of a silicon diode starts with purified silicon. Each side of

the diode is implanted with impurities (boron on the anode side, arsenic or phosphorus on the

cathode side), and the joint where the impurities meet is called the "p-n junction." Silicon

diodes have a forward-bias voltage of 0.7 Volts. Once the voltage differential between the

anode and the cathode reaches 0.7 Volts, the diode will begin to conduct electrical current

across its p-n junction. When the voltage differential drops to less than 0.7 Volts, the p-n

junction will stop conducting electrical current and the diode will cease to function as an

electrical pathway. Because silicon is relatively easy and inexpensive to obtain and process,

silicon diodes are more prevalent than germanium diodes.

Germanium diode : Germanium diodes are manufactured in a manner similar to silicon

diodes. Germanium diodes also utilize a p-n junction and are implanted with the same

impurities that silicon diodes are implanted with. Germanium diodes, however, have a

forward-bias voltage of 0.3 Volts. Germanium is a rare material that is typically found with

copper, lead or silver deposits. Because of its rarity, germanium is more expensive to work

with, thus making germanium diodes more difficult to find (and sometimes more expensive)

than silicon diodes.

Light Emitting diode (LED)

A light-emitting diode (LED) is a optoelectronic semiconductor light source. When a light-

emitting diode is forward-biased (switched on), electrons are able to recombine with electron

holes within the device, releasing energy in the form of photons. This effect is called

electroluminescence and the colour of the light (corresponding to the energy of the photon)

is determined by the energy gap of the semiconductor. A LED is often small in area (less than

1 mm2), and integrated optical components may be used to shape its radiation pattern. LEDs

present many advantages over incandescent light sources including lower energy

consumption, longer lifetime, improved physical robustness, smaller size, and faster

switching.

Diode characteristics

A semiconductor diode’s behavior in a circuit is given by its current–voltage characteristic, or

I–V graph. The shape of the curve is determined by the transport of charge carriers through

the so-called depletion layer or depletion region that exists at the p–n junction between

differing semiconductors. When a p–n junction is first created, conduction-band (mobile)

electrons from the N-doped region diffuse into the P-doped region where there is a large

population of holes (vacant places for electrons) with which the electrons "recombine". When

a mobile electron recombines with a hole, both hole and electron vanish, leaving behind an

immobile positively charged donor (dopant) on the N side and negatively charged acceptor

(dopant) on the P side. The region around the p–n junction becomes depleted of charge

carriers and thus behaves as an insulator.

However, the width of the depletion region (called the depletion width) cannot grow without

limit. For each electron–hole pair that recombines, a positively charged dopant ion is left

behind in the N-doped region, and a negatively charged dopant ion is left behind in the P-

doped region. As recombination proceeds more ions are created, an increasing electric field

develops through the depletion zone that acts to slow and then finally stop recombination. At

this point, there is a "built-in" potential across the depletion zone.

If an external voltage is placed across the diode with the same polarity as the built-in

potential, the depletion zone continues to act as an insulator, preventing any significant

electric current flow (unless electron/hole pairs are actively being created in the junction by,

for instance, light. see photodiode). This is the reverse bias phenomenon. However, if the

polarity of the external voltage opposes the built-in potential, recombination can once again

proceed, resulting in substantial electric current through the p–n junction (i.e. substantial

numbers of electrons and holes recombine at the junction). For silicon diodes, the built-in

potential is approximately 0.7 V (0.3 V for Germanium and 0.2 V for Schottky). Thus, if an

external current is passed through the diode, about 0.7 V will be developed across the diode

such that the P-doped region is positive with respect to the N-doped region and the diode is

said to be "turned on" as it has a forward bias.

At very large reverse bias, beyond the peak inverse voltage or PIV, a process called

reverse breakdown occurs that causes a large increase in current (i.e., a large number of

electrons and holes are created at, and move away from the p–n junction) that usually

damages the device permanently. The avalanche diode is deliberately designed for use in the

avalanche region. In the Zener diode, the concept of PIV is not applicable. A Zener diode

contains a heavily doped p–n junction allowing electrons to tunnel from the valence band of

the p-type material to the conduction band of the n-type material, such that the reverse

voltage is "clamped" to a known value (called the Zener voltage), and avalanche does not

occur. Both devices, however, do have a limit to the maximum current and power in the

clamped reverse-voltage region. Also, following the end of forward conduction in any diode,

there is reverse current for a short time. The device does not attain its full blocking capability

until the reverse current ceases.

The second region, at reverse biases more positive than the PIV, has only a very small reverse

saturation current. In the reverse bias region for a normal P–N rectifier diode, the current

through the device is very low (in the µA range). However, this is temperature dependent,

and at sufficiently high temperatures, a substantial amount of reverse current can be observed

(mA or more).

The third region is forward but small bias, where only a small forward current is conducted.

As the potential difference is increased above an arbitrarily defined "cut-in voltage" or "on-

voltage" or "diode forward voltage drop (Vd)", the diode current becomes appreciable (the

level of current considered "appreciable" and the value of cut-in voltage depends on the

application), and the diode presents a very low resistance. The current–voltage curve

is exponential. In a normal silicon diode at rated currents, the arbitrary cut-in voltage is

defined as 0.6 to 0.7 volts.

Diode Approximation: (Large signal operations):

1. Ideal Diode:

When diode is forward biased, resistance offered is zero,

When it is reverse biased resistance offered is infinity. It acts as a perfect switch.

The characteristic and the equivalent circuit of the diode is shown below

2. Second Approximation:

When forward voltage is more than 0.7 V, for Si diode then it conducts and offers

zero resistance. The drop across the diode is 0.7V.

When reverse biased it offers infinite resistance.

The characteristic and the equivalent circuit is shown

3rd Approximation:

• When forward voltage is more than 0.7 V, then the diode conducts and the voltage drop

across the diode becomes 0.7 V and it offers resistance Rf (slope of the current)

VD= 0.7 + ID Rf.

The output characteristic and the equivalent circuit is shown

When reverse biased resistance offered is very high & not infinity, then the diode

equivalent circuit is as shown below.

Diode rectifier circuits

One of the important applications of a semiconductor diode is in rectification of AC signals

to DC. Diodes are very commonly used for obtaining DC voltage supplies from the readily

available AC voltage. There are many possible ways to construct rectifier circuits using

diodes. The three basic types of rectifier circuits are:

The Half Wave Rectifier

The Full Wave Rectifier

The Bridge Rectifier

Diode rectifier for power supply

The purpose of a power supply is to take electrical energy in one form and convert it into

another. There are many types of power supply. Most are designed to convert high voltage

AC mains electricity to a suitable low voltage supply for electronics circuits and other

devices such as computers, fax machines and telecommunication equipment. A power supply

can by broken down into a series of blocks, each of which performs a particular function. A

transformer first steps down high voltage AC to low voltage AC. A rectifier circuit is then

used to convert AC to DC. This DC, however, contains ripples, which can be smoothened by

a filter circuit. Power supplies can be ‘regulated’ or ‘unregulated’. A regulated power supply

maintains a constant DC output voltage through ‘feedback action’. The output voltage of an

unregulated supply, on the other hand, will not remain constant. It will vary depending on

varying operating conditions, for example when the magnitude of input AC voltage changes.

Main components of a regulated supply to convert 230V AC voltage to 5V DC are shown

below:

Half wave rectifier

The easiest rectifier to understand is the half wave rectifier. A simple half-wave rectifier

using an ideal diode and a load is shown in figure.

During the positive half cycle of the source, the ideal diode is forward biased and operates as

a closed switch. The source voltage is directly connected across the load. During the negative

half cycle, the diode is reverse biased and acts as an open switch. The source voltage is

disconnected from the load. As no current flows through the load, the load voltage is zero.

Both the load voltage and current are of one polarity and hence said to be rectified. The

waveforms for source voltage vs and output voltage vo are shown in figure

We notice that the output voltage varies between the peak voltage Vm and zero in each cycle.

This variation is called “ripple”, and the corresponding voltage is called the peak-to-peak

ripple voltage, Vp-p.

Average load voltage and current

If a DC voltmeter is connected to measure the output voltage of the half-wave rectifier (i.e.,

across the load resistance), the reading obtained would be the average load voltage Vave, also

called the DC output voltage. The meter averages out the pulses and displays this average.

Average Load Current

The value of the average load current is the value that would be measured by a DC ammeter.

L

ave

LR

VI

where IL is the average current passing through the load resistance.

Peak Inverse Voltage

The maximum amount of reverse bias that a diode will be exposed to is called the peak

inverse voltage or PIV. For the half wave rectifier, the value of PIV is:

PIV = Vm

The reasoning for the above equation is that when the diode is reverse biased, there is no

voltage across the load. Therefore, all of the secondary voltage (Vm) appears across the

diode. The PIV is important because it determines the minimum allowable value of reverse

voltage for any diode used in the circuit.

Half-wave Rectifier with Capacitor Filter

The capacitor is the most basic filter type and is the most commonly used. The half-wave

rectifier for power supply application is shown below. A capacitor filter is connected in

parallel with the load. The rectifier circuit is supplied from a transformer.

Circuit operation

The operation of this circuit during positive half cycle of the source voltage is shown in

figure. During the positive half cycle, diode D1 will conduct, and the capacitor charges

rapidly. As the input starts to go negative, D1 turns off, and the capacitor will slowly

discharge through the load.

The waveform is as shown below :

The operation can be analyzed in detail using figure below.

During each positive half cycle, the capacitor charges during the interval t1 to t2. During this

interval, the diode will be forward biased. Due to this charging, the voltage across the

capacitor vo will be equal to the AC peak voltage Vm on the secondary side of the transformer

at t2 (assuming diode forward voltage drop is zero). The capacitor will supply current to load

resistor RL during time interval t2 to t3. During this interval, diode will be reverse biased since

the AC voltage is less than the output voltage vo. Due to the large energy stored in the

capacitor, the capacitor voltage will not reduce much during t2 to t3, and the voltage vo will

remain close to the peak value. As can be seen, addition of the capacitor results in much

better quality output voltage.

Full wave rectifier

The full wave rectifier can be of two types :

Full wave center tap rectifier

Full wave bridge rectifier

Full wave center tap rectifier

The rectifier consists of two diodes and a resistor as shown in figure. The transformer has a

centre-tapped secondary winding. This secondary winding has a lead attached to the centre of

the winding. The voltage from the centre tap to either end terminal on this winding is equal to

one half of the total voltage measured end-to-end.

Circuit operation:

The following figure shows the operation of the rectifier during positive half cycle. The diode

D1 is forward biased and D2 is reverse biased.

During the negative half cycle, the polarity reverses. Diode D2 is forward biased and diode

D1 is reverse biased. Note that the direction of current through the load has not changed even

though the secondary voltage has changed polarity. Thus another positive half cycle is

produced across the load.

Using the ideal diode model, the peak load voltage for the full wave rectifier is Vm. The full

wave rectifier produces twice as many output pulses as the half wave rectifier. This is the

same as saying that the full wave rectifier has twice the output frequency of a half wave

rectifier. For this reason, the average load voltage (i.e. DC output voltage) is found as:

m

ave

VV

2

Figure below illustrates the average dc voltage for a full wave rectifier.

Peak Inverse Voltage

When one of the diodes in a full-wave rectifier is reverse biased, the peak voltage across that

diode will be approximately equal to Vm. With the polarities shown, D1 is conducting and D2

is reverse biased. Thus the cathode of D1 will be at Vm. Since this point is connected directly

to the cathode of D2, its cathode will also be Vm. With –Vm applied to the anode of D2, the

total voltage across the diode D2 is 2Vm. Therefore, the maximum reverse voltage across

either diode will be twice the peak load voltage.

PIV = 2Vm

Full wave rectifier with capacitor filter

Similar to the half-wave rectifier, smoothing is performed by a large value capacitor

connected across the load resistance to act as a reservoir, supplying current to the output

when the varying DC voltage from the rectifier is falling.

The diagram below shows the unsmoothed varying DC (thin line) and the smoothed DC

(thick line). The capacitor charges quickly near the peak of the varying DC, and then

discharges as it supplies current to the output.

Full wave bridge rectifier

In many power supply circuits, the bridge rectifier is used. The bridge rectifier produces

almost double the output voltage as a full wave center-tapped transformer rectifier using the

same secondary voltage. The advantage of using this circuit is that no center-tapped

transformer is required.

Basic Circuit Operation :

During the positive half cycle, both D3 and D1 are forward biased. At the same time, both D2

and D4 are reverse biased. Note the direction of current flow through the load.

During the negative half cycle D2 and D4 are forward biased and D1 and D3 are reverse

biased. Again note that current through the load is in the same direction although the

secondary winding polarity has reversed.

Peak Inverse Voltage

In order to understand the Peak Inverse Voltage across each diode, look at the figure below. It

is a simplified version of the figure showing the circuit conditions during the positive half

cycle. The load and ground connections are removed because we are concerned with the

diode conditions only. In this circuit, diodes D1 and D3 are forward biased and act like closed

switches. They can be replaced with wires. Diodes D2 and D4 are reverse biased and act like

open switches.

The circuit can be redrawn as :

We can see that both diodes are reverse biased, in parallel, and directly across the secondary

winding. The peak inverse voltage is therefore equal to Vm.

Full Wave Bridge Rectifier With Capacitor Filter

The voltage obtained across the load resistor of the full-wave bridge rectifier described above

has a large amount of ripple. A capacitor filter may be added to smoothen the ripple in the

output, as shown below.

The rectifier circuits discussed above can be used to charge batteries and to convert AC

voltages into constant DC voltages. Full-wave and bridge rectifier are more commonly used

than half-wave rectifier.

Zener Diode:

The diodes designed to work in breakdown region are called zener diode. If the reverse

voltage exceeds the breakdown voltage, the zener diode will normally not be destroyed as

long as the current does not exceed maximum value and the device closes not over load.

When a thermally generated carrier (part of the reverse saturation current) falls down the

junction and acquires energy of the applied potential, the carrier collides with crystal ions and

imparts sufficient energy to disrupt a covalent bond. In addition to the original carrier, a new

electron-hole pair is generated. This pair may pick up sufficient energy from the applied field

to collide with another crystal ion and create still another electron-hole pair. This action

continues and thereby disrupts the covalent bonds. The process is referred to as impact

ionization, avalanche multiplication or avalanche breakdown.

There is a second mechanism that disrupts the covalent bonds. The use of a sufficiently

strong electric field at the junction can cause a direct rupture of the bond. If the electric field

exerts a strong force on a bound electron, the electron can be torn from the covalent bond

thus causing the number of electron-hole pair combinations to multiply. This mechanism is

called high field emission or Zener breakdown. The value of reverse voltage at which this

occurs is controlled by the amount ot doping of the diode. A heavily doped diode has a low

Zener breakdown voltage, while a lightly doped diode has a high Zener breakdown voltage.

At voltages above approximately 8V, the predominant mechanism is the avalanche

breakdown. Since the Zener effect (avalanche) occurs at a predictable point, the diode can be

used as a voltage reference. The reverse voltage at which the avalanche occurs is called the

breakdown or Zener voltage. A typical Zener diode characteristic is shown in fig 1. The

circuit symbol for the Zener diode is different from that of a regular diode, and is illustrated

in the figure. The maximum reverse current, IZ(max), which the Zener diode can withstand is

dependent on the design and construction of the diode. A design guideline that the minimum

Zener current, where the characteristic curve remains at VZ (near the knee of the curve), is

0.1/ IZ(max).

Zener diode characteristic

The power handling capacity of these diodes is better. The power dissipation of a zener diode

equals the product of its voltage and current.

PZ= VZ IZ

The amount of power which the zener diode can withstand ( VZ.IZ(max) ) is a limiting factor in

power supply design.

Zener Regulator:

When zener diode is forward biased it works as a diode and drop across it is 0.7 V. When it

works in breakdown region the voltage across it is constant (VZ) and the current through

diode is decided by the external resistance. Thus, zener diode can be used as a voltage

regulator in the configuration shown below for regulating the dc voltage. It maintains the

output voltage constant even through the current through it changes.

The load line of the circuit is given by Vs= Is Rs + Vz. The load line is plotted along with

zener characteristic .The intersection point of the load line and the zener characteristic gives

the output voltage and zener current.

To operate the zener in breakdown region Vs should always be greater than Vz. Rs is used to

limit the current. If the Vs voltage changes, operating point also changes simultaneously but

voltage across zener is almost constant. The first approximation of zener diode is a voltage

source of Vz magnitude and second approximation includes the resistance also. The two

approximate equivalent circuits are shown below.

If second approximation of zener diode is considered, the output voltage varies slightly as

shown in above figure. The zener ON state resistance produces more I * R drop as the current

increases. As the voltage varies form V1 to V2 the operating point shifts from Q1 to Q2.

The voltage at Q1 is

V1 = I1 RZ +VZ

and at Q2

V2 = I2 RZ +VZ

Thus, change in voltage is

V2 – V1 = ( I2 – I1 ) RZ

Δ VZ =Δ IZ RZ

BIPOLAR JUNCTION TRANSISTORS

A Bipolar Junction Transistor (BJT) is a three layer, two junction, three terminal

device. BJTs can be used as amplifiers, switches, or in oscillators. BJTs can be found either

as individual discrete components, or in large numbers as parts of integrated circuits. Bipolar

transistors are so named because their operation involves both electrons and holes.

The regions of a BJT are called emitter, collector, and base. A discrete transistor has three

leads for connection to these regions. By design, most of the BJT collector current is due to

the flow of charges injected from a high-concentration emitter into the base where there are

minority carriers that diffuse toward the collector, and so BJTs are classified as minority-

carrier devices.

BJTs come in two types, or polarities, known as PNP and NPN based on the doping types of

the three main terminal regions In an NPN transistor, a thin and lightly doped P-type material

is sandwiched between two thicker N-type materials; while in a PNP transistor, a thin and

lightly doped N-type material is sandwiched between two thicker P-type materials. In the

following we will only consider NPN BJTs.

Regions of operation

Bipolar transistors have five distinct regions of operation, defined by BJT junction biases.

The modes of operation can be described in terms of the applied voltages (this description

applies to NPN transistors; polarities are reversed for PNP transistors):

Active

Saturation

Cut-Off

Inverted

Applied voltages B-E Junction

Bias (NPN)

B-C Junction

Bias (NPN) Mode (NPN)

E < B < C Forward Reverse Forward-active

E < B > C Forward Forward Saturation

E > B < C Reverse Reverse Cut-off

E > B > C Reverse Forward Reverse-active

Applied voltages B-E Junction

Bias (PNP)

B-C Junction

Bias (PNP) Mode (PNP)

E < B < C Reverse Forward Reverse-active

E < B > C Reverse Reverse Cut-off

E > B < C Forward Forward Saturation

E > B > C Forward Reverse Forward-active

In terms of junction biasing: ('reverse biased base–collector junction' means Vbc < 0 for

NPN, opposite for PNP)

Forward-active (or simply, active): The base–emitter junction is forward biased and

the base–collector junction is reverse biased. Most bipolar transistors are designed to

afford the greatest common-emitter current gain, βF, in forward-active mode. If this is

the case, the collector–emitter current is approximately proportional to the base

current, but many times larger, for small base current variations.

Inverted: By reversing the biasing conditions of the forward-active region, a bipolar

transistor goes into reverse-active mode. In this mode, the emitter and collector

regions switch roles. Because most BJTs are designed to maximize current gain in

forward-active mode, the βF in inverted mode is several (2–3 for the ordinary

germanium transistor) times smaller. This transistor mode is seldom used, usually

being considered only for failsafe conditions and some types of bipolar logic. The

reverse bias breakdown voltage to the base may be an order of magnitude lower in

this region.

Saturation: With both junctions forward-biased, a BJT is in saturation mode and

facilitates high current conduction from the emitter to the collector (or the other

direction in the case of NPN, with negatively charged carriers flowing from emitter to

collector). This mode corresponds to a logical "on", or a closed switch.

Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse

biased) are present. There is very little current, which corresponds to a logical "off",

or an open switch.

Although these regions are well defined for sufficiently large applied voltage, they overlap

somewhat for small (less than a few hundred millivolts) biases. For example, in the typical

grounded-emitter configuration of an NPN BJT used as a pulldown switch in digital logic, the

"off" state never involves a reverse-biased junction because the base voltage never goes

below ground; nevertheless the forward bias is close enough to zero that essentially no

current flows, so this end of the forward active region can be regarded as the cutoff region.

The NPN Transistor

"Common Emitter" configuration using NPN Transistors with an example of the

construction of a NPN transistor along with the transistors current flow characteristics is

given below.

The construction and terminal voltages for an NPN transistor are shown above. The voltage

between the Base and Emitter ( VBE ), is positive at the Base and negative at the Emitter

because for an NPN transistor, the Base terminal is always positive with respect to the

Emitter. Also the Collector supply voltage is positive with respect to the Emitter ( VCE ). So

for an NPN transistor to conduct the Collector is always more positive with respect to both

the Base and the Emitter.

We know that the transistor is a "current" operated device and that a large current ( Ic )

flows freely through the device between the collector and the emitter terminals when the

transistor is switched "fully-ON". However, this only happens when a small biasing current

(Ib ) is flowing into the base terminal of the transistor at the same time thus allowing the Base

to act as a sort of current control input.

α and β Relationship in a NPN Transistor

The transistor current in an NPN transistor is the ratio of these two currents ( Ic/Ib ),

called the DC Current Gain of the device and is given the symbol Beta, ( β ). The value of β

can be large up to 200 for standard transistors, and it is this large ratio between Ic and Ib that

makes the NPN transistor a useful amplifying device when used in its active region as Ib

provides the input and Ic provides the output.

Also, the current gain of the transistor from the Collector terminal to the Emitter

terminal, Ic/Ie, is called Alpha, ( α ), and is a function of the transistor itself (electrons

diffusing across the junction). As the emitter current Ie is the product of a very small base

current plus a very large collector current, the value of alpha α, is very close to unity, and for

a typical low-power signal transistor this value ranges from about 0.950 to 0.999

By combining the two parameters α and β we can produce two mathematical expressions that

gives the relationship between the different currents flowing in the transistor.

The values of Beta vary from about 20 for high current power transistors to well over 1000

for high frequency low power type bipolar transistors. The value of Beta for most standard

NPN transistors can be found in the manufactures datasheets but generally range between 50

- 200.

The equation above for Beta can also be re-arranged to make Ic as the subject, and with a zero

base current ( Ib = 0 ) the resultant collector current Ic will also be zero, ( β x 0 ). Also when

the base current is high the corresponding collector current will also be high resulting in the

base current controlling the collector current. One of the most important properties of the

Bipolar Junction Transistor is that a small base current can control a much larger collector

current. Consider the following example.

Example No1

An NPN Transistor has a DC current gain, (Beta) value of 200. Calculate the base current Ib

required to switch a resistive load of 4mA.

Therefore, β = 200, Ic = 4mA and Ib = 20µA.

One other point to remember about NPN Transistors. The collector voltage, ( Vc ) must be

greater and positive with respect to the emitter voltage, ( Ve ) to allow current to flow through

the transistor between the collector-emitter junctions. Also, there is a voltage drop between

the Base and the Emitter terminal of about 0.7v (one diode volt drop) for silicon devices as

the input characteristics of an NPN Transistor are of a forward biased diode. Then the base

voltage, ( Vbe ) of a NPN transistor must be greater than this 0.7V otherwise the transistor

will not conduct with the base current given as.

Where: Ib is the base current, Vb is the base bias voltage, Vbe is the base-emitter volt drop

(0.7v) and Rb is the base input resistor. Increasing Ib, Vbe slowly increases to 0.7V but Ic rises

exponentially.

Example No2

An NPN Transistor has a DC base bias voltage, Vb of 10v and an input base resistor, Rb of

100kΩ. What will be the value of the base current into the transistor.

Therefore, Ib = 93µA.

Bipolar Transistor Configurations

As the Bipolar Transistor is a three terminal device, there are basically three possible

ways to connect it within an electronic circuit with one terminal being common to both the

input and output. Each method of connection responding differently to its input signal within

a circuit as the static characteristics of the transistor varies with each circuit arrangement.

• Common Base Configuration - has Voltage Gain but no Current Gain.

• Common Emitter Configuration - has both Current and Voltage Gain.

• Common Collector Configuration - has Current Gain but no Voltage Gain.

The Common Base (CB) Configuration

As its name suggests, in the Common Base or grounded base configuration, the

BASE connection is common to both the input signal AND the output signal with the input

signal being applied between the base and the emitter terminals. The corresponding output

signal is taken from between the base and the collector terminals as shown with the base

terminal grounded or connected to a fixed reference voltage point.

The input current flowing into the emitter is quite large as its the sum of both the base

current and collector current respectively therefore, the collector current output is less than

the emitter current input resulting in a current gain for this type of circuit of "1" (unity) or

less, in other words the common base configuration "attenuates" the input signal.

The behavior of the NPN-transistor is determined by its two PN-junctions:

o The forward biased base-emitter (BE) PN-junction allows the free electrons to

flow from the emitter through the PN-junction to form the emitter current IE.

o As the P-type base is thin and lightly doped, only a small number of the

electrons from the emitter are combined with the holes in base to form the

base current IB, while most of the electrons go through the base to reach the

collector-base junction to form the collector current IC.

The ratio of the output current IC and the input current IE is defined as the CB

current gain or current transfer ratio:

o The reverse biased collector-base (CB) PN-junction blocks the majority

carriers (holes in the P-type base and electrons in N-type collector), but lets

the minority carriers to go through, including the free electrons in the base

coming from the emitter αIE , and the reverse saturation current of the

collector-base PN-junction ICBO (much smaller than EI .

The relationship between the output IC and the input IE can be found as:

The base current IB is:

In summary:

The Common Base Transistor Circuit

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in

that the signal voltages Vin and Vout are "in-phase". This type of transistor arrangement is not

very common due to its unusually high voltage gain characteristics. Its output characteristics

represent that of a forward biased diode while the input characteristics represent that of an

illuminated photo-diode.

Also this type of bipolar transistor configuration has a high ratio of output to input

resistance or more importantly "load" resistance ( RL ) to "input" resistance ( Rin ) giving it a

value of "Resistance Gain". Then the voltage gain ( Av ) for a common base configuration is

therefore given as:

The Common Emitter (CE) Configuration

In the Common Emitter or grounded emitter configuration, the input signal is applied

between the base, while the output is taken from between the collector and the emitter as

shown. This type of configuration is the most commonly used circuit for transistor based

amplifiers and which represents the "normal" method of bipolar transistor connection.

The common emitter amplifier configuration produces the highest current and power gain of

all the three bipolar transistor configurations. This is mainly because the input impedance is

LOW as it is connected to a forward-biased PN-junction, while the output impedance is

HIGH as it is taken from a reverse-biased PN-junction.

Two voltages VBE and VCE are applied to the base and collector of the transistor with

respect to the common emitter . The BE junction is forward biased while the CB junction

is reverse biased. The voltages of CB and CE configurations are related by:

The Common Emitter Amplifier Circuit

In this type of configuration, the current flowing out of the transistor must be equal to the

currents flowing into the transistor as the emitter current is given as Ie = Ic + Ib. Also, as the

load resistance ( RL ) is connected in series with the collector, the current gain of the common

emitter transistor configuration is quite large as it is the ratio of Ic/Ib and is given the Greek

symbol of Beta, ( β ). As the emitter current for a common emitter configuration is defined as

Ie = Ic + Ib, the ratio of Ic/Ie is called Alpha, given the Greek symbol of α. Note: that the value

of Alpha will always be less than unity.

Since the electrical relationship between these three currents, Ib, Ic and Ie is determined by the

physical construction of the transistor itself, any small change in the base current ( Ib ), will

result in a much larger change in the collector current ( Ic ). Then, small changes in current

flowing in the base will thus control the current in the emitter-collector circuit. Typically,

Beta has a value between 20 and 200 for most general purpose transistors.

By combining the expressions for both Alpha, α and Beta, β the mathematical relationship

between these parameters and therefore the current gain of the transistor can be given as:

Hence, to summarise, this type of bipolar transistor configuration has a greater input

impedance, current and power gain than that of the common base configuration but its

voltage gain is much lower. The common emitter configuration is an inverting amplifier

circuit. This means that the resulting output signal is 180o "out-of-phase" with the input

voltage signal.

The input current is IB, and the output current is

Solving this equation for IC, we get the relationship between the output IC and the input IB:

Here the ratio of the output current IC and the input current IB is defined as the CE current

gain or current transfer ratio:

and CBOCEO II )1( is the reverse saturation current between collector and emitter.

In summary:

The CB and CE current gains are related by:

The Common Collector (CC) Configuration

In the Common Collector or grounded collector configuration, the collector is now common

through the supply. The input signal is connected directly to the base, while the output is

taken from the emitter load as shown. This type of configuration is commonly known as a

Voltage Follower or Emitter Follower circuit. The emitter follower configuration is very

useful for impedance matching applications because of the very high input impedance, in the

region of hundreds of thousands of Ohms while having relatively low output impedance.

The Common Collector Transistor Circuit

The common emitter configuration has a current gain approximately equal to the β value of

the transistor itself. In the common collector configuration the load resistance is situated in

series with the emitter so its current is equal to that of the emitter current. As the emitter

current is the combination of the collector AND the base current combined, the load

resistance in this type of transistor configuration also has both the collector current and the

input current of the base flowing through it. Then the current gain of the circuit is given as:

The Common Collector Current Gain

This type of bipolar transistor configuration is a non-inverting circuit in that the signal

voltages of Vin and Vout are "in-phase". It has a voltage gain that is always less than "1"

(unity). The load resistance of the common collector transistor receives both the base and

collector currents giving a large current gain (as with the common emitter configuration)

therefore, providing good current amplification with very little voltage gain.

COMPARISON OF THREE CONFIGURATIONS

Characteristic Common

Base

Common

Emitter

Common

Collector

Input Impedance Low Medium High

Output

Impedance Very High High Low

Phase Angle 0o 180

o 0

o

Voltage Gain High Medium Low

Current Gain Low Medium High

Power Gain Low Very High Medium

Transistor as an amplifier

Amplifier is a circuit that is used for amplifying a signal. The input signal to an amplifier will

be a current or voltage and the output will be an amplified version of the input signal. An

amplifier circuit which is purely based on a transistor or transistors is called a transistor

amplifier. Transistors amplifiers are commonly used in applications like RF (radio

frequency), audio, OFC (optic fibre communication) etc. Anyway the most common

application we see in our day to day life is the usage of transistor as an audio amplifier. As

you know there are three transistor configurations that are used commonly i.e. common base

(CB), common collector (CC) and common emitter (CE). In common base configuration has

a gain less than unity and common collector configuration (emitter follower) has a gain

almost equal to unity). Common emitter follower has a gain that is positive and greater than

unity. So, common emitter configuration is most commonly used in audio amplifier

applications.

A good transistor amplifier must have the following parameters; high input impedance, high

band width, high gain, high slew rate, high linearity, high efficiency, high stability etc. In

order to prevent the transistor amplifier circuit from loading the input voltage source, the

transistor amplifier circuit must have high input impedance.

Common emitter RC coupled amplifier.

The common emitter RC coupled amplifier is one of the simplest and elementary transistor

amplifier that can be made. Don’t expect much boom from this little circuit, the main purpose

of this circuit is pre-amplification i.e to make weak signals strong enough for further

processing or amplification. If designed properly, this amplifier can provide excellent signal

characteristics. The circuit diagram of a single stage common emitter RC coupled amplifier

using transistor is shown in Fig1.

Capacitor Cin is the input DC decoupling capacitor which blocks any DC component if

present in the input signal from reaching the Q1 base. If any external DC voltage reaches the

base of Q1, it will alter the biasing conditions and affects the performance of the amplifier.

R1 and R2 are the biasing resistors. This network provides the transistor Q1′s base with the

necessary bias voltage to drive it into the active region. The region of operation where the

transistor is completely switched of is called cut-off region and the region of operation where

the transistor is completely switched ON (like a closed switch) is called saturation region.

The region in between cut-off and saturation is called active region. For a transistor amplifier

to function properly, it should operate in the active region. Let us consider this simple

situation where there is no biasing for the transistor. As we all know, a silicon transistor

requires 0.7 volts for switch ON and surely this 0.7 V will be taken from the input audio

signal by the transistor. So all parts of there input wave form with amplitude ≤ 0.7V will be

absent in the output waveform. In the other hand if the transistor is given with a heavy bias at

the base ,it will enter into saturation (fully ON) and behaves like a closed switch so that any

further change in the base current due to the input audio signal will not cause any change in

the output. The voltage across collector and emitter will be 0.2V at this condition (Vce sat =

0.2V). That is why proper biasing is required for the proper operation of a transistor

amplifier.

Cout is the output DC decoupling capacitor. It prevents any DC voltage from entering into the

succeeding stage from the present stage. If this capacitor is not used the output of the

amplifier (Vout) will be clamped by the DC level present at the transistors collector.

Rc is the collector resistor and Re is the emitter resistor. Values of Rc and Re are so selected

that 50% of Vcc gets dropped across the collector & emitter of the transistor.This is done to

ensure that the operating point is positioned at the center of the load line. 40% of Vcc is

dropped across Rc and 10% of Vcc is dropped across Re. A higher voltage drop across Re

will reduce the output voltage swing and so it is a common practice to keep the voltage drop

across Re = 10%Vcc . Ce is the emitter by-pass capacitor. At zero signal condition (i.e, no

input) only the quiescent current (set by the biasing resistors R1 and R2 flows through the

Re). This current is a direct current of magnitude few milli amperes and Ce does nothing.

When input signal is applied, the transistor amplifies it and as a result a corresponding

alternating current flows through the Re. The job of Ce is to bypass this alternating

component of the emitter current. If Ce is not there , the entire emitter current will flow

through Re and that causes a large voltage drop across it. This voltage drop gets added to the

Vbe of the transistor and the bias settings will be altered. It reality, it is just like giving a

heavy negative feedback and so it drastically reduces the gain.

Bandwidth.

The range of frequency that an amplifier can amplify properly is called the bandwidth of that

particular amplifier. Usually the bandwidth is measured based on the half power points i.e.

the points where the output power becomes half the peak output power in the frequency Vs

output graph. In simple words, bandwidth is the difference between the lower and upper half

power points. The band width of a good audio amplifier must be from 20 Hz to 20 KHz

because that is the frequency range that is audible to the human ear. The frequency response

of a single stage RC coupled transistor is shown in the figure below. Points tagged P1 and P2

are the lower and upper half power points respectively.

Gain : Gain of an amplifier is the ratio of output power to the input power. It represents how

much an amplifier can amplify a given signal. Gain can be simply expressed in numbers or in

decibel (dB). Gain in number is expressed by the equation G = Pout / Pin. In decibel the gain

is expressed by the equation Gain in dB = 10 log (Pout / Pin). Here Pout is the power output

and Pin is the power input. Gain can be also expressed in terms of output voltage / input

voltage or output current / input current. Voltage gain in decibel can be expressed using the

equation, Av in dB = 20 log ( Vout / Vin) and current gain in dB can be expressed using the

equation Ai = 20 log (Iout / Iin).

COMPARISON BETWEEN BJT, FET, MOSFET, IGBT

Sl

No BJT FET MOSFET IGBT

1 BJT (Bipolar

Junction

Transistor) are

transistors.

FET (Field Effect

Transistor) is

transistors.

MOSFET is a

Metal Oxide

Semiconductor

Field-Effect

Transistor

IGBT (Insulated Gate

Bipolar Transistor) has

a complex device

structure. It is a type of

transistor. IGBT has the

combined features of

both MOSFET and

bipolar junction

transistor

2 Consists of two

PN junctions.

Terminals of BJT

are known as

emitter, collector

and base

made of three

terminals known as

‘Gate’, ‘Source’ and

‘Drain’

MOSFET has a

gate, source and

drain

Terminals of IGBT are

known as emitter,

collector and gate

3 BJT is a bipolar

transistor

JFET is unipolar

transistor

4 there are two

types of charge

carriers-electrons

and holes

JFET there is only

one type of carrier-

n-channel case the

carrier is electron

and in p-channel

hole

5 large collector

emitter (Ic)

current is

controlled by the

small base

emitter current

(IB)

Source to drain

current flow is

controlled by

adjusting the channel

width by applying an

appropriate voltage

to gate

The operation of

MOSFET depends

on the voltage at

the oxide-insulated

gate electrode

IGBT is driven by the

gate voltage

6 current controlled

device

voltage controlled

device

voltage controlled

device

has current voltage

characteristics

7 switching speed

is low; switching

loss is more;

conduction loss

is less

switching speed is

high; so loss is less

higher switching speed

8 less power

efficient

FETs are more

power efficient

In digital and

analog circuits,

MOSFETs are

considered to be

more commonly

used than BJT

IGBTs are better in

power handling

9 Difficult

fabrication

Easy fabrication

10 More noise Less noise

11 Emitter and

collector

terminals are not

interchangeable

Source and drain

terminals are

interchangeable

12 Low input

impedance

High input

impedance

Very high input

impedance

13 BJTs are

preferred for low

current

applications,

while MOSFETs

are for high power

functions.

14 Generally the

input terminal is

forward biased.

Generally the input

terminal is reverse

biased. Conductivity

is controlled by the

reverse biasing of

the gate

Conductivity is

controlled by the

carriers induced in

the channel.

IGBT has a lower

forward voltage drop

15 Possible to

thermal runaway

Not possible to

thermal runaway

16 It can be operated

only in depletion

mode

It can be operated

in both depletion

mode and

enhancement mode

17 High gate current.

High drain

resistance

Low gate current

Low drain

resistance.

INTEGRATED CIRCUITS

An IC consists of interconnected low cost electronic circuit consisting of active and passive

components in a single piece (“chip”) of semiconductor material called silicon. By

connecting a large number of components, each performing simple operations, an IC that

performs very complex tasks can be built. The active components are transistors and diodes

and passive components are resistors and capacitors.

Advantages of IC:

1. Low cost of producing integrated circuits.

2. ICs can be made very compact, having up to several billion transistors and other

electronic components in an area the size.

3. IC’s production capability, reliability, and building-block approach to circuit design

ensured the rapid adoption of standardized Integrated Circuits in place of designs

using discrete transistors.

4. Performance is high because the components switch quickly

5. consume little power

6. Miniaturization and hence increased equipment density.

7. Cost reduction due to batch processing.

8. Increased system reliability due to the elimination of soldered joints.

9. Improved functional performance.

10. Matched devices.

11. Increased operating speeds.

12. Reduction in power consumption

Classifications of IC’s:

1. SSI, MSI, LSI and VLSI:

SSI: Integrated circuits contained only a few transistors called "small-scale integration"

(SSI)

MSI: devices which contained hundreds of transistors on each chip, called "medium-scale

integration" (MSI).

LSI: with tens of thousands of transistors per chip.

VLSI: beyond several billion transistors

2. Analog and digital IC

Digital integrated circuits can contain one to millions of logic gates, flip-flops ,

multiplexers, and other circuits in a few square millimetres.

Analog ICs, such as sensors, power management circuits, and operational amplifiers, work

by processing continuous signals. They perform functions like amplification, active filtering,

demodulation, and mixing.

3. Monolithic IC and Hybrid IC:

Monolithic IC: all circuit components, both active and passive elements and their

interconnections are manufactured into or on top of a single chip of silicon.

In hybrid circuits, separate component parts are attached to a ceramic substrate and

interconnected by means of either metallization pattern or wire bounds.

Ideal Operational Amplifiers

As well as resistors and capacitors, Operational Amplifiers, or Op-amps as they are more

commonly called, are one of the basic building blocks of Analogue Electronic

Circuits. Operational amplifiers are linear devices that have all the properties required for

nearly ideal DC amplification and are therefore used extensively in signal conditioning,

filtering or to perform mathematical operations such as add, subtract, integration and

differentiation.

An ideal Operational Amplifier is basically a three-terminal device which consists of two

high impedance inputs, one called the Inverting Input, marked with a negative or "minus"

sign, ( - ) and the other one called the Non-inverting Input, marked with a positive or "plus"

sign ( + ).

The third terminal represents the op-amps output port which can both sink and source either a

voltage or a current. In a linear operational amplifier, the output signal is the amplification

factor, known as the amplifiers gain ( A ) multiplied by the value of the input signal and

depending on the nature of these input and output signals, there can be four different

classifications of operational amplifier gain.

Voltage – Voltage "in" and Voltage "out"

Current – Current "in" and Current "out"

Transconductance – Voltage "in" and Current "out"

Transresistance – Current "in" and Voltage "out"

Since most of the circuits dealing with operational amplifiers are voltage amplifiers, we will

limit the tutorials in this section to voltage amplifiers only, (Vin and Vout).

The amplified output signal of an Operational Amplifier is the difference between the two

signals being applied to the two inputs. In other words the output signal is a differential signal

between the two inputs and the input stage of an Operational Amplifier is in fact a differential

amplifier as shown below.

Differential Amplifier

The circuit below shows a generalized form of a differential amplifier with two inputs

marked V1 and V2. The two identical transistors TR1 and TR2 are both biased at the same

operating point with their emitters connected together and returned to the common rail, -

Vee by way of resistor Re.

Differential Amplifier

The circuit operates from a dual supply+Vcc and -Vee which ensures a constant supply. The

voltage that appears at the output, Vout of the amplifier is the difference between the two

input signals as the two base inputs are in anti-phase with each other. So as the forward bias

of transistor,TR1 is increased, the forward bias of transistor TR2 is reduced and vice versa.

Then if the two transistors are perfectly matched, the current flowing through the common

emitter resistor, Re will remain constant.

Like the input signal, the output signal is also balanced and since the collector voltages either

swing in opposite directions (anti-phase) or in the same direction (in-phase) the output

voltage signal, taken from between the two collectors is, assuming a perfectly balanced

circuit the zero difference between the two collector voltages. This is known as the Common

Mode of Operation with the common mode gain of the amplifier being the output gain when

the input is zero.

Ideal Operational Amplifiers also have one output (although there are ones with an additional

differential output) of low impedance that is referenced to a common ground terminal and it

should ignore any common mode signals that is, if an identical signal is applied to both the

inverting and non-inverting inputs there should no change to the output. However, in real

amplifiers there is always some variation and the ratio of the change to the output voltage

with regards to the change in the common mode input voltage is called the Common Mode

Rejection Ratio or CMRR.

Operational Amplifiers on their own have a very high open loop DC gain and by applying

some form ofNegative Feedback we can produce an operational amplifier circuit that has a

very precise gain characteristic that is dependant only on the feedback used. An operational

amplifier only responds to the difference between the voltages on its two input terminals,

known commonly as the "Differential Input Voltage" and not to their common potential.

Then if the same voltage potential is applied to both terminals the resultant output will be

zero. An Operational Amplifiers gain is commonly known as theOpen Loop Differential

Gain, and is given the symbol (Ao).

Equivalent Circuit for Ideal Operational Amplifiers

Op-amp Idealized Characteristics

PARAMETER IDEALIZED CHARACTERISTIC

Open Loop

Gain, (Avo)

Infinite - The main function of an operational amplifier is to

amplify the input signal and the more open loop gain it has the

better. Open-loop gain is the gain of the op-amp without positive

or negative feedback and for an ideal amplifier the gain will be

infinite but typical real values range from about 20,000 to

200,000.

Input

impedance, (Zin)

Infinite - Input impedance is the ratio of input voltage to input

current and is assumed to be infinite to prevent any current

flowing from the source supply into the amplifiers input circuitry

(Iin =0). Real op-amps have input leakage currents from a few

pico-amps to a few milli-amps.

Output

impedance,

(Zout)

Zero - The output impedance of the ideal operational amplifier is

assumed to be zero acting as a perfect internal voltage source with

no internal resistance so that it can supply as much current as

necessary to the load. This internal resistance is effectively in

series with the load thereby reducing the output voltage available

to the load. Real op-amps have output-impedance in the 100-20Ω

range.

Bandwidth,

(BW)

Infinite - An ideal operational amplifier has an infinite frequency

response and can amplify any frequency signal from DC to the

highest AC frequencies so it is therefore assumed to have an

infinite bandwidth. With real op-amps, the bandwidth is limited

by the Gain-Bandwidth product (GB), which is equal to the

frequency where the amplifiers gain becomes unity.

Offset Voltage,

(Vio)

Zero - The amplifiers output will be zero when the voltage

difference between the inverting and the non-inverting inputs is

zero, the same or when both inputs are grounded. Real op-amps

have some amount of output offset voltage.

From these "idealized" characteristics above, we can see that the input resistance is infinite,

so no current flows into either input terminal (the "current rule") and that the differential

input offset voltage is zero (the "voltage rule"). It is important to remember these two

properties as they will help us understand the workings of the Operational Amplifier with

regards to the analysis and design of op-amp circuits.

However, real Operational Amplifiers such as the commonly available uA741, for example

do not have infinite gain or bandwidth but have a typical "Open Loop Gain" which is defined

as the amplifiers output amplification without any external feedback signals connected to it

and for a typical operational amplifier is about 100dB at DC (zero Hz). This output gain

decreases linearly with frequency down to "Unity Gain" or 1, at about 1MHz and this is

shown in the following open loop gain response curve.

Open-loop Frequency Response Curve

From this frequency response curve we can see that the product of the gain against frequency

is constant at any point along the curve. Also that the unity gain (0dB) frequency also

determines the gain of the amplifier at any point along the curve. This constant is generally

known as the Gain Bandwidth Product or GBP.

Therefore, GBP = Gain x Bandwidth or A x BW.

For example, from the graph above the gain of the amplifier at 100kHz = 20dB or 10, then

the

GBP = 100,000Hz x 10 = 1,000,000.

Similarly, a gain at 1kHz = 60dB or 1000, therefore the

GBP = 1,000 x 1,000 = 1,000,000.

The Voltage Gain (A) of the amplifier can be found using the following formula:

and in Decibels or (dB) is given as:

An Operational Amplifiers Bandwidth

The operational amplifiers bandwidth is the frequency range over which the voltage gain of

the amplifier is above 70.7% or -3dB (where 0dB is the maximum) of its maximum output

value as shown below.

Here we have used the 40dB line as an example. The -3dB or 70.7% of Vmax down point

from the frequency response curve is given as 37dB. Taking a line across until it intersects

with the main GBP curve gives us a frequency point just above the 10kHz line at about 12 to

15kHz. We can now calculate this more accurately as we already know the GBP of the

amplifier, in this particular case 1MHz.

Example No1.

Using the formula 20 log (A), we can calculate the bandwidth of the amplifier as:

37 = 20 log A therefore, A = anti-log (37 ÷ 20) = 70.8

GBP ÷ A = Bandwidth, therefore, 1,000,000 ÷ 70.8 = 14,124Hz, or 14kHz

Then the bandwidth of the amplifier at a gain of 40dB is given as 14kHz as previously

predicted from the graph.

Example No 2.

If the operational amplifiers gain was reduced by half to say 20dB in the above frequency

response curve, the -3dB point would now be at 17dB. This would then give us an overall

gain of 7.08, therefore A = 7.08. If we use the same formula as above this new gain would

give us a bandwidth of 141.2kHz, ten times more than at 40dB. It can therefore be seen that

by reducing the overall open loop gain of an operational amplifier its bandwidth is increased

and visa versa. The -3dB point is also known as the "half power point", as the output power

of the amplifier is at half its maximum value at this point.

Operational Amplifiers Summary

We know now that an Operational amplifiers is a very high gain DC differential amplifier

that uses one or more external feedback networks to control its response and characteristics.

We can connect external resistors or capacitors to the op-amp in a number of different ways

to form basic "building Block" circuits such as, Inverting, Non-Inverting, Voltage Follower,

Summing, Differential, Integrator and Differentiator type amplifiers.

Op-amp Symbol

An "ideal" or perfect Operational Amplifier is a device with certain special characteristics

such as infinite open-loop gain Ao, infinite input resistance Rin, zero output resistance Rout,

infinite bandwidth 0 to ∞and zero offset (the output is exactly zero when the input is zero).

There are a very large number of operational amplifier IC's available to suit every possible

application from standard bipolar, precision, high-speed, low-noise, high-voltage, etc in

either standard configuration or with internal JFET transistors. Operational amplifiers are

available in IC packages of either single, dual or quad op-amps within one single device. The

most commonly available and used of all operational amplifiers in basic electronic kits and

projects is the industry standard μA-741.

The Inverting Amplifier

We saw in the last tutorial that the Open Loop Gain, (Avo) of an ideal operational amplifier

can be very high, as much as 1,000,000 (120dB) or more. However, this very high gain is of

no real use to us as it makes the amplifier both unstable and hard to control as the smallest of

input signals, just a few micro-volts, (μV) would be enough to cause the output voltage to

saturate and swing towards one or the other of the voltage supply rails losing complete

control.

As the open loop DC gain of an operational amplifier is extremely high we can therefore

afford to lose some of this high gain by connecting a suitable resistor across the amplifier

from the output terminal back to the inverting input terminal to both reduce and control the

overall gain of the amplifier. This then produces and effect known commonly as Negative

Feedback, and thus produces a very stable Operational Amplifier based system.

Negative Feedback is the process of "feeding back" a fraction of the output signal back to

the input, but to make the feedback negative, we must feed it back to the negative or

"inverting input" terminal of the op-amp using an external Feedback Resistor called Rf. This

feedback connection between the output and the inverting input terminal forces the

differential input voltage towards zero.

This effect produces a closed loop circuit to the amplifier resulting in the gain of the amplifier

now being called its Closed-loop Gain. Then a closed-loop inverting amplifier uses negative

feedback to accurately control the overall gain of the amplifier, but at a cost in the reduction

of the amplifiers bandwidth.

This negative feedback results in the inverting input terminal having a different signal on it

than the actual input voltage as it will be the sum of the input voltage plus the negative

feedback voltage giving it the label or term of a Summing Point. We must therefore separate

the real input signal from the inverting input by using an Input Resistor, Rin.

As we are not using the positive non-inverting input this is connected to a common ground or

zero voltage terminal as shown below, but the effect of this closed loop feedback circuit

results in the voltage potential at the inverting input being equal to that at the non-inverting

input producing a Virtual Earthsumming point because it will be at the same potential as the

grounded reference input. In other words, the op-amp becomes a "differential amplifier".

Inverting Amplifier Configuration

In this Inverting Amplifier circuit the operational amplifier is connected with feedback to

produce a closed loop operation. For ideal op-amps there are two very important rules to

remember about inverting amplifiers, these are: "no current flows into the input terminal" and

that "V1 equals V2", (in real world op-amps both of these rules are broken).

This is because the junction of the input and feedback signal ( X ) is at the same potential as

the positive ( + ) input which is at zero volts or ground then, the junction is a "Virtual

Earth". Because of this virtual earth node the input resistance of the amplifier is equal to the

value of the input resistor, Rin and the closed loop gain of the inverting amplifier can be set

by the ratio of the two external resistors.

We said above that there are two very important rules to remember about Inverting

Amplifiers or any operational amplifier for that matter and these are.

1. No Current Flows into the Input Terminals

2. The Differential Input Voltage is Zero as V1 = V2 = 0 (Virtual Earth)

Then by using these two rules we can derive the equation for calculating the closed-loop gain

of an inverting amplifier, using first principles.

Current ( i ) flows through the resistor network as shown.

Then, the Closed-Loop Voltage Gain of an Inverting Amplifier is given as.

and this can be transposed to give Vout as:

Linear Output

The negative sign in the equation indicates an inversion of the output signal with respect to

the input as it is 180o out of phase. This is due to the feedback being negative in value.

The equation for the output voltage Vout also shows that the circuit is linear in nature for a

fixed amplifier gain as Vout = Vin x Gain. This property can be very useful for converting a

smaller sensor signal to a much larger voltage.

Another useful application of an inverting amplifier is that of a "transresistance amplifier"

circuit. A Transresistance Amplifier also known as a "transimpedance amplifier", is

basically a current-to-voltage converter (Current "in" and Voltage "out"). They can be used in

low-power applications to convert a very small current generated by a photo-diode or photo-

detecting device etc, into a usable output voltage which is proportional to the input current as

shown.

Transresistance Amplifier Circuit

The simple light-activated circuit above, converts a current generated by the photo-diode

into a voltage. The feedback resistor Rf sets the operating voltage point at the inverting input

and controls the amount of output. The output voltage is given as Vout = Is x Rf. Therefore,

the output voltage is proportional to the amount of input current generated by the photo-

diode.

Example No1

Find the closed loop gain of the following inverting amplifier circuit.

Using the previously found formula for the gain of the circuit

we can now substitute the values of the resistors in the circuit as follows,

Rin = 10kΩ and Rf = 100kΩ.

and the gain of the circuit is calculated as -Rf/Rin = 100k/10k = 10.

Therefore, the closed loop gain of the inverting amplifier circuit above is

given 10 or 20dB (20log(10)).

Example No2

The gain of the original circuit is to be increased to 40 (32dB), find the new values of the

resistors required.

Assume that the input resistor is to remain at the same value of 10KΩ, then by re-arranging

the closed loop voltage gain formula we can find the new value required for the feedback

resistor Rf.

Gain = -Rf/Rin

therefore, Rf = Gain x Rin

Rf = 40 x 10,000

Rf = 400,000 or 400KΩ

The new values of resistors required for the circuit to have a gain of 40 would be,

Rin = 10KΩ and Rf = 400KΩ.

The formula could also be rearranged to give a new value of Rin, keeping the same value

of Rf.

One final point to note about the Inverting Amplifier configuration for an operational

amplifier, if the two resistors are of equal value, Rin = Rf then the gain of the amplifier will

be -1 producing a complementary form of the input voltage at its output as Vout = -Vin. This

type of inverting amplifier configuration is generally called a Unity Gain Inverter of simply

an Inverting Buffer.

In the next tutorial about Operational Amplifiers, we will analyse the complement of

the Inverting Amplifier operational amplifier circuit called the Non-inverting

Amplifier that produces an output signal which is "in-phase" with the input.

The Non-inverting Amplifier

The second basic configuration of an operational amplifier circuit is that of a Non-inverting

Amplifier. In this configuration, the input voltage signal, (Vin) is applied directly to the non-

inverting ( + ) input terminal which means that the output gain of the amplifier becomes

"Positive" in value in contrast to the "Inverting Amplifier" circuit we saw in the last tutorial

whose output gain is negative in value. The result of this is that the output signal is "in-phase"

with the input signal.

Feedback control of the non-inverting amplifier is achieved by applying a small part of the

output voltage signal back to the inverting ( - ) input terminal via a Rf - R2 voltage divider

network, again producing negative feedback. This closed-loop configuration produces a non-

inverting amplifier circuit with very good stability, very high input

impedance, Rin approaching infinity, as no current flows into the positive input terminal,

(ideal conditions) and a low output impedance, Rout as shown below.

Non-inverting Amplifier Configuration

In the previous Inverting Amplifier tutorial, we said that "no current flows into the input"

of the amplifier and that "V1 equals V2". This was because the junction of the input and

feedback signal ( V1 ) are at the same potential. In other words the junction is a "virtual

earth" summing point. Because of this virtual earth node the resistors, Rf and R2 form a

simple potential divider network across the non-inverting amplifier with the voltage gain of

the circuit being determined by the ratios of R2 and Rf as shown below.

Equivalent Potential Divider Network

Then using the formula to calculate the output voltage of a potential divider network, we can

calculate the closed-loop voltage gain ( A V ) of the Non-inverting Amplifier as follows:

Then the closed loop voltage gain of a Non-inverting Amplifier is given as:

We can see from the equation above, that the overall closed-loop gain of a non-inverting

amplifier will always be greater but never less than one (unity), it is positive in nature and is

determined by the ratio of the values of Rf and R2. If the value of the feedback resistor Rf is

zero, the gain of the amplifier will be exactly equal to one (unity). If resistor R2 is zero the

gain will approach infinity, but in practice it will be limited to the operational amplifiers

open-loop differential gain, ( Ao ).

We can easily convert an inverting operational amplifier configuration into a non-inverting

amplifier configuration by simply changing the input connections as shown.

Voltage Follower (Unity Gain Buffer)

If we made the feedback resistor, Rf equal to zero, (Rf = 0), and resistor R2 equal to

infinity, (R2 = ∞), then the circuit would have a fixed gain of "1" as all the output voltage

would be present on the inverting input terminal (negative feedback). This would then

produce a special type of the non-inverting amplifier circuit called a Voltage Follower or

also called a "unity gain buffer".

As the input signal is connected directly to the non-inverting input of the amplifier

the output signal is not inverted resulting in the output voltage being equal to the input

voltage, Vout = Vin. This then makes thevoltage follower circuit ideal as a Unity Gain

Buffer circuit because of its isolation properties as impedance or circuit isolation is more

important than amplification while maintaining the signal voltage. The input impedance of

the voltage follower circuit is very high, typically above 1MΩ as it is equal to that of the

operational amplifiers input resistance times its gain ( Rin x Ao ). Also its output impedance

is very low since an ideal op-amp condition is assumed.

Voltage Follower

In this non-inverting circuit configuration, the input impedance Rin has increased to infinity

and the feedback impedance Rf reduced to zero. The output is connected directly back to the

negative inverting input so the feedback is 100% and Vin is exactly equal to Vout giving it a

fixed gain of 1 or unity. As the input voltage Vin is applied to the non-inverting input the

gain of the amplifier is given as:

Since no current flows into the non-inverting input terminal the input impedance is infinite

(ideal op-amp) and also no current flows through the feedback loop so any value of resistance

may be placed in the feedback loop without affecting the characteristics of the circuit as no

voltage is dissipated across it, zero current flows, zero voltage drop, zero power loss.

Since the input current is zero giving zero input power, the voltage follower can provide a

large power gain. However in most real unity gain buffer circuits a low value (typically 1kΩ)

resistor is required to reduce any offset input leakage currents, and also if the operational

amplifier is of a current feedback type.

The voltage follower or unity gain buffer is a special and very useful type of Non-inverting

amplifiercircuit that is commonly used in electronics to isolated circuits from each other

especially in High-order state variable or Sallen-Key type active filters to separate one filter

stage from the other. Typical digital buffer IC's available are the 74LS125 Quad 3-state

buffer or the more common 74LS244 Octal buffer.

One final thought, the output voltage gain of the voltage follower circuit with closed loop

gain is Unity, the voltage gain of an ideal operational amplifier with open loop gain (no

feedback) is Infinite. Then by carefully selecting the feedback components we can control

the amount of gain produced by an operational amplifier anywhere from one to infinity.

The Summing Amplifier

The Summing Amplifier is a very flexible circuit based upon the standard Inverting

Operational Amplifier configuration that can be used for combining multiple inputs. We saw

previously in the inverting amplifier tutorial that the inverting amplifier has a single input

voltage, ( Vin ) applied to the inverting input terminal. If we add more input resistors to the

input, each equal in value to the original input resistor, Rin we end up with another

operational amplifier circuit called a Summing Amplifier, "summing inverter" or even a

"voltage adder" circuit as shown below.

Summing Amplifier Circuit

The output voltage, ( Vout ) now becomes proportional to the sum of the input

voltages, V1, V2, V3 etc. Then we can modify the original equation for the inverting

amplifier to take account of these new inputs thus:

However, if all the input impedances, (Rin) are equal in value the final equation for the

output voltage is given as:

Summing Amplifier Equation

We now have an operational amplifier circuit that will amplify each individual input voltage

and produce an output voltage signal that is proportional to the algebraic "SUM" of the three

individual input voltagesV1, V2 and V3. We can also add more inputs if required as each

individual input "see's" their respective resistance, Rin as the only input impedance.

This is because the input signals are effectively isolated from each other by the "virtual earth"

node at the inverting input of the op-amp. A direct voltage addition can also be obtained

when all the resistances are of equal value and Rf is equal to Rin.

A Scaling Summing Amplifier can be made if the individual input resistors are "NOT"

equal. Then the equation would have to be modified to:

To make the math's a little easier, we can rearrange the above formula to make the feedback

resistorRF the subject of the equation giving the output voltage as:

This allows the output voltage to be easily calculated if more input resistors are connected to

the amplifiers inverting input terminal. The input impedance of each individual channel is the

value of their respective input resistors, ie, R1, R2, R3 ... etc.

The Summing Amplifier is a very flexible circuit indeed, enabling us to effectively "Add" or

"Sum" together several individual input signals. If the inputs resistors, R1, R2, R3 etc, are all

equal a unity gain inverting adder can be made. However, if the input resistors are of different

values a "scaling summing amplifier" is produced which gives a weighted sum of the input

signals.

Example No1

Find the output voltage of the following Summing Amplifier circuit.

Summing Amplifier

Using the previously found formula for the gain of the circuit

we can now substitute the values of the resistors in the circuit as follows,

we know that the output voltage is the sum of the two amplified input signals and is

calculated as:

then the output voltage of the Summing Amplifier circuit above is given as -45 mV and is

negative as its an inverting amplifier.

Summing Amplifier Applications

If the input resistances of a summing amplifier are connected to potentiometers the individual

input signals can be mixed together by varying amounts. For example, measuring

temperature, you could add a negative offset voltage to make the display read "0" at the

freezing point or produce an audio mixer for adding or mixing together individual waveforms

(sounds) from different source channels (vocals, instruments, etc) before sending them

combined to an audio amplifier.

Summing Amplifier Audio Mixer

Another useful application of a Summing Amplifier is as a weighted sum digital-to-

analogue converter. If the input resistors, Rin of the summing amplifier double in value for

each input, for example, 1kΩ, 2kΩ, 4kΩ, 8kΩ, 16kΩ, etc, then a digital logical voltage, either

a logic level "0" or a logic level "1" on these inputs will produce an output which is the

weighted sum of the digital inputs. Consider the circuit below.

Digital to Analogue Converter

Of course this is a simple example. In this DAC summing amplifier circuit, the number of

individual bits that make up the input data word, and in this example 4-bits, will ultimately

determine the output step voltage as a percentage of the full-scale analogue output voltage.

Also, the accuracy of this full-scale analogue output depends on voltage levels of the input

bits being consistently 0V for "0" and consistently 5V for "1" as well as the accuracy of the

resistance values used for the input resistors, Rin. Fortunately to overcome these errors,

commercial available Digital-to Analogue and Analogue-to Digital devices are available.

In the next tutorial about Operational Amplifiers, we will examine the effect of the output

voltage, Voutwhen a signal voltage is connected to the inverting input and the non-inverting

input at the same time to produce another common type of operational amplifier circuit called

a Differential Amplifier which can be used to "subtract" the voltages present on its inputs.

Differential Amplifier

Thus far we have used only one of the operational amplifiers inputs to connect to the

amplifier, using either the "inverting" or the "non-inverting" input terminal to amplify a

single input signal with the other input being connected to ground. But we can also connect

signals to both of the inputs at the same time producing another common type of operational

amplifier circuit called a Differential Amplifier.

Basically, as we saw in the first tutorial about operational amplifiers, all op-amps are

"Differential Amplifiers" due to their input configuration. But by connecting one voltage

signal onto one input terminal and another voltage signal onto the other input terminal the

resultant output voltage will be proportional to the "Difference" between the two input

voltage signals of V1 and V2.

Then differential amplifiers amplify the difference between two voltages making this type of

operational amplifier circuit a Subtractor unlike a summing amplifier which adds or sums

together the input voltages. This type of operational amplifier circuit is commonly known as

a Differential Amplifier configuration and is shown below:

By connecting each input inturn to 0v ground we can use superposition to solve for the output

voltageVout. Then the transfer function for a Differential Amplifier circuit is given as:

When resistors, R1 = R2 and R3 = R4 the above transfer function for the differential

amplifier can be simplified to the following expression:

Differential Amplifier Equation

If all the resistors are all of the same ohmic value, that is: R1 = R2 = R3 = R4 then the circuit

will become a Unity Gain Differential Amplifier and the voltage gain of the amplifier will

be exactly one or unity. Then the output expression would simply be Vout = V2 - V1. Also

note that if input V1 is higher than input V2 the ouput voltage sum will be negative, and

if V2 is higher than V1, the output voltage sum will be positive.

The Differential Amplifier circuit is a very useful op-amp circuit and by adding more

resistors in parallel with the input resistors R1 and R3, the resultant circuit can be made to

either "Add" or "Subtract" the voltages applied to their respective inputs. One of the most

common ways of doing this is to connect a "Resistive Bridge" commonly called a Wheatstone

Bridge to the input of the amplifier as shown below.

Transistor Transistor Logic

TTL is the short form of Transistor Transistor Logic. As the name suggests they refers to the

digital integrated circuits that employ logic gates consisting primarily of bipolar transistors.

The most basic TTL circuit is an inverter

Most TTL circuits use a totem pole output circuit instead of the pull-up resistor as shown in

Fig.1. It has a two transistors TC connected to Vcc and TD connected to GND. The emitter of

the TC is connected to the collector of TD by a diode. The output is taken from the collector of

transistor TD. TA is a multiple emitter transistor having only one collector and base but with

multiple emitters. The multiple base emitter junction behaves just like an independent diodes.

Applying a logic '1' input voltage to both emitter inputs of TA reverse-biases both base-

emitter junctions, causing current to flow through R A into the base of TB, which is driven

into saturation. When TB starts conducting, the stored base charge of TC dissipates through

the TB collector, driving TC into cut-off. On the other hand, current flows into the base of TD ,

causing it to saturate and its collector emitter voltage is 0.2 V and the output is equal to 0.2 V,

i.e. at logic 0.T B always provides complementary inputs to the bases of TC and TD, so that

either TC or TD will be ON. So the output impedance is low.

When at least one of inputs are at 0 V, the multiple emitter base junctions of transistor TA are

forward biased whereas the base collector is reverse biased and transistor TB remains off and

therefore the output voltage is equal to VCC . Since the base voltage for transistor TC is VCC ,

this transistor is on and the output is also VCC . And the input to transistor TD is 0 V, hence it

remains off.

Advantages of TTL:

1. High Speed

2. Exhibits a low input resistance.

Disadvantages of TTL:

1. Vulnerable to stray signals.

CMOS Logic

The term 'Complementary Metal-Oxide-Semiconductor' (CMOS), refers to the device

technology for fabricating integrated circuits using both n- and p-channel MOSFET's.The

input to a CMOS circuit is always to the gate of the input MOS transistor. The gate offers a

very high resistance because it is isolated from the channel by an oxide layer. The current

flowing through a CMOS input is virtually zero, and the device is operated mainly by the

voltage applied to the gate, which controls the conductivity of the device channel. The low

input currents required by a CMOS circuit results in lower power consumption, which is the

major advantage of CMOS over TTL.

2 input CMOS NAND Gate:

Figure above shows a 2 input CMOS NAND Gate where the TN1 and TP1 have the same input

A and TN2and TP2 has the same input B. When both the A and B are high, TN1 and TN2 are

ON and TP1 and TP2 are OFF. Therefore, the output is at logic 0. On the other hand, when

both input A and input B are logic 0, TN1 and TN2 is OFF and T P1 and TP2 are ON. Hence, the

output is at V CC , logic 1. Similar situation arises when any one of the input is logic 0. In

such a case, one of the bottom series transistors i.e. either TN1 or TN2 would be OFF forcing

the output to logic 1.

Comparison between CMOS and TTL:

Sl no. CMOS TTL

1 CMOS circuits utilize FETs. TTL circuits utilize BJTs

2 CMOS allows a much higher density of

logic functions in a single chip

TTL allows a less higher density of logic

functions in a single chip

3 CMOS components are more expensive TTL components are less expensive

4 CMOS technology is usually less

expensive because CMOS chips being

smaller and requiring less regulation

TTL technology are more expensive

because TTL chips being larger and

requiring more regulation

5 CMOS circuits draw less power as TTL TTL circuits draw power

6 A simpler and cheaper design Costilier design

7 The transmission of digital signals

becomes simpler and less expensive with

CMOS chips

The transmission of digital signals

becomes Complicated and expensive with

CMOS chips

8 CMOS components are more susceptible

to damage from electrostatic

TTL components are Less susceptible to

damage from electrostatic discharge

9 less current handling capability higher speed, better current handling

10 Better voltage output, better input voltage

tolerance, variable supply voltages,

higher density

output high voltage drops and fixed supply

voltages

Characteristics of CMOS logic:

1. Dissipates low power: The power dissipation is dependent on the power supply

voltage, frequency, output load, and input rise time. At 1 MHz and 50 pF load, the

power dissipation is typically 10 nW per gate.

2. Short propagation delays: Depending on the power supply, the propagation delays are

usually around 25 nS to 50 nS.

3. Rise and fall times are controlled: The rise and falls are usually ramps instead of step

functions, and they are 20 - 40% longer than the propagation delays.

4. Noise immunity approaches 50% or 45% of the full logic swing.

5. Levels of the logic signal will be essentially equal to the power supplied since the

input impedance is so high.

Characteristics of TTL logic:

1. Power dissipation is usually 10 mW per gate.

2. Propagation delays are 10 nS when driving a 15 pF/400 ohm load.

3. Voltage levels range from 0 to Vcc where Vcc is typically 4.75V - 5.25V. Voltage

range 0V - 0.8V creates logic level 0. Voltage range 2V - Vcc creates logic level 1.

Functional diagram of CD4011

IC Family: CMOS

Description: Quad 2- input NAND gate

Functional diagram of 7400

Truth table of NAND Gate:

Input1 Input2 Output

0 0 1

0 1 1

1 0 1

1 1 0

MODULE II

BASIC COMMUNICATION ENGINEERING

Basic Communications System

The basic communications system has:

Transmitter: The sub-system that takes the information signal and processes it prior to

transmission. The transmitter modulates the information onto a carrier signal, amplifies the

signal and broadcasts it over the channel

Channel: The medium which transports the modulated signal to the receiver. Air acts as the

channel for broadcasts like radio. May also be a wiring system like cable TV or the Internet.

Receiver: The sub-system that takes in the transmitted signal from the channel and processes

it to retrieve the information signal. The receiver must be able to discriminate the signal from

other signals which may use the same channel, amplify the signal for processing and

demodulate to retrieve the information. It also then processes the information for reception .

Often, the message is itself a signal, e.g., an audio signal, and to produce a signal that is

suitable for transmission through the channel, we effect some transformation on the message

signal.

Modulation: The process by which some characteristics of a carrier signal is varied in

accordance with message signal.

Modulation is the process whereby some characteristic of one wave is varied in accordance

with some characteristic of another wave. The basic types of modulation are angular

modulation (including the special cases of phase and frequency modulation) and amplitude

modulation

Information

Source

Signal

Modulator

Propagation

Channel

Information

Destination

Signal

Demodulator

Why is Modulation Required?

To achieve easy radiation: If the communication channel consists of free space, antennas

are required to radiate and receive the signal. Dimension of the antennas is limited by the

corresponding wavelength. It is necessary to keep antenna height equal to wavelength (l)

of electrical signal connected to it. Now wavelength (l) will be –

It means that we need height of antenna equal to 100 km! This is practically impossible!

Therefore, we cannot transmit low frequency signals directly. As per equation, if we

increase frequency of electrical signal then wavelength will reduce and hence the height.

The highest frequency of electrical signals in audio range is audio frequency is 20 kHz.

For this, height of antenna will be around 15km. This height is also impossible!

The audio signals are transmitted in the frequency 20Hz to 20 KHz. So all signals will

be inseparably mixed up. In order to separate them they have to be given its own

frequency location. Once the signal is translated a tuned circuit is used in front end of the

receiver to ensure that unwanted signals are rejected. This will overcome the difficulties

of poor radiation at low frequencies and reduces interference

But the carrier frequency will have constant amplitude, frequency and phase

relationship. But the message signal will have all the factors varying. These factors are

impossible to be represented by three set of constant parameters. So un modulated carrier

frequency cannot be used.

To overcome the above, modulation is required where the amplitude or frequency or

phase of the carrier wave is made proportional to the varying amplitude, frequency,

phase of the information or message.

Amplitude modulation is a process in which amplitude of carrier wave is changed in

accordance with instantaneous amplitude of modulating signal.

Frequency modulation (FM) – frequency modulation is a process in which frequency of

carrier wave is changed in accordance with instantaneous amplitude of modulating signal

Equation of modulated wave:

Let the carrier wave of amplitude VC and frequency fc be modulated by a sinusoidal wave of

amplitude Vs and frequency fs. Expressions for instantaneous value of original carrier wave,

signal wave and amplitude modulated (AM) wave are given as

Vc = vc sin Ɯct …(i)

Vs = vs sin Ɯs t …(ii)

Amplitude of the modulated wave is given by

v = vc + Vs

= vc + vs sin Ɯs t

= vc(1 + vs/ vc sin Ɯs t )

= vc(1 + m sin Ɯs t )

Where m is the modulation index which is given by m= vs/ vC

Modulation indeed, m is very important as it determines the quality of the transmitted signal.

The greater the degree of modulation (i.e. m), the stronger and clear will be the audio-signal.

But if m exceeds unity, distortion will occur during reception. m lies between 0 and 1.

The instanteaneous voltage of AM wave will be

V= v sinɵ

= vc(1 + m sin Ɯs t ) sin Ɯct

= vc sin Ɯct + m vc sin Ɯc t sin Ɯs t

= vc sin Ɯct + mVc/2 {cos (Ɯc t - Ɯs t ) - cos (Ɯs t + Ɯc t) } …(iii)

AM wave actually consists of three sinusoidal waves, one having the frequency of carrier fc,

the frequency of carrier wave, another having a frequency (fc + fs) and third having a

frequency (fc-fs). The frequencies (fc + fs) and (fc – fs) are called the side frequencies. The

fig. below shows a portion of the frequency spectrum. The amplitudes of these three waves

are directly related to the amplitudes of carrier wave and audio frequency signal, as shown by

ordinates.

The bandwidth of AM signals can be easily predicted using the now familiar formula:

BW =( fc + fm) - ( fc – fm) = 2 fm

Wave form representation of Amplitude modulated wave:

Waveform of AM wave with 50% (m=0.5), 100% (m=1)and 150% (m=1.5) of

modulation

Power of modulated wave:

The power carried by a voltage wave is proportional to the square of its amplitude. So the

total power carried by AM wave is given as

PT=(Vc/√2)2 + (mVc/2√2)

2 + mVc/2√2)

2 =V

2c/2R (1 + m

2/2) = Pc (1+ m

2/2)

Where Pc= power of the carrier

Power carried by side bands is given as

Psb = (mVc/2R√2) + (mVc/2R√2)2 =V

2c/2R (m

2/2) = m

2Pc/2

Efficiency:

SB / PTOT

where: PSB = the power in all the side-bands

PTOT = the total transmitted power (includes carrier and side-bands)

Advantages of Amplitude Modulation, AM

1. There are several advantages of amplitude modulation, and some of these reasons

have meant that it is still in widespread use today:

2. It is simple to implement

3. it can be demodulated using a circuit consisting of very few components

4. AM receivers are very cheap as no specialized components are needed.

Limitations:

1. Reception is generally noisy as a radio receiver is not capable of distinguishing the

amplitude variations that represent noise and those which contain desired signal

2. Low efficiency as in amplitude modulation, useful power is in the side bands

containing the signal and which is quite low (for example side band power is one-third of

total power of AM wave for 100%. Modulation).

3. Small operating range because of low efficiency of AM.

4. Poor audio quality. Broadcasting stations are usually assigned a band width of 10 kHz

where as for minimizing interference from adjacent stations minimum required bandwidth is

30 kHz. So in AM broadcasting stations audio-quality is usually poor.

Frequency Modulation:

In frequency modulation scheme, the frequency of the carrier signal is modulated or changed

in accordance with the instantaneous value of the information carrying modulating signal. In

phase modulation, on the other hand, it is the phase angle of the carrier signal which is

modulated in accordance with the instantaneous value of the modulating signal.

The carrier signal

With a sinusoidally varying signal called the 'carrier ‘can be expressed as

is the amplitude and is the angular frequency of the carrier signal. is the time.

is the linear frequency.

The modulating signal

The message signal containing the information to be transmitted is called the modulating

signal as it is used to modulate or change some characteristic of the carrier signal. We

consider here a cosinusoidal modulating signal expressed as

is the amplitude and is the angular frequency of the modulating signal. is the time.

fm is the corresponding linear frequency.

The Frequency-Modulated (FM) signal

To obtain an expression for the frequency-modulated signal, the instantaneous value of the

modulated frequency of the carrier, , keeping in mind that in frequency modulation the

carrier frequency is modulated by the instantaneous value of the modulating signal. By

instantaneous, we mean, that which changes with time. The relevant expression would then

be:

is the frequency of the unmodulated carrier, is the instanteneous modulating voltage

and is a proportionality constant.

Since the amplitude of the carrier remains fixed in frequency modulation, the form of the

instanteneous frequency-modulated signal would be:

θ(t) is the total instanteneous phase angle of the frequency-modulated voltage waveform. It is

correlated with fFM(t) as follows:

Δfcmax Is called the maximum frequency deviation in FM andm

c

ff

fm max

is called the

modulation index for FM. So finally we arrive at the expression for the frequency-modulated

(FM) wave,

Waveform representation of a Frequency modulated wave:

Side-frequency components of the FM signal

A look at the expression for the frequency-modulated wave,

which is a sine of a sine function. The frequency-components can be determined, by

expressing the frequency-modulated wave in terms of the so called 'Bessel-functions':

)( fn mJ are the Bessel-functions of the first kind of order (n=1,2,…,∞).

The values of the Bessel-functions can be obtained from tabulated values and graphs for

different values of the modulation-index mf. From the expression above we observe that the

FM wave comprises of a modulated carrier component of frequency ωc and an infinite

number of side-frequency components which may be grouped into, respectively, a pair of 1st-

order side-frequency components called 1st-order side-bands, having frequencies ωc+ωm and

ωc-ωm , a pair of 2nd-order side-frequency components called 2nd-order sidebands having

frequencies and , and so on and so forth. The amplitude of each side-

frequency component is proportional to the Bess el-function of the corresponding order.

The amplitude of the modulated carrier component is decreased from that of the unmodulated

carrier but the decrease is fully compensated by contributions from other side-frequency

components. As a result, the amplitude of the FM wave equals that of the unmodulated

carrier.

The Frequency-Spectrum

The frequency components in a frequency-modulated wave are expressed as vertical lines

spaced apart on both sides of the carrier frequency in what is called a frequency-

spectrum of the FM wave or the FM spectrogram.

The height of each line is proportional to the amplitude of the respective component. The

central line depicts the spectral component at the carrier frequency which we call the

modulated carrier component. The lines on both sides represent the odd-order and even-order

side-frequency component pairs.

Bandwidth of FM wave

As a frequency modulated signal has sidebands that extend out to infinity, it is normal

accepted practice to determine the bandwidth as that which contains approximately 98% of

the signal power.

A rule of thumb, often termed Carsons' Rule states that 98% of the signal power is contained

within a bandwidth equal to the deviation frequency, plus the modulation frequency doubled,

i.e.:

Advantages of frequency modulation, FM

Resilience to noise: One particular advantage of frequency modulation is its

resilience to signal level variations. The modulation is carried only as

variations in frequency not in signal strength

The other advantage of FM is its resilience to noise and interference. It is for

this reason that FM is used for high quality broadcast transmissions.

Easy to apply modulation at a low power stage of the transmitter:

Another advantage of frequency modulation is associated with the

transmitters. It is possible to apply the modulation to a low power stage of the

transmitter.

It is possible to use efficient RF amplifiers with frequency modulated signals:

It is possible to use non-linear RF amplifiers to amplify FM signals in a

transmitter and these are more efficient than the linear ones required for

signals with any amplitude variations. This means that for a given power

output, less battery power is required and this makes the use of FM more

viable for portable two-way radio applications.

Comparison Between AM and FM:

Sl No Amplitude Modulation Frequency Modulation

1. Amplitude of the carrier is varied in

accordance with the instantaneous

amplitude of the modulating signal

Frequency of the carrier is varied in

accordance with the instantaneous

amplitude of the modulating signal

2. AM is simpler FM is complex

3. Only two side bands Infinite no side bands

4. Bandwidth requirement is less Bandwidth requirement is large

5. BW is independent of Modulation index BW is dependent of Modulation index

6. As depth of MI increases the power

transmitted increases

Power transmitted remains constant

7. In the modulated signal carrier Change with amplitude of sidebands

component is constant

8. AM is more prone to signal distortion

and degradation.

FM is less prone to signal distortion

and degradation Amplitude limiters

are used at FM receivers in order to

detect the noise. Also by increasing

deviation the noise can be reduced

9. Sky wave transmission is used Space wave is used

10. Simple equipments are required for

transmission and reception

Complicated circuits are for

modulation and demodulation.

11. Distance of transmission is more Less due to space wave transmission

12. AM uses medium wave and high

frequency ranges for broad casting

AM uses Very high frequency and

ultra high frequency ranges where

noises are less

Pulse Modulation:

A pulse is an abruptly changing voltage or current wave which may or may not repeat

itself. The simplest non repetitive pulse is a stepped up voltage or current shown in Figure 1

(a) which can be obtained by connecting a voltmeter across a battery through a switch and

then suddenly closing the switch. The voltmeter will read Zero upto a time when the switch is

closed, where upon the voltage will suddenly rise to its maximum value and will stay there.

The Figure (b) Shows a repetitive pulse train and Figure (c) shows a pulse with its trailing

and leading edge.

Different types of pulses:

Pulse modulation: It may be defined as a modulation system in which some parameter of a

train of pulse is varied in accordance with the instantaneous value of the modulating signal.

In this system, waveforms are sampled at regular intervals and the information is transmitted

through the sampling rate. The parameters of the pulses which may be varied are: amplitude,

width (or duration), position and time etc.

In pulse modulation, pulses result from sampling the modulating sine wave, In other words,

the modulating wave is sliced into small units, and the process is called quantizing or

quantization. These quantum prints are then converted into digital binary codes, which

represent amplitude of the wave at that print.

In pulse communication, we use rectangular pulses as carrier and one of the parameters of the

pulse (amplitude, width, position) is varied according to the signal. A characteristic of pulse

communication is that the information remains at base band and is not translated to higher

frequency.

Basic block diagram of Pulse modulation

Classification of Pulse Modulation:

Families of pulse modulation

1. Analog pulse modulation

2. Digital pulse modulation

Pulse Amplitude Modulation (PAM)

If a message waveform is adequately described by periodic sample values, it can be

transmitted using analogue pulse modulation wherein the sample values modulate the

amplitude of pulse train. Therefore, the amplitudes of regularly spaced pulses are varied in

proportion to the corresponding sample values of a continuous message signal x(t).This

technique is termed Pulse Amplitude Modulation.

Waveform of PAM:

Pulse‐Time Modulation (PTM)

The sample values of a message can also modulate the time 12 parameters of a pulse train:

1. Pulse‐duration modulation (PDM) (or Pulse-Width Modulation)

Here samples of the message signal are used to vary the

duration of the pulses.

2. Pulse position – pulse‐position modulation (PPM)

The position of a pulse relative to its unmodulated time of occurrence is varied in

accordance with the message signal.

Waveform of PDM and PPM:

Advantages of Pulse modulation:

1. Main advantages of pulse modulation are that when we combine pulse modulation

with continuous modulation (AM, FM, PM), we can obtain “multi channel”

communication system, a desirable feature for “data transmission”.

2. The transmitted power can be concentrated into short bursts instead of being

generated continuously.

AM RECEIVER

The Super heterodyne Receiver

The electromagnetic signal received at the antenna is a sum of the broadcast signal of the

desired station and the signals of other AM and FM radio stations. The AM receiver must

separate the desired signal from all others; this is done in the frequency domain by

eliminating all frequency components that are not from the desired station. The separation of

the desired signal from unwanted signals is performed by the RF amplifier, IF mixer, and IF

amplifier. After the desired signal is separated from the received signal, it is demodulated to

recover the information signal. Demodulation is performed by the envelope detector. The

dotted lines connecting the local oscillator, RF amplifier, and the mixer in block diagrams

and schematics to indicate GANGED TUNING. Ganged tuning is the process used to tune

two or more circuits with a single control. When the frequency of the receiver is changed all

three stages change by the same amount. There is a fixed difference in frequency between the

local oscillator and the RF amplifier at all times. This difference in frequency is the IF. This

fixed difference and ganged tuning ensures a constant IF over the frequency range of the

receiver.

Antenna: The antenna captures electromagnetic energy. The output of the antenna is a

voltage or current that is typically very small (on the order of micro-amps or micro-volts).

This voltage signal contains the desired signal as well as signals from other sources.

RF Amplifier or Pre Amplifier: RF stands for radio frequency which amplifies the very

small signal from the antenna to a voltage level that is appropriate for transistor circuits. The

RF amplifier contains bandpass filters and amplifier section. A bandpass filter attenuates

frequency components outside a particular frequency band. Here the RF amplifier attenuates

frequency components outside the frequency band containing the desired station. The RF

amplifier does not eliminate all frequency components outside the desired band; this is

accomplished by the IF amplifier.

IF Mixer: Heterodyning takes place here. Heterodyning is the combining of the incoming

signal with the local oscillator signal. When heterodyning the incoming signal and the local

oscillator signal in the mixer stage, four frequencies are produced. They are the two basic

input frequencies and the sum and the difference of those two frequencies. The amplifier that

follows (IF amplifier) will be tuned to the difference frequency. This difference frequency is

known as the intermediate frequency (IF). A typical value of IF for an AM communications

receiver is 455kilohertz. The difference frequency is a lower frequency than either the RF

input or oscillator frequencies.

IF Amplifier: The IF amplifier bandpass filters the output of the IF Mixer, eliminating

essentially all of the frequency components outside the frequency interval from 450 kHz to

460 kHz. The desired signal is the envelope of the IF signal.

Demodulator or Envelope Detector: Once the IF stages have amplified the intermediate

frequency to a sufficient level, it is fed to the detector. When the mixer is referred to as the

first detector, this stage would be called the second detector. The detector extracts the

modulating audio signal. The detector stage consists of a rectifying device and filter, which

respond only to the amplitude variations of the IF signal. This develops an output voltage

varying at an audio-frequency rate. The output from the detector is further amplified in the

audio amplifier and is used to drive a speaker or earphones.

Audio Amplifier: The envelope detector cannot supply enough power at its output to

generate the desired sound intensity at the speaker. Thus, an audio amplifier is used to

provide the power to drive the speaker(s); this power may range from mill watts to several

hundred watts.

The output of an ideal audio amplifier is either a voltage or current proportional to its input.

Speaker: The speaker converts its input current or voltage into sound energy.

FM RECEIVER:

RF

Amplifie

r

Mixer

Local

Oscillator

If

Amplifie

r

Demodulator or

Envelop

detector

Audio

Amplifier

Antenna

Automatic Gain Control

(AGC)

Ganged tuning

The function of a frequency-modulated receiver is the same as that of an AM

superheterodyne receiver. Figure above shows the block diagram showing waveforms of a

typical FM superheterodyne receiver. In fm receivers, the amplitude of the incoming signal is

increased in the RF stages. The mixer combines the incoming RF with the local oscillator

signal to produce the intermediate frequency, which is then amplified by one or more IF

amplifier stages. FM receiver has a wide-band IF amplifier. The bandwidth for any type of

modulation must be wide enough to receive and pass all the side-frequency components of

the modulated signal without distortion. The IF amplifier of a FM receiver must have a

broader bandpass than an AM receiver. An FM signal has a wider bandwidth than AM

because the number of extra sidebands that occur in an FM transmission is directly related to

the amplitude and frequency of the audio signal. FM demodulation is the process of detecting

variations in the frequency of the signal. In FM receivers a DISCRIMINATOR is a circuit

designed to respond to frequency shift variations. A discriminator is preceded by a LIMITER

circuit, which limits all signals to the same amplitude level to minimize noise interference.

The audio frequency component is then extracted by the discriminator, amplified in the AF

amplifier, and used to drive the speaker.

ADVANTAGES —

1. FM signals are less susceptible of noises while AM signals are subject to cracking

noises and whistles.

2. FM has the advantage of operating at a higher frequency where a greater amount of

frequencies are available.

3. Fm signals provide much more realistic sound reproduction because of an increase in

the number of sidebands. This increase in the number of sidebands allows more of the

original audio signal to be transmitted and, therefore, a greater range of frequencies

for you to hear.

4. Fm requires a wide bandpass to transmit signals. Each transmitting station must be

assigned a wide band in the fm frequency spectrum. During fm transmissions, the

number of significant sidebands that must be transmitted to obtain the desired fidelity

AM Low Level Modulator Transmitter

Block - diagram of a simple AM Low level signal transmitter is shown on above fig.. The

amplitude modulation is being performed in a stage called the modulator. Two signals are

entering it: high frequency signal called the carrier (or the signal carrier), being created into

the HF oscillator namely crystal oscillator and amplified in the HF amplifier to the required

signal level, and the low frequency (modulating) signal coming from the microphone or some

other LF signal source (cassette player, record player, CD player etc.), being amplified in the

LF amplifier. On modulator's output the amplitude modulated signal is acquired. This signal

is then amplified in the power amplifier, and then led to the transmission antenna.

High Level Modulated AM transmitter:

Carrier Chain or Excitation section:

Carrier oscillator: The carrier oscillator generates the carrier signal, which lies in the RF

range. The frequency of the carrier is always very high. Because it is very difficult to

generate high frequencies with good frequency stability, the carrier oscillator generates a sub

multiple with the required carrier frequency. This sub multiple frequency is multiplied by the

frequency multiplier stage to get the required carrier frequency. Further, a crystal oscillator

can be used in this stage to generate a low frequency carrier with the best frequency stability.

The frequency multiplier stage then increases the frequency of the carrier to its required

value.

Buffer Amplifier: The purpose of the buffer amplifier is twofold. It first matches the output

impedance of the carrier oscillator with the input impedance of the frequency multiplier, the

next stage of the carrier oscillator. It then isolates the carrier oscillator and frequency

multiplier. This is required so that the multiplier does not draw a large current from the

carrier oscillator. If this occurs, the frequency of the carrier oscillator will not remain stable.

Frequency Multiplier: The sub-multiple frequency of the carrier signal, generated by the

carrier oscillator, is now applied to the frequency multiplier through the buffer amplifier. This

stage is also known as harmonic generator. The frequency multiplier generates higher

harmonics of carrier oscillator frequency. The frequency multiplier is a tuned circuit that can

be tuned to the requisite carrier frequency that is to be transmitted.

Power Amplifier: The power of the carrier signal is then amplified in the power amplifier

stage. This is the basic requirement of a high-level transmitter. A class C power amplifier

gives high power current pulses of the carrier signal at its output.

Audio Chain or audio section:

The audio signal to be transmitted is obtained from the microphone, as shown in figure. The

audio driver amplifier amplifies the voltage of this signal. This amplification is necessary to

drive the audio power amplifier. Next, a class A or a class B power amplifier amplifies the

power of the audio signal.

Modulated Class C Amplifier: This is the output stage of the transmitter. The modulating

audio signal and the carrier signal, after power amplification, are applied to this modulating

stage. The modulation takes place at this stage. The class C amplifier also amplifies the

power of the AM signal to the reacquired transmitting power. This signal is finally passed to

the antenna, which radiates the signal into space of transmission. In the absence of a

modulating signal, a continuous RF carrier is radiated by the antenna.

Comparison between High level and Low level Modulation

Wireless communication

Wireless communication is the transfer of information over a distance without the use of

electrical conductors or "wires". The distances involved may be short (a few meters as in

television remote control) or long (thousands or millions of kilometers for radio

communications). When the context is clear, the term is often shortened to "wireless".

Wireless communication is generally considered to be a branch of telecommunications.

Wireless operations permits services, such as long range communications, that are

impossible or impractical to implement with the use of wires. The term is commonly used in

the telecommunications industry to refer to telecommunications systems (e.g. radio

transmitters and receivers, remote controls, computer networks, network terminals, etc.)

which use some form of energy (e.g. radio frequency (RF), infrared light, laser light, visible

light, acoustic energy, etc.) to transfer information without the use of wires.Information is

transferred in this manner over both short and long distances.

It encompasses various types of fixed, mobile, and portable two-way radios, cellular

telephones, personal digital assistants (PDAs), and wireless networking. Other examples of

wireless technology include GPS units, garage door openers and or garage doors, wireless

computer mice, keyboards and headsets, satellite television and cordless telephones.

Satellite

A Satellite is a solid object which revolves around some heavenly body due to the effect of

gravitational forces which are mutual in nature. We can categorize satellites in two types,

namely Passive Satellites and Active satellites. Passive satellites are not like active satellites.

Even a moon can be a passive satellite. Thus passive satellites are relay stations in space. A

passive satellite can be further subdivided into two types, namely Natural satellites and

artificial satellites. A moon is a natural satellite of earth. But spherical balloon with metal

coated plastic serve as artificial satellites.

Active satellites are complicated structures having a processing equipment called

Transponder which is very vital for functioning of the satellite. These transponders serve dual

purpose i.e. provides amplification of the incoming signal and performs the frequency

translation of the incoming signal to avoid interference between the two signals.

Satellite Orbits

Satellites are launched into orbit, which is to say that they are shot up into the sky on rockets

to get them up above the atmosphere where there is no friction. The idea is to get them flying

so fast, that when they fall back to earth, they fall towards earth at the same rate as the earth's

surface falls away from them. When an object's path around the earth, when it's "trajectory"

matches the earth's curvature, the object is said to be "in orbit".

Orbital Distances

Any satellite can achive orbit at any distance from the earth if its velocity is sufficient to keep

it from falling to earth and it is free of friction from earth's atmosphere. The farter the satellite

is from the earth, the longer it takes for a radio or microwave frequency transmission to reach

the satellite. The altitudes at which satellites can orbit are split into two categories:

Low Earth Orbit (LEO)

Medium Earth Orbit (MEO)

Satellites can orbit around the equator or the poles, though technically they can orbit the earth

on any eliptical or circular path.

Equatorial Orbit

Polar Orbit

When a satellite's orbit matches the rotation of the earth, and it's position over the earth

remains fixed, it's called Geostationary or geosynchronous orbit.

Orbit Distance Miles Km 1-way

Delay

Low Earth Orbit (LEO) 100-500 160 - 1,400 50 ms

Medium Earth Orbit (MEO) 6,000 - 12,000 10 -15,000 100 ms

Geostationary Earth Orbit (GEO) ~22,300 36,000 250 ms

Satellite Orbits

Fig. 1 - Satellite Orbital Paths

LOW EARTH (LEO)

Typical Uses: Satellite phone, Military, Observation

Satellites in low earth orbit (LEO) satellites complete one orbit roughly every 90 minutes at a

height of between 100 and 500 miles above the earth's surface. This means that they are fast

moving ( >17,000mph) and sophisticated ground equipment must be used to track the

satellite. This makes for expensive antennas that must track the satellite and lock to the signal

while moving.

Satellites in this orbital range also have a very small 'footprint'--that is, the surface of the

earth that can be covered by the signal broadcast from the satellite is small. Thus, lots of

satellites (35 or more) are required to make worldwide communication possible. At several

million dollars per satellite, this is a very expensive satellite network to build and maintain.

MIDDLE EARTH (MEO)

Typical Uses: Weather Satellites, Observation, spy satellites

Most of the satellites in middle earth orbit circle the earth at approximately 6,000 to 12,000

miles above the earth in an elliptical orbit around the poles of the earth. Any orbit that circles

around the poles is refered to as a 'polar orbit'. Polar orbits have the advantage of covering a

different section of the earth's surface as they circle the earth. As the earth rotates, satellites in

polar orbits can cover the entire surface of the earth. Fewer satellites are required to create

coverage for the entire earth, as these satellites are higher and have a larger footprint. Spy

satellites typically use middle earth, polar orbits to cover as much of the earth's surface as

possible from one satellite.

GEOSTATIONARY and GEOSYNCHRONOUS (GEO)

Typical Uses for satellites in Geostationary Orbits: Television satellites, Long Distance

Communications satellites, Internet, Global Positioning Systems (GPS)

At 22,240 miles above the earth, craft inserted into orbit over the equator and traveling at

approximately 6,880 miles per hour around the equator following the earths rotation. This

allows these satellites to maintain their relative position over the earth's surface. Since the

satellite follows the earth, and takes 24 hours to complete it's orbit around the earth,

geostationary orbits are also called geosynchronous.

Craft in geostationary orbit don't need to be tracked, reducing the cost of earthstation

antennas. Geostationary craft have the advantage of height, giving them the broadest footprint

(the signal broadcast covers the most earth surface), but this same height makes them

unsuitable for Voice, Voice over IP and other latency-sensitive services due to the ground-

satellite-ground propagation times (225 ms round trip or more). Additional power and larger

dishes are also required to boost the signal to the satellite and receive the signal on the

ground. Signals in geostationary systems also must pass through the entire atmosphere and

suffer the greatest dissipation of all three orbital systems.

Groundstations in the northern hemisphere point south to the equator to send and receive to

satellites. Groundstations in the southern hemisphere point north to communicate with the

same satellites. Geostationary satellites do have one small limitation. Groundstations that are

too far north or south (at the poles) cannot 'see' the geostationary satellite as the curve of the

earth is between the groundstation and the satellite. Thus, satellites in other orbits must be

used.

Geostationary satellites are 'parked' in positions over the equator to maximize coverage over

the inhabited portions of the earth. This area in space forms a belt and is referred to as the

'Clarke Belt' because it was noted science-fiction author Arthur C. Clarke that was the first to

propose the idea.

POLAR ORBIT

A satellite in this orbit flies over the earth from pole to pole. They are typically inserted at

lower orbits. Many polar orbits are elliptical in nature, and most polar craft are in the MEO

altitude. This orbit is most commonly used in surface mapping and observation satellites as

they allow a satellite which orbits the earth to take advantage of the earth's rotation to cause

the entire surface of the earth to pass below the satellite. Many of the pictures of the earth's

surface in applications such as Google Earth come from satellites in these polar orbits.

ELLIPTICAL

An elliptical orbit is an oval shaped orbit used to place the orbit close to earth in specific

locations and to orbit at specific intervals. An elliptical orbit has two critical distances called

apogee and perigee. Perigee is when an orbital object is closest to the earth. Apogee is when

it is farthest away. Elliptical orbit satellites cover the polar regions where the geostationary

satellites cannot reach.

All ABOUT SATELLITE COMMUNICATION

Earth station

Terrestrial

system

User

Earth station

Terrestrial

system

User

Satellite

Uplink

Antenna

Downlink

Antenna

Earth station

Terrestrial

system

User

Earth station

Terrestrial

system

User

Earth station

Terrestrial

system

User

Earth station

Terrestrial

system

User

SatelliteSatellite

Uplink

Antenna

Uplink

Antenna

Downlink

Antenna

Downlink

Antenna

The term Satellite communication is very frequently used, but what is satellite

communication? It is simply the communication of the satellite in space with large number of

earth stations on the ground. Users are the ones who generate baseband signals, which is

processed at the earth station and then transmitted to the satellite through dish antennas. Now

the user is connected to the earth station via some telephone switch or some dedicated link.

The satellite receives the uplink frequency and the transponder present inside the satellite

does the processing function and frequency down conversion in order to transmit the

downlink signal at different frequency. The earth station then receives the signal from the

satellite through parabolic dish antenna and processes it to get back the baseband signal. This

baseband signal is then transmitted to the respective user via dedicated link or other terrestrial

system. Previously satellite communication system used large sized parabolic antennas with

diameters around 30 meters because of the very faint and weak signals received. But

nowadays satellites have become much stronger, bigger and powerful due to which antennas

used have become automatically smaller in size. Thus the earth station antennas are now not

large in size as the antennas used in olden days. A satellite communication system operates

and works in the millimeter and microwave wave frequency bands from 1 Ghz to 50 Ghz.

There are various frequency bands utilized by satellites but the most recognized of them is

the uplink frequency of 6 Ghz and the downlink frequency of 4 Ghz. Actually the uplink

frequency band is 5.725 to 7.075 Ghz and the actual downlink frequency band is from 3.4 to

4.8 Ghz. The major components of a Satellite Communication system is spacecraft and one or

more earth stations.

Satellite Communication Earth Station

Earth station is the common name for every installation located on the Earth's surface and

intended for communication (transmission and/or reception) with one or more satellites. Earth

stations include all devices and installations for satellite communications: handheld devices

for mobile satellite telephony, briefcase satellite phones, satellite TV reception, as well as

installations that are less familiar, eg VSAT stations and satellite broadcast TV stations. The

term Earth station refers to the collection of equipment that is needed to perform

communications via satellite: the antenna (often a dish) and the associated equipment

(receiver/decoder, transmitter).

The equipment used in satellite earth station are shown in fig , the earth station consist of a

dish antenna transmitter which can transmit a high frequencies (5.9—6.4GHZ) micro wave

signals, some earth stations also called ground station , which can transmit and receive the

signals while others can only receive signals.

A high directive and a high gain antenna is necessary at the earth station , because the losses

over the long T/N path is very high , the signals power reaching back to the earth station from

satellite is very small . there fore at receiving end a parabolic dish antenna with 61m diameter

provides a high gain and thus amplify the signal power , it is important to have a low noise

amplifier before the mixer stage in the receiver C,K,T at the satellite earth terminal.

SATELLITE TRANSPONDER

The transponder is the key component for satellite communications: it is the part of the

payload that takes the signals received from the transmitting Earth station, filters and

translates these signals and then redirects them to the transmitting antenna on board.

Communications satellites carry a large number of transponders on board (normally from six

to more than 24), enabling them to deliver multiple channels of communication at the same

time. These channels are called carriers.

The figure shows a simplified configuration of the block diagram of a

communications/broadcasting satellite transponder. The configuration of an actual satellite is

much more complex than this figure because multiple transponders are mounted, switching

between active and redundant pieces of equipment is required in the case of failure and

switching of transmission paths is also necessary to provide various services.

The main functions of a transponder are 1) receiving a faint signal from the ground; 2)

amplifying it with low noise; 3) converting it to a frequency to be transmitted to the ground;

4) limiting bandwidth to prevent unnecessary signal emission; 5) amplifying power to a level

for transmission to the ground; 6) transmitting the signal to the ground.The transponder is

generally organized by combining multiple pieces of equipment, each of which provides one

of the above functions. When we refer to transponder equipment, we mean these individual

pieces of equipment.

Fig.2 Basic configuration of a satellite transponder.

Low-Noise Amplifier (LNA)

At present, one of the leading products among transponder equipment is the low-noise

amplifier (LNA). The LNA is used to amplify a received faint signal with low noise and is

therefore required to have excellent noise and wideband characteristics. The LNA is a single-

function equipment with a compact size and its specifications are not much dependent on the

specifications of individual satellites.

Current LNAs use the GaAs FET as an amplification device, but we are studying

standardization and cost reduction using MMICs as well as a device that can meet the

increasingly severe over power input requirements.

Frequency Converter

A frequency converter is used to convert the frequency of a received signal into a frequency

for use in transmission to the ground. For instance, the Ku-band frequency converter converts

signal frequency from the 14-GHz band to the 12-GHz band. The main characteristics

required for a frequency converter include low spurious emissions, linearity and a stable local

frequency. Particularly in the case of frequency conversion for satellite equipment, the input

and output frequencies are close, so reducing spurious emissions from the mixer becomes a

key design issue. In addition, since frequencies often vary due to satellites and

communication channels, the manufacturing of frequency converters is characterized by

high-mix, low-volume production.

Antenna

The antenna is the equipment characterizing each communications/broadcasting satellite's

mission. Its functions include the formation of multibeams and contoured beams and the

tracking of aircraft or other satellites. antennas that receive the original signal from the

transmitting Earth station and re-transmit this signal to the receive stations on Earth. The

antennas that were used in the past to do this were omni-directional (transmitting signals in

every direction) and not very effective. They were replaced by more efficient high-gain

antennas (most often dish shaped) pointing quite precisely towards the areas they were

servicing. To allow for flexibility in services or areas covered, later developments allowed

the re-pointing of the so-called steerable antenna to cover a different area or to reshape or

reformat the beam. Future developments will allow for a highly precise and efficient

reshaping of the transmitted beam in order to cover very small areas (pencil beams). This will

greatly facilitate the differentiation of services within large regions. The antennas on board

the satellite are typically limited in size to around 2-3 m by the space that is available on the

satellite structure.

Differences between satellite-based and terrestrial wireless communications:

1. The area of coverage of a satellite system far exceeds that of a terrestrial system. In

the case of a geostationary satellite, a single antenna is visible to about one-fourth of

the earth's surface.

2. Spacecraft power and allocated bandwidth are limited resources that call for careful

tradeoffs in earth station/satellite design parameters.

3. Conditions between communicating satellites are more time invariant than those

between satellite and earth station or between two terrestrial wireless antennas. Thus,

satellite-to-satellite communication links can be designed with great precision.

4. Transmission cost is independent of distance, within the satellite's area of coverage.

5. Broadcast, multicast, and point-to-point applications are readily accommodated.

6. Very high bandwidths or data rates are available to the user.

7. Although satellite links are subject to short-term outages or degradations, the quality

of transmission is normally extremely high.

8. For a geostationary satellite, there is an earth-satellite-earth propagation delay of

about one-fourth of a second.

APPLICATIONS OF SATELLITES:

1. Weather Forecasting

2. Radio and TV Broadcast

3. Military Satellites

4. Navigation Satellites

5. Global Telephone

6. Connecting Remote Areas

7. Global Mobile Communication

History of GSM

During the early 1980s, analog cellular telephone systems were experiencing rapid growth in

Europe, particularly in Scandinavia and the United Kingdom, but also in France and

Germany. Each country developed its own system, which was incompatible with everyone

else's in equipment and operation. This was an undesirable situation, because not only was

the mobile equipment limited to operation within national boundaries, which in a unified

Europe were increasingly unimportant, but there was also a very limited market for each type

of equipment, so economies of scale and the subsequent savings could not be realized.

The Europeans realized this early on, and in 1982 the Conference of European Posts and

Telegraphs (CEPT) formed a study group called the Groupe Spécial Mobile (GSM) to study

and develop a pan-European public land mobile system. The proposed system had to meet

certain criteria:

Good subjective speech quality

Low terminal and service cost

Support for international roaming

Ability to support handheld terminals

Support for range of new services and facilities

Spectral efficiency

ISDN compatibility

In 1989, GSM responsibility was transferred to the European Telecommunication Standards

Institute (ETSI), and phase I of the GSM specifications were published in 1990. Commercial

service was started in mid-1991, and by 1993 there were 36 GSM networks in 22 countries.

Although standardized in Europe, GSM is not only a European standard. Over 200 GSM

networks (including DCS1800 and PCS1900) are operational in 110 countries around the

world. In the beginning of 1994, there were 1.3 million subscribers worldwide, which had

grown to more than 55 million by October 1997. With North America making a delayed entry

into the GSM field with a derivative of GSM called PCS1900, GSM systems exist on every

continent, and the acronym GSM now aptly stands for Global System for Mobile

communications.

The developers of GSM chose an unproven (at the time) digital system, as opposed to the

then-standard analog cellular systems like AMPS in the United States and TACS in the

United Kingdom. They had faith that advancements in compression algorithms and digital

signal processors would allow the fulfillment of the original criteria and the continual

improvement of the system in terms of quality and cost. The over 8000 pages of GSM

recommendations try to allow flexibility and competitive innovation among suppliers, but

provide enough standardization to guarantee proper interworking between the components of

the system. This is done by providing functional and interface descriptions for each of the

functional entities defined in the system.

Services provided by GSM

From the beginning, the planners of GSM wanted ISDN compatibility in terms of the services

offered and the control signalling used. However, radio transmission limitations, in terms of

bandwidth and cost, do not allow the standard ISDN B-channel bit rate of 64 kbps to be

practically achieved.

Using the ITU-T definitions, telecommunication services can be divided into bearer services,

teleservices, and supplementary services. The most basic teleservice supported by GSM is

telephony. As with all other communications, speech is digitally encoded and transmitted

through the GSM network as a digital stream. There is also an emergency service, where the

nearest emergency-service provider is notified by dialing three digits (similar to 911).

A variety of data services is offered. GSM users can send and receive data, at rates up to 9600

bps, to users on POTS (Plain Old Telephone Service), ISDN, Packet Switched Public Data

Networks, and Circuit Switched Public Data Networks using a variety of access methods and

protocols, such as X.25 or X.32. Since GSM is a digital network, a modem is not required

between the user and GSM network, although an audio modem is required inside the GSM

network to interwork with POTS.

Other data services include Group 3 facsimile, as described in ITU-T recommendation T.30,

which is supported by use of an appropriate fax adaptor. A unique feature of GSM, not found

in older analog systems, is the Short Message Service (SMS). SMS is a bidirectional service

for short alphanumeric (up to 160 bytes) messages. Messages are transported in a store-and-

forward fashion. For point-to-point SMS, a message can be sent to another subscriber to the

service, and an acknowledgement of receipt is provided to the sender. SMS can also be used

in a cell-broadcast mode, for sending messages such as traffic updates or news updates.

Messages can also be stored in the SIM card for later retrieval.

Supplementary services are provided on top of teleservices or bearer services. In the current

(Phase I) specifications, they include several forms of call forward (such as call forwarding

when the mobile subscriber is unreachable by the network), and call barring of outgoing or

incoming calls, for example when roaming in another country. Many additional

supplementary services will be provided in the Phase 2 specifications, such as caller

identification, call waiting, multi-party conversations.

Architecture of the GSM network

A GSM network is composed of several functional entities, whose functions and interfaces

are specified. Figure 1 shows the layout of a generic GSM network. The GSM network can

be divided into three broad parts. The Mobile Station is carried by the subscriber. The Base

Station Subsystem controls the radio link with the Mobile Station. The Network Subsystem,

the main part of which is the Mobile services Switching Center (MSC), performs the

switching of calls between the mobile users, and between mobile and fixed network users.

The MSC also handles the mobility management operations. Not shown is the Operations and

Maintenance Center, which oversees the proper operation and setup of the network. The

Mobile Station and the Base Station Subsystem communicate across the Um interface, also

known as the air interface or radio link. The Base Station Subsystem communicates with the

Mobile services Switching Center across the A interface.

Figure 1. General architecture of a GSM network

Mobile Station

The mobile station (MS) consists of the mobile equipment (the terminal) and a smart card

called the Subscriber Identity Module (SIM). The SIM provides personal mobility, so that the

user can have access to subscribed services irrespective of a specific terminal. By inserting

the SIM card into another GSM terminal, the user is able to receive calls at that terminal,

make calls from that terminal, and receive other subscribed services.

The mobile equipment is uniquely identified by the International Mobile Equipment Identity

(IMEI). The SIM card contains the International Mobile Subscriber Identity (IMSI) used to

identify the subscriber to the system, a secret key for authentication, and other information.

The IMEI and the IMSI are independent, thereby allowing personal mobility. The SIM card

may be protected against unauthorized use by a password or personal identity number.

Base Station Subsystem

The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS)

and the Base Station Controller (BSC). These communicate across the standardized Abis

interface, allowing (as in the rest of the system) operation between components made by

different suppliers.

The Base Transceiver Station houses the radio tranceivers that define a cell and handles the

radio-link protocols with the Mobile Station. In a large urban area, there will potentially be a

large number of BTSs deployed, thus the requirements for a BTS are ruggedness, reliability,

portability, and minimum cost.

The Base Station Controller manages the radio resources for one or more BTSs. It

handles radio-channel setup, frequency hopping, and handovers, as described below. The

BSC is the connection between the mobile station and the Mobile service Switching Center

(MSC).

Network Subsystem

The central component of the Network Subsystem is the Mobile services Switching

Center (MSC). It acts like a normal switching node of the PSTN or ISDN, and additionally

provides all the functionality needed to handle a mobile subscriber, such as registration,

authentication, location updating, handovers, and call routing to a roaming subscriber. These

services are provided in conjuction with several functional entities, which together form the

Network Subsystem. The MSC provides the connection to the fixed networks (such as the

PSTN or ISDN). Signalling between functional entities in the Network Subsystem uses

Signalling System Number 7 (SS7), used for trunk signalling in ISDN and widely used in

current public networks.

The Home Location Register (HLR) and Visitor Location Register (VLR), together

with the MSC, provide the call-routing and roaming capabilities of GSM. The HLR contains

all the administrative information of each subscriber registered in the corresponding GSM

network, along with the current location of the mobile. The location of the mobile is typically

in the form of the signalling address of the VLR associated with the mobile station. The

actual routing procedure will be described later. There is logically one HLR per GSM

network, although it may be implemented as a distributed database.

The Visitor Location Register (VLR) contains selected administrative information

from the HLR, necessary for call control and provision of the subscribed services, for each

mobile currently located in the geographical area controlled by the VLR. Although each

functional entity can be implemented as an independent unit, all manufacturers of switching

equipment to date implement the VLR together with the MSC, so that the geographical area

controlled by the MSC corresponds to that controlled by the VLR, thus simplifying the

signalling required. Note that the MSC contains no information about particular mobile

stations --- this information is stored in the location registers.

The other two registers are used for authentication and security purposes. The

Equipment Identity Register (EIR) is a database that contains a list of all valid mobile

equipment on the network, where each mobile station is identified by its International Mobile

Equipment Identity (IMEI). An IMEI is marked as invalid if it has been reported stolen or is

not type approved. The Authentication Center (AuC) is a protected database that stores a

copy of the secret key stored in each subscriber's SIM card, which is used for authentication

and encryption over the radio channel.

Radio link aspects

The International Telecommunication Union (ITU), which manages the international

allocation of radio spectrum (among many other functions), allocated the bands 890-915

MHz for the uplink (mobile station to base station) and 935-960 MHz for the downlink (base

station to mobile station) for mobile networks in Europe. Since this range was already being

used in the early 1980s by the analog systems of the day, the CEPT had the foresight to

reserve the top 10 MHz of each band for the GSM network that was still being developed.

Eventually, GSM will be allocated the entire 2x25 MHz bandwidth.

CELLULAR CONCEPT

Introduction

The cellular concept was a major breakthrough in solving the problem of spectral congestion

and user capacity. It offered very high capacity in a limited spectrum allocation without any

major technological changes. The cellular concept is a system-level idea which calls for

replacing a single, high power transmitter (large cell) with many low power transmitters

(small cells), each providing coverage to only a small portion of the service area. Each base

station is allocated a portion of the total number of channels available to the entire system,

and nearby base stations are assigned different groups of channels so that all the available

channels are assigned to a relatively small number of neighbouring base stations.

Neighboring base stations are assigned different groups of channels so that the interference

between base stations (and the mobile users under their control) is minimized. By

systematically spacing base stations and their channel groups throughout a market, the

available channels are distributed throughout the geographic region and may be reused as

many times as necessary so long as the interference between co-channel stations is kept

below acceptable levels.

As the demand for service increases (i.e., as more channels are needed within a particular

market), the number of base stations may be increased (along with a corresponding decrease

in transmitter power to avoid added interference), thereby providing additional radio capacity

with no additional increase in radio spectrum. This fundamental principle is the foundation

for all modern wireless communication systems, since it enables a fixed number of channels

to serve an arbitrarily large number of subscribers by reusing the channels throughout the

coverage region. Furthermore, the cellular concept allows every piece of subscriber

equipment within a country or continent to be manufactured with the same set of channels so

that any mobile may be used anywhere within the region.

A cellular network or mobile network is a radio network distributed over land areas called

cells, each served by at least one fixed-location transceiver, known as a cell site or base

station. In a cellular network, each cell uses a different set of frequencies from neighbouring

cells, to avoid interference and provide guaranteed bandwidth within each cell.

When joined together these cells provide radio coverage over a wide geographic area. This

enables a large number of portable transceivers (e.g., mobile phones, pagers, etc.) to

communicate with each other and with fixed transceivers and telephones anywhere in the

network, via base stations, even if some of the transceivers are moving through more than

one cell during transmission.

Cellular networks offer a number of advantages over alternative solutions:

flexible enough to use the features and functions of almost all public and private

networks

increased capacity

reduced power use

larger coverage area

reduced interference from other signals

Example of frequency reuse factor or pattern 1/4

In a cellular radio system, a land area to be supplied with radio service is divided into regular

shaped cells, which can be hexagonal, square, circular or some other regular shapes, although

hexagonal cells are conventional. Each of these cells is assigned multiple frequencies (f1 - f6)

which have corresponding radio base stations. The group of frequencies can be reused in

other cells, provided that the same frequencies are not reused in adjacent neighboring cells as

that would cause co-channel interference.

The increased capacity in a cellular network, compared with a network with a single

transmitter, comes from the fact that the same radio frequency can be reused in a different

area for a completely different transmission. If there is a single plain transmitter, only one

transmission can be used on any given frequency. Unfortunately, there is inevitably some

level of interference from the signal from the other cells which use the same frequency. This

means that, in a standard FDMA system, there must be at least a one cell gap between cells

which reuse the same frequency.

MODULE III

Basic instrumentation and Consumer electronics

TRANSDUCERS

A transducer is a device that converts one form of energy to another. Energy types include

(but are not limited to) electrical, mechanical, electromagnetic (including light), chemical,

acoustic or thermal energy. While the term transducer commonly implies the use of a

sensor/detector, any device which converts energy can be considered a transducer.

Transducers are widely used in measuring instruments.

Common examples for a transducer include microphones, loudspeakers, thermometers,

position and pressure sensors, strain gauges and antenna. Although not generally thought of

as transducers, photocells, LEDs (light-emitting diodes), and even common light bulbs are

transducers.

STRAIN GAUGE

A strain gauge is a device used to measure the strain of an object. Invented by Edward E.

Simmons and Arthur C. Ruge in 1938, the most common type of strain gauge consists of an

insulating flexible backing which supports a metallic foil pattern. The gauge is attached to the

object by a suitable adhesive. As the object is deformed, the foil is deformed, causing its

electrical resistance to change. This resistance change, usually measured using a Wheatstone

bridge, is related to the strain by the quantity known as the gauge factor.

A strain gauge takes advantage of the physical property of electrical conductance and its

dependence on the conductor's geometry. When an electrical conductor is stretched, it will

become narrower and longer, which increases its electrical resistance. Conversely, when a

conductor is compressed, it will broaden and shorten, and electrical resistance decreases. By

measuring the electrical resistance of the strain gauge, the amount of applied stress may be

inferred. A typical strain gauge arranges a long, thin conductive strip in a zigzag pattern of

parallel lines such that a small amount of stress in the direction of the orientation of the

parallel lines results in a multiplicatively larger strain over the effective length of the

conductor and hence a multiplicatively larger change in resistance—than would be observed

with a single straight-line conductive wire.

Working:

The working of a strain gauge is based on the fact that when stress is applied on a metal

semiconductor, its resistance changes owing to change in length and cross-sectional area of

the conductor. Resistance of conductor under stress is also changed due to change in

resistivity of the conductor. This property is called piezo-resistive effect.

If a conductor of length L, area of cross-section A, and resistivity ρ is subjected to axial

tension, the resistance will change because of change in length, area of cross-section and

resistivity of the material as:

A

LR

The Gauge factor G indicated the strain sensitivity of the gauge in terms of the change in

resistance per unit resistance per unit strain.

The term L

L is called strain ε and is usually measured in micro-strains (1μ mm/m).

RR

LL

RRG

Strain gauge based technology is utilized commonly in the manufacture of pressure sensors.

The gauges used in pressure sensors themselves are commonly made from silicon,

polysilicon, metal film, thick film, and bonded foil.

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)

LVDT is a differential transformer used for translating linear motion into an electrical signal.

LVDT linear position sensors are readily available that can measure movements as small as a

few millionths of an inch up to several inches, but are also capable of measuring positions up

to ±20 inches.

Figure shows the components of a typical LVDT.

A normal Strain Gauge

When the gauge is stressed,

resistance increases

When the gauge is

compressed, resistance is

reduced.

The transformer's internal structure consists of a primary winding centred between a pair of

identically wound secondary windings, symmetrically spaced about the primary. The coils

are wound one one-piece hollow form of thermally stable glass reinforced polymer, secured

in a cylindrical stainless steel housing. This coil assembly is usually the stationary element of

the position sensor.

The moving element of an LVDT is a separate tubular armature of magnetically permeable

material called the core, which is free to move axially within the coil's hollow bore, and

mechanically coupled to the object whose position is being measured. In operation, the

LVDT's primary winding is energized by alternating current of appropriate amplitude and

frequency, known as the primary excitation. The LVDT's electrical output signal is the

differential AC voltage between the two secondary windings, which varies with the axial

position of the core within the LVDT coil. Usually this AC output voltage is converted by

suitable electronic circuitry to high level DC voltage or current that is more convenient to

use.

Working

The LVDT's primary winding, P, is energized by a constant amplitude AC source. The

magnetic flux thus developed is coupled by the core to the adjacent secondary windings, S1

and S2.

Case I: If the core is located midway between S1 and S2, equal flux is coupled to each

secondary so the voltages, V1 and V2, induced in windings S1 and S2 respectively are equal.

At this reference midway core position, known as the null point, the differential voltage

output, (V1 - V2), is essentially zero.

Case II: If the core is moved closer to S1 than to S2, more flux is coupled to S1 and less to S2,

so the induced voltage V1 is increased while V2 is decreased, resulting in the differential

voltage (V1 - V2).

Case III: If the core is moved closer to S2, more flux is coupled to S2 and less to S1, so V2 is

increased as V1 is decreased, resulting in the differential voltage (V2 - V1).

Merits

Infinite resolution

Has Linear characteristics within its prescribed range

High sensitivity

Output is very high which ,in some cases, eliminates the need for amplification

devices

Consumes less power

Can be used on high frequencies upto 20kHz

Output impedance remains constant

Absence of sliding contact makes the LVDT a more reliable device

Has very low hysteresis

Rugged device

Very stable and easy to align and maintain due to simplicity of construction , small

size and light weight

Demerits

Shielding is required since it is sensitive to magnetic field

Relatively large displacements are needed for appreciable differential output

Sometimes, the performance is affected by vibrations

THERMISTOR

A thermistor is a type of resistor whose resistance varies significantly with temperature.

Thermistors are constructed of semiconductor material with a resistivity that is especially

sensitive to temperature. However, unlike most other resistive devices, the resistance of a

thermistor decreases with increasing temperature.

Since the resistance of thermistors is dependent on the temperature, they can be connected in

the electrical circuit to measure the temperature of the body. The thermistors are made up of

ceramic like semiconducting materials. They are mostly composed of oxides of manganese,

nickel and cobalt having the resistivity of about 100 to 450,000 ohm-cm. Since the resistivity

of the thermistors is very high the resistance of the circuit in which they are connected for

measurement of temperature can be measured easily. This resistance is calibrated against, the

input quantity, which is the temperature, and its value can be obtained easily. Thermistors are

available in various shapes like disc, rod, washer, bead etc. They are of small size and they all

can be fitted easily to the body whose temperature has to be measured and also can be

connected to the circuit easily. Most of the thermistors are quite cheap.

Principle of Working of Thermistors

As mentioned earlier the resistance of the thermistors decreases with the increase its

temperature. The resistance of thermistor is given by:

keRR 0

where

0

11

TTk

where R is the resistance of the thermistor at any temperature T in oK (degree Kelvin)

R0 is the resistance of the thermistor at particular reference temperature T0 in oK

β is a constant whose value ranges from 3400 to 3900 depending on the material used for the

thermistors and its composition.

The thermistor acts as the temperature sensor and it is placed on the body whose temperature

is to be measured. It is also connected in the electric circuit. When the temperature of the

body changes, the resistance of the thermistor also changes, which is indicated by the circuit

directly as the temperature since resistance is calibrated against the temperature. The

thermistor can also be used for some control which is dependent on the temperature.

Merits

1) When the resistors are connected in the electrical circuit, heat is dissipated in the circuit

due to flow of current. This heat tends to increase the temperature of the resistor due to which

their resistance changes. For the thermistor the definite value of the resistance is reached at

the given ambient conditions due to which the effect of this heat is reduced.

2) In certain cases even the ambient conditions keep on changing, this is compensated by the

negative temperature characteristics of the thermistor. This is quite convenient against the

materials that have positive resistance characteristics for the temperature.

3) The thermistors are used not only for the measurement of temperature, but also for the

measurement of pressure, liquid level, power etc.

4) They are also used as the controls, overload protectors, giving warnings etc.

5) The size of the thermistors is very small and they are very low in cost. However, since

their size is small they have to be operated at lower current levels.

PHOTODIODES

A photodiode is a type of photodetector capable of converting light into either current or

voltage, depending upon the mode of operation. The common, traditional solar cell used to

generate electric solar power is a large area photodiode.

Photodiodes are similar to regular semiconductor diodes except that they may be either

exposed (to detect vacuum UV or X-rays) or packaged with a window or optical fibre

connection to allow light to reach the sensitive part of the device. Many diodes designed for

use specifically as a photodiode use a PIN junction rather than a p-n junction, to increase the

speed of response. A photodiode is designed to operate in reverse bias.

A photodiode is a p-n junction or PIN structure. When a photon of sufficient energy strikes

the diode, it excites an electron, thereby creating a free electron (and a positively charged

electron hole). This mechanism is also known as the inner photoelectric effect. If the

absorption occurs in the junction's depletion region these carriers are swept from the junction

by the built-in field of the depletion region. Thus holes move toward the anode, and electrons

toward the cathode, and a photocurrent is produced. This photocurrent is the sum of both the

dark current (without light) and the light current, so the dark current must be minimized to

enhance the sensitivity of the device. The photodiode can be operated in different modes:

Photovoltaic mode: When used in zero bias or photovoltaic mode, the flow of

photocurrent out of the device is restricted and a voltage builds up. This mode

exploits the photovoltaic effect, which is the basis for solar cells – a traditional solar

cell is just a large area photodiode.

Photoconductive mode:

A photodiode can be operated as a photodetector when it is reverse biased. This

increases the width of the depletion layer, which decreases the junction's capacitance

resulting in faster response times. The reverse bias induces only a small amount of

current (known as saturation or back current) along its direction while the

photocurrent remains virtually the same.

A photodiode is operated as an LED when it is forward biased. This supplies a current

through the junction and emits photons of the specified wavelength. Since the electron

energy drop across the junction can be chosen by design and the use of different

elements, we can create different wavelengths.

Merits

1. Excellent linearity of output current as a function of incident light

2. Spectral response from 190 nm to 1100 nm (silicon), longer wavelengths with other

semiconductor materials

3. Low noise

4. Ruggedized to mechanical stress

5. Low cost

6. Compact and light weight

7. Long lifetime

8. High quantum efficiency, typically 80%

9. No high voltage required

Demerits

1. Small area

2. No internal gain

3. Much lower overall sensitivity

4. Photon counting only possible with specially designed, usually cooled photodiodes,

with special electronic circuits

5. Response time for many designs is slower

MOVING COIL MICROPHONE

A microphone is a transducer that converts acoustic energy to electrical energy. It produces

an electrical analog output signal which is proportional to the "acoustic" sound wave acting

upon its flexible diaphragm. This signal is an "electrical image" representing the

characteristics of the acoustic waveform. Generally, the output signal from a microphone is

an analog signal either in the form of a voltage or current which is proportional to the actual

sound wave.

The most common types of microphones available as sound transducers are Dynamic,

Condenser, Ribbon, Electret, and Piezo-electric Crystal types. Typical applications for

microphones as a sound transducer include audio recording, reproduction, broadcasting as

well as telephones, television, digital computer recording and body scanners, where

ultrasound is used in medical applications. An example of a simple "Dynamic" microphone is

shown here.

The moving coil microphone works on the principle of Electromagnetic Induction. As a

copper wire coil moves in the magnetic field a voltage is generated as given by:

where V is resulting voltage from B is magnetic field, l is the length of the copper wire and u

is the velocity at which it passes thought the field. The microphone has a very small coil of

thin wire suspended within the magnetic field of a permanent magnet. As the sound wave hits

the flexible diaphragm, the diaphragm moves back and forth in response to the sound

pressure acting upon it causing the attached coil of wire to move within the magnetic field of

the magnet.

The movement of the coil within the magnetic field causes a voltage to be induced in the coil

as defined by Faraday's Law of Electromagnetic Induction. The resultant output voltage

signal from the coil is proportional to the pressure of the sound wave acting upon the

diaphragm so the louder or stronger the sound wave the larger the output signal will be,

making this type of microphone design pressure sensitive.

BluV

As the coil of wire is usually very small the range of movement of the coil and attached

diaphragm is also very small producing a very linear output signal which is 900 out of phase

to the sound signal. Also, because the coil is a low impedance inductor, the output voltage

signal is also very low so some form of "pre-amplification" of the signal is required.

Moving coil microphones are cheap and robust making them good for the rigors of live

performance and touring. They are especially suited for the close miking of Bass and Guitar

speaker cabinets and Drum kits. They are also good for live vocals as their resonance peak of

around 5 kHz provides an inbuilt presence boost that improves speech/singing intelligibility.

MOVING COIL LOUDSPEAKER

A loudspeaker is an electro-acoustic transducer that produces sound in response to an

electrical audio signal input. The most common form of loudspeaker uses a paper cone

supporting a voice coil electromagnet acting on a permanent magnet (moving coil

loudspeaker). Where accurate reproduction of sound is required, multiple loudspeakers may

be used, each reproducing a part of the audible frequency range.

The term "loudspeaker" may refer to individual transducers (known as "drivers") or to

complete speaker systems consisting of an enclosure including one or more drivers. To

adequately reproduce a wide range of frequencies, most loudspeaker systems employ more

than one driver, particularly for higher sound pressure level or maximum accuracy. Individual

drivers are used to reproduce different frequency ranges. The drivers are named subwoofers

(for very low frequencies); woofers (low frequencies); mid-range speakers (middle

frequencies); tweeters (high frequencies); and sometimes super-tweeters, optimized for the

highest audible frequencies

A moving coil loudspeaker is as shown. It uses a lightweight diaphragm, or cone, connected

to a rigid basket, or frame, via a flexible suspension, commonly called a spider, that

constrains a coil of fine tinsel wire to move axially through a cylindrical magnetic gap. When

an electrical signal is applied to the voice coil, a magnetic field is created by the electric

current in the voice coil, making it a variable electromagnet.

When an analogue signal passes through the voice coil of the speaker, an electro-magnetic

field is produced and whose strength is determined by the current flowing through the "voice"

coil, which in turn is determined by the volume control setting of the driving amplifier. The

electro-magnetic force produced by this field opposes the main permanent magnetic field

around it and tries to push the coil in one direction or the other depending upon the

interaction between the north and south poles.

As the voice coil is permanently attached to the cone/diaphragm this also moves in tandem

and its movement causes a disturbance in the air around it thus producing a sound or note. If

the input signal is a continuous sine wave then the cone will move in and out acting like a

piston pushing and pulling the air as it moves and a continuous single tone will be heard

representing the frequency of the signal. The strength and therefore its velocity, by which the

cone moves and pushes the surrounding air produces the loudness of the sound.

As the speech or voice coil is essentially a coil of wire it has, like an inductor an impedance

value. This value for most loudspeakers is between 4 and 16Ω's and is called the "nominal

impedance" value of the speaker measured at 0Hz, or DC It is important to always match the

output impedance of the amplifier with the nominal impedance of the speaker to obtain

maximum power transfer between the amplifier and speaker with most amplifier-speaker

combinations having and efficiency rating as low as 1 or 2%.

The human ear can generally hear sounds from between 20Hz to 20kHz, and the frequency

response of modern loudspeakers called general purpose speakers are tailored to operate

within this frequency range as well as headphones, earphones and other types of

commercially available headsets used as sound transducers. However, for high performance

High Fidelity (Hi-Fi) type audio systems, the frequency response of the sound is split up into

different smaller sub-frequencies thereby improving both the loudspeakers efficiency and

overall sound quality as follows:

Descriptive Unit Frequency Range

Sub-Woofer 10Hz to 100Hz

Bass 20Hz to 3kHz

Mid-Range 1kHz to 10kHz

Tweeter 3kHz to 30kHz

DIGITAL MULTIMETERS

A DMM is very important laboratory instrument. It is used to measure AC/DC voltage,

AC/DC current and resistance. Since it gives digital display, it is very accurate. The accuracy

is sometimes called as resolution of digital multimeter. Resolution is also related with

sensitivity of multimeter. Greater is the sensitivity higher is the resolution. It has one more

advantage: it has very high input resistance. Hence minimum loading effects are produced on

the electrical quantity like voltage or current under measurement. It also provides over

ranging indicator. Its block diagram is given below :

The block diagram of digital multimeter

1) To measure resistance – connect an unknown resistor across its input probes. Keep rotary

switch in position–1. Some current flows through the resistor, from constant current source.

Now according to Ohm’s law voltage is produced across it. This voltage is directly

proportional to its resistance. This voltage is buffered by the buffer amplifier and then fed to

A/D converter, to get digital display in Ohms.

2) To measure AC voltage – connect an unknown AC voltage across input probes. Keep

rotary switch in position–2. The voltage is attenuated, if it is above the selected range and

then rectified to convert it into proportional DC voltage. It is then fed to A/D converter to get

the digital display in Volts.

3) To measure AC current – this circuit measures the current indirectly. Because the circuit

can measure only voltage and the A/D converter can convert voltage into proportional digital

signals. So the current is converted into proportional voltage first and then measured.

Connect an unknown AC current across input probes. Keep the switch in position–3. The

current is converted proportionally into voltage with the help of I–V converter and then

rectified. Now the voltage in terms of AC current is fed to A/D converter to get digital

display in Amperes.

4) To measure DC current – here also the circuit measures the current indirectly. Connect

an unknown DC current across input probes. Keep the switch in position–4. The current is

converted proportionally into voltage with the help of I–V converter. Now the voltage in

terms of DC current is fed to A/D converter to get the digital display in Amperes.

5) To measure DC voltage – connect an unknown DC voltage across input probes. Keep the

switch in position–5. The voltage is attenuated, if it is above the selected range and then

directly fed to A/D converter to get the digital display in Volts.

CONSUMER ELECTRONICS

Basic principles of TV

Video is a combination of light and sound, both of which are made up of vibrations or

frequencies. We are surrounded by various forms of vibrations: visible, tangible, audible, and

many other kinds that our senses are unable to perceive. We are in the midst of a wide

spectrum which extends from zero to many millions of vibrations per second. The unit we use

to measure vibrations per second is Hertz (Hz). Sound vibrations occur in the lower regions

of the spectrum, whereas light vibrations can be found in the higher frequency areas. The

sound spectrum ranges from 20 to 20,000 Hertz (Hz). Light vibrations range from 370 trillion

to 750 trillion Hz. When referring to light, we speak of wavelengths rather than vibrations.

The technique of compatible colour television utilizes two transmissions. One of

these carries information about the brightness, or luminance, of the televised scene, and the

other carries the colour, or chrominance, information. Since the ability of the human eye to

perceive detail is most acute when viewing white light, the luminance transmission carries the

impression of fine detail. Because it employs methods essentially identical to those of a

monochrome television system, it can be picked up by black-and-white receivers. The

chrominance transmission has no appreciable effect on black-and-white receivers, yet, when

used with the luminance transmission in a colour receiver, it produces an image in full colour.

By transforming the primary-colour values, it is possible to specify any coloured light by

three quantities: (1) its luminance (brightness or “brilliance”); (2) its hue (the redness,

orangeness, blueness, or greenness, etc., of the light); and (3) its saturation (vivid versus

pastel quality). Since the intended luminance value of each point in the scanning pattern is

transmitted by the methods of monochrome television, it is only necessary to transmit, via an

additional two-valued signal, supplementary information giving the hue and saturation of the

intended colour at the respective points.

The Human Eye

The eye tends to retain an image for about 80 milliseconds after it has disappeared.

Advantage is taken of this in television and cinematography, where a series of still pictures

(25 per second) create the illusion of a continuously moving picture. Other characteristics of

the human eye are that it is less sensitive to color detail than to black-and-white detail, and

that the human eye does not respond equally to all colors. The eye is most sensitive to the

yellow/green region, and less in the areas of red and (particularly) blue.

PICTURE TRANSMISSION

The picture information is optical in character and may be thought of as an assemblage of a

large number of tiny areas representing picture details. These elementary areas into which

picture details may be broken up are known as ‘picture elements’ or ‘pixels’, which when

viewed together represent visual information of the scene. Thus, at any instant there are

almost an infinite number of pieces of information that need to be picked up simultaneously

for transmitting picture details. However, simultaneous pick-up is not practicable because it is

not feasible to provide a separate signal path (channel) for the signal obtained from each

picture element. In practice, this problem is solved by a method known as ‘scanning’ where

conversion of optical information to electrical form is carried out element by element, one at

a time and in a sequential manner to cover the entire picture. Besides, scanning is done at a

very fast rate and repeated a large number of times per second to create an illusion

(impression at the eye) of simultaneous reception from all the elements, though using only

one signal path.

Interlaced Scan

Interlaced scan is a technique used for "painting" an image on a screen, which was

originally designed for the analog NTSC television system. Interlaced scan uses two fields to

create a frame. One field contains all the odd lines in the image; the second field contains all

the even lines of the image. Figure 1 depicts the composed frame with its two sets of odd and

even fields. For example, a television scans 60 fields every second, which are divided into 30

odd and 30 even lines. These two sets of 30 lines are combined to create a full frame every

1/30th

second, resulting in a display of 30 frames per second (30hz).

The meaning of interlaced signals is that one picture is broken up into alternating

fields. The total image is conjunct in the human brain such that the two fields are combined

together and create a single image, even when they really are separated. Therefore, there is a

minimum refresh rate for interlace scanning, otherwise flickering may be observed. It is

common to use 30Hz as such refresh rate to avoid flickering, which is the standard rate in

CRT technology.

Main TV broadcasting system

Analogue television systems:

–NTSC

–PAL

–SECAM

Digital television systems:

–Japan: ISDB

–Europe: DVB

–America: ATSC DTV

PAL (Phase Alternating Line)

The name "Phase Alternating Line" describes the way that the phase of part of the

colour information on the video signal is reversed with each line, which automatically

corrects phase errors in the transmission of the signal by cancelling them out, at the expense

of vertical frame colour resolution. Lines where the colour phase is reversed compared to

NTSC are often called PAL or phase-alternation lines, which justifies one of the expansions

of the acronym, while the other lines are called NTSC lines. Early PAL receivers relied on

the human eye to do that cancelling; however, this resulted in a comb-like effect known as

Hanover bars on larger phase errors. Thus, most receivers now use a chrominance delay line,

which stores the received colour information on each line of display; an average of the colour

information from the previous line and the current line is then used to drive the picture tube.

The effect is that phase errors result in saturation changes, which are less

objectionable than the equivalent hue changes of NTSC. A minor drawback is that the

vertical colour resolution is poorer than the NTSC system's, but since the human eye also has

a colour resolution that is much lower than its brightness resolution, this effect is not visible.

In any case, NTSC, PAL, and SECAM all have chrominance bandwidth (horizontal colour

detail) reduced greatly compared to the luminance signal.

A colour receiver is similar to the black and white receiver as shown in Fig. 1.7. The main

difference between the two is the need of a colour or chroma subsystem. It accepts only the

colour signal and processes it to recover (B-Y) and (R-Y) signals. These are combined with

the Y signal to obtain VR, VG and VB signals as developed by the camera at the transmitting

end. VG becomes available as it is contained in the Y signal. The three colour signals are fed

after sufficient amplification to the colour picture tube to produce a colour picture on its

screen.

PAL is based on a 625 line, 50 field/25 frames a second, at 50HZ system. The signal

is interlaced, like NTSC, into two fields, composed of 312 lines each. PAL has a frame rate

closer to that of film. PAL has 25 frames per second rate, while film has a frame rate of 24

frames per second. Countries on the PAL system include the U.K., Germany, Spain, Portugal,

Italy, China, India, most of Africa, and the Middle East.

Block Diagram of PAL TV receiver (colour)

The simplest functional block diagram of the PAL D receiver subsystem is shown in Figure.

As in any receiver, the output from the IF section is detector. The output is the Y signal

mixed with chrominance signal. It is amplified by the first video amplifier before feeding to

the different sections of the receiver. The output from the first video amplifier feeds both a

4.43MHz band pass amplifier and a gated burst amplifier located in the chroma section of the

receiver.

The purpose of the color killer circuit is to make the chrominance band pass amplifier

inoperative when the receiver is tuned to receive a black and white program. The(R-Y) and

(B-Y) color difference signals as obtained from the two modulators are fed to the matrixing

circuit which not only generate the(G-Y) signal but also provide necessary amplitude

correction to the three color difference signals that is necessary because of the weighting

factor applied at the transmitting end.

Basic principles of DTH

Today, most satellite TV customers in developed television markets get their programming

through a direct broadcast satellite (DBS) provider, such as DISH TV or DTH platform. The

provider selects programs and broadcasts them to subscribers as a set package. Basically, the

provider’s goal is to bring dozens or even hundreds of channels to the customer’s television in a

form that approximates the competition from Cable TV.

Unlike earlier programming, the provider’s broadcast is completely digital, which means it has

high picture and stereo sound quality. Early satellite television was broadcast in C-band - radio in

the 3.4-gigahertz (GHz) to 7-GHz frequency range. Digital broadcast satellite transmits

programming in the Ku frequency range (10 GHz to 14 GHz ). There are five major components

involved in a direct to home (DTH) satellite system: the programming source, the broadcast

center, the satellite, the satellite dish and the receiver.

THE COMPONENTS

Programming sources are simply the channels that provide programming for

broadcast. The provider (the DTH platform) doesn’t create original programming itself; it

pays other companies (HBO, for example, or ESPN or STAR TV or Sahara etc.) for the right

to broadcast their content via satellite. In this way, the provider is kind of like a broker

between the viewer and the actual programming sources. (Cable television networks also

work on the same principle.) The broadcast center is the central hub of the system. At the

broadcast center or the Playout & Uplink location, the television provider receives signals

from various programming sources, compreses I using digital compression, if necessary

scrambles it and beams a broadcast signal to the satellite being used by it. The satellites

receive the signals from the broadcast station and rebroadcast them to the ground. The

viewer’s dish picks up the signal from the satellite (or multiple satellites in the same part of

the sky) and passes it on to the receiver in the viewer’s house. The receiver processes the

signal and passes it on to a standard television. Lets look at each step in the process in greater

detail.

THE PROGRAMMING

Satellite TV providers get programming from two major sources: International

turnaround channels (such as HBO, ESPN and CNN, STAR TV, SET, B4U etc) and various

local channels (SaBe TV, Sahara TV, Doordarshan, etc). Most of the turnaround channels

also provide programming for cable television, so sometimes some of the DTH platforms will

ad in some special channels exclusive to itself to attract more subscriptions.

Turnaround channels usually have a distribution center that beams their programming to a

geostationary satellite. The broadcast center uses large satellite dishes to pick up these analog

and digital signals from several sources.

THE BROADCAST CENTER

The broadcast center converts all of this programming into a high-quality, uncompressed

digital stream. At this point, the stream contains a vast quantity of data — about 270 megabits

per second (Mbps) for each channel. In order to transmit the signal from there, the broadcast

center has to compress it. Otherwise, it would be too big for the satellite to handle. The providers

use the MPEG-2 compressed video format — the same format used to store movies on DVDs.

With MPEG-2 compression, the provider can reduce the 270-Mbps stream to about 3 or 10 Mbps

(depending on the type of programming). This is the crucial step that has made DTH service a

success. With digital compression, a typical satellite can transmit about 200 channels. Without

digital compression, it can transmit about 30 channels. At the broadcast center, the high-quality

digital stream of video goes through an MPEG-2 encoder, which converts the programming to

MPEG-2 video of the correct size and format for the satellite receiver in your house.

ENCRYPTION & TRANSMISION

After the video is compressed, the provider needs to encrypt it in order to keep people

from accessing it for free. Encryption scrambles the digital data in such a way that it can only be

decrypted (converted back into usable data) if the receiver has the correct decoding satellite

receiver with decryption algorithm and security keys. Once the signal is compressed and

encrypted, the broadcast center beams it directly to one of its satellites. The satellite picks up the

signal, amplifies it and beams it back to Earth, where viewers can pick it up.

THE DISH

A satellite dish is just a special kind of antenna designed to focus on a specific broadcast

source. The standard dish consists of a parabolic (bowl-shaped) surface and a central feed horn.

To transmit a signal, a controller sends it through the horn, and the dish focuses the signal into a

relatively narrow beam. The dish on the receiving end can’t transmit information; it can only

receive it. The receiving dish works in the exact opposite way of the transmitter. When a beam

hits the curved dish, the parabola shape reflects the radio signal inward onto a particular point,

just like a concave mirror focuses light onto a Particular point.

THE RECEIVER

The end component in the entire satellite TV system is the receiver. The receiver has four

essential jobs: It de-scrambles the encrypted signal. In order to unlock the signal, the receiver

needs the proper decoder chip for that programming package. The provider can communicate

with the chip, via the satellite signal, to make necessary adjustments to its decoding programs.

The provider may occasionally send signals that disrupt illegal de-scramblers, as an electronic

counter measure (ECM) against illegal users.

MP3

The first mp3 player was invented by a South Korean company in 1997 and it had a

data storage capacity of just 32 megabytes. Several improvements have been made in it since

then; especially its storage space has been increased to 160 gigabytes in the latest ones.

Several companies are in market and are competing with each other to attract attention of the

customers by adding new features in their latest players.

There are several kinds of mp3 players in the market but they can be classified in three major

groups. First type is hard drive based and it is heavy as compared to others because a large

sized hard drive is incorporated in it to provide a large data storage capacity of about 10

gigabytes in which a person can store about 3000 mp3 files. Their only drawback is their

heavy size.

Second type is micro hard drive based and in this type a small hard drive is incorporated to

reduce weight as well as the physical size. It has a storage capacity of about 6 gigabytes and

can store about 1750 mp3 files. Most of the customers prefer this type because of its light

weight and moderate storage capacity. In addition it has rechargeable batteries to supply

power to the device.

Third type is known as flash based and it has no moving parts in it which makes it less energy

consuming causing the batteries to last for a longer time. It has less storage space as

compared to the two previous types, ranging from 32 megabytes to about 2 megabytes. Non-

rechargeable batteries are usually used in it and can be replaced by the new ones even in

middle of the use. Most of the people with huge music collection don't like this type because

of low storage capacity.

Multichannel audio

The term "multichannel audio" refers to the use of multiple audio tracks to reconstruct

sound on a multi-speaker sound system.

Two digits separated by a decimal point (2.1, 5.1, 6.1, 7.1, etc.) are used to classify the

various kinds of speaker set-ups, depending on how many audio tracks are used.

The first digit shows the number of primary channels, each of which are reproduced on a

single speaker, while the second refers to the presence of a Low Frequency Effect (LFE for

short), which reproduced on a subwoofer. Thus, 1.0 corresponds to mono sound (meaning

one-channel) and 2.0 correspond to stereo sound.

5.1 Setup

The physical configuration of the speakers in a 5.1 surround system is of utmost importance,

as it directly influences sound quality and the realizm of audio effects.

For best results, there are a number of rules to follow when placing each speaker:

1. Front speakers should preferably be placed at the height of the ears of a seated

listener. The rear (surround) speakers must be positioned slightly above this height.

2. The left and right front speakers must be placed one on each side of the television set,

both at the same distance. In practice, they should each form a 25° to 45° angle

with the listener.

3. The central speaker must be placed directly above or below the TV set, as it is

primarily used to relay the main actors' dialogue.

4. The subwoofer may be placed anywhere in the room, but preferably on the ground, so

as to better transmit the vibrations. It is best to try out different locations in the room.

5. The optimal position for the rear speakers is a short way back from the listener,

forming a 90° to 110° angle with him or her.

7.1 Setup

A 7.1 setup bridges the gap between the two rear speakers, using not one, but two speakers.