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3.0 BIPOLAR JUNCTION TRANSISTOR (BJT)

Physical structure and schematic symbols. Basic transistor operation. Configurations andcurrent relationships. I-V characteristics. Operating regions. The DC load line. Data sheets.Analysis and design of biasing circuit. Biasing stability. Classification of Amplifier and oncepts.Other application of BJT

Upon completion of viewing this presentation, you should able to:3.1 Understand the basic of bipolar junction transistor (BJT)3.2 Know the methods of connecting a transistor circuit and its characteristics.3.3 Understand Frequency Response Curve3.4 Understand the classification of amplifier3.5 Know other biasing techniques of common emitter transistor configuration.3.6 Understand other applications of BJT

3.1 Basic of bipolar junction transistor (BJT)

Physical structure and schematic symbols for BJT.

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The construction and circuit symbols for both the NPN and PNP bipolar transistor are givenabove with the arrow in the circuit symbol always showing the direction of "conventional currentflow" between the base terminal and its emitter terminal. The direction of the arrow always points from thepositive P-type region to the negative N-type region for both transistor types, exactly the same as for thestandard diode symbol.

(BJT) is a three-terminal electronic device constructed ofdoped semiconductormaterial and maybe used in amplifying or switching applications. Bipolartransistors are so named because theiroperation involves both electrons and holes. Charge flow in a BJT is due to bidirectional diffusionofcharge carriers across a junction between two regions of different charge concentrations.

NPN Transistor

NPN is one of the two types of bipolar transistors, consisting of a layer of P-doped semiconductor(the "base") between two N-doped layers. A small current entering the base is amplified to

produce a large collector and emitter current. That is, an NPN transistor is "on" when its base ispulled high relative to the emitter.

Most of the NPN current is carried by electrons, moving from emitter to collector as minoritycarriers in the P-type base region. Most bipolar transistors used today are NPN, because electronmobility is higher than hole mobility in semiconductors, allowing greater currents and fasteroperation.

A mnemonic device for the remembering the symbol for an NPN transistor is not pointing in,based on the arrows in the symbol and the letters in the name. That is, the NPN transistor is theBJT transistor that is "not pointing in".

PNP Transistor

The other type of BJT is the PNP, consisting of a layer of N-doped semiconductor between twolayers of P-doped material. A small current leaving the base is amplified in the collector output.That is, a PNP transistor is "on" when its base is pulled low relative to the emitter.

The arrows in the NPN and PNP transistor symbols are on the emitter legs and point in thedirection of the conventional current flow when the device is in forward active mode.

A mnemonic device for the remembering the symbol for a PNP transistor ispointing in (proudly),based on the arrows in the symbol and the letters in the name. That is, the PNP transistor is theBJT transistor that is "pointing in".

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Basic transistor operation.

An NPN transistor can be considered as two diodeswith a sharedanode. In typical operation, thebase-emitterjunction isforward biased and the basecollector junction isreverse biased. In anNPN transistor, for example, when a positive voltage is applied to the baseemitter junction, theequilibrium between thermally generated carriersand the repelling electric field of the depletionregion becomes unbalanced, allowing thermally excited electrons to inject into the base region.These electrons wander (or "diffuse") through the base from the region of high concentration nearthe emitter towards the region of low concentration near the collector. The electrons in the baseare called minority carriersbecause the base is doped p-type which would make holes themajority carrierin the base.

Operation of NPN junction transistor

To minimize the percentage of carriers that recombinebefore reaching the collectorbasejunction, the transistor's base region must be thin enough that carriers can diffuse across it inmuch less time than the semiconductor's minority carrier lifetime. In particular, the thickness ofthe base must be much less than the diffusion lengthof the electrons. The collectorbase junctionis reverse-biased, and so little electron injection occurs from the collector to the base, butelectrons that diffuse through the base towards the collector are swept into the collector by theelectric field in the depletion region of the collectorbase junction. The thin shared base andasymmetric collectoremitter doping is what differentiates a bipolar transistor from two separateand oppositely biased diodes connected in series.

A large current (electron) flows from the collector through the base and into the emitter. Thecurrent in the emitter IE is the sum of the base current IB and the collector current IC

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IE = IB + IC
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Characteristic curve and operating regions of BJT.

In most practical situations we can expect the Collector current to be set almost entirely by the chosen Base-Emitter voltage. However, this is only true when the the Base-Collector voltage we are applying is 'bigenough' to quickly draw over to the Collector any free electrons which enter the Base region from theEmitter.

Characteristic curve of BJT.

The above plot of characteristic curves gives a more complete picture of what we can expect from a workingBipolar Transistor. Each curve shows how the colletor current, IC, varies with the Collector-Emitter voltage,VCE, for a specific fixed value of the Base current, IB. This kind of characteristic curve 'family' is one of themost useful ones when it comes to building amplifiers, etc, using Bipolar Transistors as it contains quite a lotof detailed information.

When the applied VCE level is 'large enough' (typically above two or three volts) the Collector is able to toremove free electrons from the Base almost as quickly as they Emitter injects them. Hence we get a currentwhich is set by the Base-Emitter voltage and see a current gain value which doesn't alter very much if wechange either the base current or the applied Collector potential.

However, when we reduce the Collector potential so that VCE is less than a couple of volts, we find that it isno longer able to efficiently remove electrons from the Base. This produces a sort of partial 'roadblock' effect

where free electrons tend to hang about in the Base region. These makes the Base region seem 'morenegative' to any electrons in the Emitter and tends to reduce the overall flow of current through the device.As we lower the Collector potential to become almost the same as that of the Base and Emitter it eventuallystops drawing any electrons out of the device and the Collector current falls towards zero.

The precise voltage at which the Collector ceases to be an effective 'collector of electrons' depends on thetemperature and the manufacturing details of the transisor. In general we can expect most BipolarTransistors to work efficiently provided that we arrange for a VCEvalue of at least two or three volts - andpreferrably five volts or more. Such a device can be used as an effective amplifier. Lower voltages mayprevent it from working correctly.

The second set of characteristics were going to be interested in is illustrated to the right as a family of iC-vCEcurves. Each of the curves in this family illustrates the dependence of the collector current (iC) on thecollector emitter voltage (vCE) when the base current (iB) has a constant value (i.e., vBE is held constant).

There are three distinct regions of these characteristics that are of importance: As the magnitude of vCE decreases, there comes a point when the collector voltage becomes less than

the base voltage. When this happens, the transistor leaves the linear region of operation and enters thesaturation region, which is highly nonlinear and is not usable for amplification.

The cutoff region of operation occurs for base currents near zero. In the cutoff region, the collectorcurrent approaches zero in a nonlinear manner and is also avoided for amplification applications.

The linear region is where we want to be for amplification. In the linear(oractive) region the curveswould ideally be horizontal straight lines, indicating that the collector behaves as a constant currentsource independent of the collector voltage (iC = iB). Practically, these curves have a slight positive

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slope. If these curves are extended to the left along the vCE axis, they will converge to a point known asthe Early voltage, shown as VA in the figure below.

Operating regions of BJT.

BJT operates as an amplifier.

The bipolar transistor, usually used as an amplifier. Most amplifiers use the common-emitter circuitconfiguration because the circuit offers both voltage and current gain resulting in a much higher power gainthan can be obtained by either the emitter follower or the common-base amplifier.

BJT operates as a switch.

Thebipolar transistor, whether NPN or PNP, may be used as a switch. Recall that the bipolar transistorhas three regions of operation: the cut-off region, the linear or active region, and the saturation region.When used as a switch, the bipolar transistor is operated in the cut-off region (the region where in thetransistor is not conducting, and therefore makes the circuit 'open') and saturation region (the regionwherein the transistor is in full conduction, thereby closing the circuit).

The bipolar transistor is a good switch because of its large transconductance Gm, with Gm = Ic/Vbewhere Ic is the collector-to-emitter (output) current and Vbe is the base-emitter (input) voltage. Its highGm allows large collector-to-emitter currents to be easily achieved if sufficient excitation is applied at thebase.

To illustrate this, the simplest way to use an NPN bipolar transistor as a switch is to insert the loadbetween the positive supply and its collector, with the emitter terminal grounded (as shown in Figure 1).

Applying no voltage at the base of the transistor will put it in the cut-offregion, preventing current fromflowing through it and through the load, which is a resistor in this example. In this state, the load is 'off'.

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Input signal

Output signal
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A Simple Switch Using an NPNTransistor

Applying enough voltage at the base of the transistor will cause it to saturate and become fullyconductive, effectively pulling the collector of the transistor to near ground. This causes acollector-to-emitter current to flow through the load that's limited only by the impedance of theload. In this state, the load is 'on'.

One limitation of this simple design is that the switch-off time of the transistor is slower than itsswitch-on time if the load is a resistor. This is because of the stray capacitance across the

collector of the transistor and ground, which needs to charge through the load resistor duringswitch-off. On the other hand, this stray capacitance is easily discharged to ground by thelarge collector current flow when the transistor is switched on. There are, of course, otherbetter designsfor using the bipolar transistor as a switch.

(a) mechanical switch, (b) NPN transistor switch, (c) PNP transistor switch.

Transistor: (a) cutoff, lamp off; (b) saturated, lamp on.

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If the switch is open as in (Figure above (a), the base wire of the transistor will be left floating(not connected to anything) and there will be no current through it. In this state, the transistorissaid to be cutoff. If the switch is closed as in (Figure above (b), however, electrons will be able toflow from the emitter through to the base of the transistor, through the switch and up to the leftside of the lamp, back to the positive side of the battery. This base current will enable a muchlarger flow of electrons from the emitter through to the collector, thus lighting up the lamp. In thisstate of maximum circuit current, the transistor is said to be saturated.

Of course, it may seem pointless to use a transistor in this capacity to control the lamp. After all,we're still using a switch in the circuit, aren't we? If we're still using a switch to control the lamp --if only indirectly -- then what's the point of having a transistor to control the current? Why not justgo back to our original circuit and use the switch directly to control the lamp current?

Two points can be made here, actually. First is the fact that when used in this manner, the switchcontacts need only handle what little base current is necessary to turn the transistor on; thetransistor itself handles most of the lamp's current. This may be an important advantage if theswitch has a low current rating: a small switch may be used to control a relatively high-currentload. More important, the current-controlling behavior of the transistor enables us to usesomething completely different to turn the lamp on or off.

3.2 Methods of connecting a transistor circuit and its characteristic

Transistor circuits may be classified into three configurations based on which terminal iscommon to both the input and the output of the circuit. These configurations are: the common-emitter configuration, the common-base configuration and the common-collector configuration.

Bipolar Transistor Configurations

As the Bipolar Transistoris 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 andoutput. Each method of connection responding differently to its input signal within a circuit as thestatic characteristics of the transistor vary with each circuit arrangement.

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

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

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

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a) The Common emitter (CE) configuration

The common-emitter (CE)transistor configuration, the transistor terminal common to boththe input and the output of the circuit is the emitter. The common-emitter configuration, whichis also known as the 'grounded-emitter' configuration, is the most widely used among thethree configurations.In the Common Emitteror grounded emitter configuration, the input signal is applied betweenthe base, while the output is taken from between the collector and the emitter as shown. Thistype of configuration is the most commonly used circuit for transistor basedamplifiers andwhich represents the "normal" method of bipolar transistor connection. The common emitteramplifier configuration produces the highest current and power gain of all the three bipolartransistor configurations. This is mainly because the input impedance is LOW as it isconnected to a forward-biased PN-junction, while the output impedance is HIGH as it is takenfrom a reverse-biased PN-junction.

The Common Emitter Transistor Circuit

In this type of configuration, the current flowing out of the transistor must be equal to the currentsflowing into the transistor as the emitter current is given as IE = IB + IC. 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 ofIc/Ib and is given the Greek symbol ofBeta, (). Asthe emitter current for a common emitter configuration is defined as Ie = Ic + Ib, Since theelectrical relationship between these three currents, Ib, Ic and Ie is determined by the physicalconstruction of the transistor itself, any small change in the base current (Ib), will result in a muchlarger change in the collector current (Ic). Then, small changes in current flowing in the base willthus control the current in the emitter-collector circuit. Typically, Beta has a value between 20 and200 for most general purpose transistors.

the mathematical relationship between these parameters and therefore the current gain of thetransistor can be given as:Beta, () = IC/IB

Where: "Ic" is the current flowing into the collector terminal, "Ib" is the current flowing into thebase terminal and "Ie" is the current flowing out of the emitter terminal.

Then 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 muchlower. The common emitter configuration is an inverting amplifier circuit resulting in the outputsignal being 180o out-of-phase with the input voltage signal.

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b) The Common Base (CB) configuration

As its name suggests, in the Common Base or grounded base configuration, the BASEconnection is common to both the input signal AND the output signal with the input signal beingapplied between the base and the emitter terminals. The corresponding output signal is takenfrom 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 quitelarge as its the sum of both the base current and collector current respectively therefore, thecollector current output is less than the emitter current input resulting in a current gain for thistype of circuit of "1" (unity) or less, in other words the common base configuration "attenuates"the input signal.

The Common Base Transistor Circuit

This type of amplifier configuration is a non-inverting voltage amplifier circuit, in that the signalvoltages Vin and Vout are in-phase. This type of transistor arrangement is not very common dueto its unusually high voltage gain characteristics. Its output characteristics represent that of aforward 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 ormore importantly "load" resistance (RL) to "input" resistance (Rin) giving it a value of "ResistanceGain". Then the voltage gain (Av for a common base configuration is therefore given as:

Common Base Voltage Gain ;

The common base circuit is generally only used in single stage amplifier circuits such asmicrophone pre-amplifier or radio frequency (Rf) amplifiers due to its very good high frequencyresponse.

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c) The Common collector configuration (CC) Configuration

In the Common Collectoror grounded collector configuration, the collector is now commonthrough the supply. The input signal is connected directly to the base, while the output is takenfrom the emitter load as shown. This type of configuration is commonly known as a VoltageFollowerorEmitter Followercircuit. The emitter follower configuration is very useful forimpedance matching applications because of the very high input impedance, in the region ofhundreds of thousands of Ohms while having a relatively low output impedance.

The Common Collector Transistor Circuit

The common emitter configuration has a current gain approximately equal to the value of thetransistor itself. In the common collector configuration the load resistance is situated in series withthe emitter so its current is equal to that of the emitter current. As the emitter current is thecombination of the collector AND the base current combined, the load resistance in this type oftransistor configuration also has both the collector current and the input current of the baseflowing 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 loadresistance of the common collector transistor receives both the base and collector currents givinga large current gain (as with the common emitter configuration) therefore, providing good currentamplification with very little voltage gain.

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Characteristic curve (IC VC) of common emitter amplifier.

VCC Ic

RB RC

Rin Ib

RL Vin Vout

Common-Emitter Transistor Configuration

The input current and output voltage of the common-emitter configuration, which are the base

current Ib and the collector-emitter voltage Vce, respectively, are often considered as theindependent variables in this circuit. Its dependent variables, on the other hand, are the base-emitter voltage Vbe (which is the input voltage) and the collector current Ic (which is the outputcurrent). A plot of the output current Ic against the collector-emitter voltage Vce for differentvalues of Ib may be drawn for easier analysis of a transistor's input/output characteristics, asshown in thisDiagram of Vce-Ic Curves.

A signal source Vin having a source resistance Rin is connected to the common-emitter (Common-

Emitter Transistor Configuration). When Vin is raised from zero, an ac signal current is producedin the base. Inturn, this ac signal current in the base produces an ac signal current in thecollector. The ac signal current in Rc produces the ac voltage drop across Rc that we observeas the output signal Vout.

When the RB was adjusted in the circuit to a value of IB that locates the operating point of thetransistor at point Q on the load line (Characteristic curve (Ic Vc)) bellow. The small sinusoidalsignal current fed into the base varies the base current sinusoidally from Q to M to Q to N and toQ. The sinusoidal variation of base current causes the collector current vary sinusoidally.

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Characteristic curve (Ic Vc) of common emitter amplifier with AC signals obtained from a load line.

Define DC and AC

In the dc mode the level of IC and IB are related by a quantity called beta (). The dc Beta oftransistor, dc is defined as the ratio of IC to IB at a given operating point and defined by thefollowing equation:

Where IC and IB are determined at a particular operating point on the characteristics.

The ac Beta, ac is defined as the ratio of a change in collector current IC to a change in basecurrent IB at a given operating point for a constant VCE.For ac situations an ac beta has been defined as follow:

ac = ICIB VCE = constant

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dc = ICIB

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Calculation the currents and voltages of common emitter amplifier.

+Vcc

RB RC

VCE

Vin RL Vout

Forward Bias of Base-Emiter;

Consider first the base-emitter circuit loop, writing Kirchhffs voltage equation in the clockwise direction forthe loop, we obtain

Vcc

+VCC VRB VBE= 0

+VCC IBRB VBE= 0 RB RC+VCC = IBRB + VBE

IBRB= VCC VBE

Collector-Emitter Loop;

The magnitude of the collector current is related directly to IB through;

Applying Kirchhoffs voltage law in the clockwise direction around the indicated closedloop, will result in the following:

VccVCC = VRC + VCEVCE = VCC VRC

RC

IC VCE

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IB = VCC VBERB

IC = IB

V

VCE = VCC - ICRC

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DC load line.

A typical CE circuit is shown below:

The current IC and the voltage VCE are determined by both the transistor to satisfy its outputcharacteristics, and the external circuit including the voltage source VCC and resistor RC tosatisfy the equation VCE = VCC - ICRC. This linear equation is a straight line, called the load line,in the output current-voltage characteristics plot. The load line can be found as the straight linethat passes through the two special points:

Characteristic curve (Ic Vc) of common emitter amplifier with a DC load line.

DC load line consists with 3 parts:

i) Operating point (Q-point)ii) Saturation Level

iii) Cut off Level.

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i) Operating Point (Q-point).

For transistor amplifiers the resulting dc current and voltage establish an operating point on thecharacteristics that define the region that will be employed for amplification of the applied signal.Since the operating point is a fixed point on the characteristics, it is also called the quiescent

point(abbreviated Q-point). By definition, quiescentmeans quiet, still, inactive.

The operating pointof the circuit in this configuration is generally designed to be in the active

region, approximately between middle of the load line and close to saturation point. In this region,the collector current is proportional to the base current, and hence useful foramplifierapplications. Q-point consist from two item: ICQ and VCQ.

ICQ = IBVCQ = VCC - ICRC

ii) Saturation Level

The point on the load line where it intersects the collector current axis is referred to as saturationpoint. At this point, the transistor current is maximum and voltage across collector is minimum, fora given load. For this circuit, IC-SAT= VCC/RC

iii) Cut off Level

The cutoff pointis the point where the load line intersects with the collector voltage axis. Here thetransistor current is minimum (approximately zero) and emitter is grounded. Hence VCE-VCUTOFF=Vcc.

The actual current IC and voltage VCE, called the DC operating point orQ-point, can beobtained as the intersection of the load line and the curve in the current-voltage characteristics,corresponding to the given base current IB, to satisfy both the internal I-V characteristics of thetransistor and the external circuit parameters.

Voltage Gain

The Voltage Gain of the common emitter amplifier is equal to the ratio of the change in the inputvoltage to the change in the amplifiers output voltage. But voltage gain is also equal to the ratio of

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Voltage Gain (AV) = VoutVin
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the signal resistance in the Collector to the signal resistance in the Emitter and is given as:

Amplifier Distortion

Voltage or Current Gain, (amplification) provided by the amplifier is the ratio of the peak inputvalue to its peak output value. However, if we incorrectly design our amplifier circuit and set thebiasing Q-point at the wrong position on the load line or apply too large an input signal, theresultant output signal may not be an exact reproduction of the original input signal waveform. Inother words the amplifier will suffer from distortion. Consider the common emitter amplifier circuitbelow.

Common Emitter Amplifier:

Distortion of the signal waveform may take place because:

1. Amplification may not be taking place over the whole signal cycle due to incorrect biasing.

2. The input signal may be too large, causing the amplifier to limit.

3. The amplification may not be linear over the entire frequency range of inputs.

This means then that during the amplification process of the signal waveform, some form ofAmplifierDistortion has occurred. Amplifiers are basically designed to amplify small voltage input signals intomuch larger output signals and this means that the output signal is constantly changing by somefactor or value times the input signal for all input frequencies. We saw previously that thismultiplication factor is called the Beta, value of the transistor. Common emitter or even commonsource type transistor circuits work fine for small AC input signals but suffer from one majordisadvantage, the bias Q-point of a bipolar amplifier depends on the same Beta value which may varyfrom transistors of the same type, ie. the Q-point for one transistor is not necessarily the same as theQ-point for another transistor of the same type due to the inherent manufacturing tolerances. If thisoccurs the amplifier may not be linear and Amplitude Distortion will result but careful choice of thetransistor and biasing components can minimise the effect of amplifier distortion.

Amplitude Distortion

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Amplitude distortion occurs when the peak values of the frequency waveform are attenuated causingdistortion due to a shift in the Q-point and amplification may not take place over the whole signalcycle. This non-linearity of the output waveform is shown below.

Amplitude Distortion due to Incorrect Biasing

If the bias is correct the output waveform should look like that of the input waveform only bigger,(amplified). If there is insufficient bias the output waveform will look like the one on the right withthe negative part of the output waveform "cut-off". If there is too much bias the output waveformwill look like the one on the left with the positive part "cut-off". When the bias voltage is too small,during the negative part of the cycle the transistor does not conduct fully so the output is set bythe supply voltage. When the bias is too great the positive part of the cycle saturates thetransistor and the output drops almost to zero.

Even with the correct biasing voltage level set, it is still possible for the output waveform to

become distorted due to a large input signal being amplified by the circuits gain. The outputvoltage signal becomes clipped in both the positive and negative parts of the waveform an nolonger resembles a sine wave, even when the bias is correct. This type of amplitude distortion iscalled Clipping and is the result of "Over-driving" the input of the amplifier.

When the input amplitude becomes too large, the clipping becomes substantial and forces theoutput waveform signal to exceed the power supply voltage rails with the peak (+ve half) and thetrough (-ve half) parts of the waveform signal becoming flattened or "Clipped-off". To avoid thisthe maximum value of the input signal must be limited to a level that will prevent this clippingeffect as shown above.

Amplitude Distortion due to Clipping

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Amplitude Distortion greatly reduces the efficiency of an amplifier circuit. These "flat tops" of thedistorted output waveform either due to incorrect biasing or over driving the input do not contributeanything to the strength of the output signal at the desired frequency. Having said all that, some wellknown guitarist and rock bands actually prefer that their distinctive sound is highly distorted or"overdriven" by heavily clipping the output waveform to both the +ve and -ve power supply rails. Also,excessive amounts of clipping can also produce an output which resembles a "square wave" shapewhich can then be used in electronic or digital circuits.

We have seen that with a DC signal the level of gain of the amplifier can vary with signalamplitude, but as well as Amplitude Distortion, other types of distortion can occur with AC signalsin amplifier circuits, such as Frequency Distortion and Phase Distortion.

Common Emitter Amplifier Summary

Then to summarize. The Common Emitter Amplifiercircuit has a resistor in its Collector circuit.The current flowing through this resistor produces the voltage output of the amplifier. The value ofthis resistor is chosen so that at the amplifiers quiescent operating point, Q-point this outputvoltage lies half way along the transistors load line.

The Base of the transistor used in a common emitter amplifier is biased using two resistors as apotential divider network. This type of biasing arrangement is commonly used in the design ofbipolar transistor amplifier circuits and greatly reduces the effects of varying Beta, ( ) by holdingthe Base bias at a constant steady voltage. This type of biasing produces the greatest stability.

3.3 Frequency Response Curve

FREQUENCY RESPONSE CURVE.

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The frequency response curve is a visual representation of the quality ofamplitude overfrequency generated by specific components. The graph depicting a frequency response curvewill have a vertical axis and a horizontal axis. The vertical axis is usually labeled as the level ofsound, also called amplitude, in decibels (dB), while the horizontal axis is labeled as thefrequency, the vibration that is captured by your ear and is measured in hertz (Hz).

A frequency-response curve is a graphical representation of the relationship between amplifiergain and operating frequency. A generic frequency response curve is shown in Figure 1. This

particular curve illustrates the relationship between power gain and frequency. As shown:

The circuit power gain remains relatively constant across the midbandrange offrequencies.

As operating frequency decreases from the midband area of the curve, a point is reachedwhere the power gain begins to drop off. The frequency at which power gain equals 50%

of its midband value is called the lower cutoff frequency ( ).

As operating frequency increases from the midband area of the curve, a point is reachedwhere the power gain begins to drop off again. The frequency at which power gain equals

50% of its midband value is called the upper cutoff frequency ( ).

Note that the bandwidth of the circuit is found as the difference between the cutoff frequencies.

By formula,

A generic frequency-response curve

The decibel (dB) is a logarithmic unit that indicates the ratio of a physical quantity (usually poweror intensity) relative to a specified or implied reference level. A ratio in decibels is ten times thelogarithm to base 10 of the ratio of two power quantities. Being a ratio of two measurements of aphysical quantity in the same units, it is a dimensionless unit. A decibel is one tenth of a bel, aseldom-used unit.

The decibel is widely known as a measure of sound pressure level, but is also used for a widevariety of other measurements in science and engineering, most prominently in acoustics,electronics, and control theory. In electronics, the gain of amplifiers, attenuation of signals, andsignal to noise ratios are often expressed in decibels. It confers a number of advantages, such asthe ability to conveniently represent very large or small numbers, a logarithmic scaling thatroughly corresponds to the human perception of sound and light, and the ability to carry outmultiplication of ratios by simple addition and subtraction.

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The calculation of the ratio in decibels varies depending on whether the quantity being measuredis apower quantityor a field quantity.

A field quantityis a quantity such as voltage, current, sound pressure, electric field strength,velocity and charge density, the square of which in linear systems is proportional to power. A

power quantityis a power or a quantity directly proportional to power, e.g. energy density,acoustic intensity and luminous intensity.

Voltage gain

When power gain is calculated using voltage instead of power, making the substitution (P=V2/R),the formula is:

In many cases, the input and output impedances are equal, so the above equation can besimplified to:

and then the 20 log rule :

This simplified formula is used to calculate a voltage gain in decibels, and is equivalent to apower gain only if the impedances at input and output are equal.

For example, peak voltage measurements of input and output show an input of 1.5 volts and anoutput of 4.418 volts. This gives us a voltage gain ratio of 2.9453 (4.418 V / 1.5 V), or 9.3827 dB.

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Vout 2

Gain = 10 log Rout dB

Vin 2

Rin

Gain = 10 log Vout 2 dB

Vin

Gain = 20 log Vout dBVin

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Because the current gain of the common-emitter amplifier is fixed by , and since the input andoutput voltages will be equal to the input and output currents multiplied by their respectiveresistors, we can derive an equation for approximate voltage gain:

An Operational Amplifiers Bandwidth

The operational amplifiers bandwidth is the frequency range over which the voltage gain of theamplifier is above 70.7% or-3dB (where 0dB is the maximum) of its maximum output value asshown below.

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Here we have used the 40dB line as an example. The -3dB or 70.7% of Vmax down point fromthe frequency response curve is given as 37dB. Taking a line across until it intersects with themain GBP curve gives us a frequency point just above the 10kHz line at about 12 to 15kHz. Wecan now calculate this more accurately as we already know the GBP of the amplifier, in thisparticular case 1MHz.

3.4 Classification of Amplifier

AMPLIFIER CLASSES OF OPERATION

Every portion of the input signal there was an output from the amplifier. This is not always thecase with amplifiers. It may be desirable to have the transistor conducting for only a portion of theinput signal. The portion of the input for which there is an output determines the class of operationof the amplifier. There are four classes of amplifier operations. They are class A, class AB, classB, and class C.

Class A Amplifier Operation

Class A amplifiers are biased so that variations in input signal polarities occur within the limits ofCUTOFF and SATURATION. In a PNP transistor, for example, if the base becomes positive withrespect to the emitter, holes will be repelled at the PN junction and no current can flow in thecollector circuit. This condition is known as cutoff. Saturation occurs when the base becomes sonegative with respect to the emitter that changes in the signal are not reflected in collector-currentflow.

Biasing an amplifier in this manner places the dc operating point between cutoff and saturationand allows collector current to flow during the complete cycle (360 degrees) of the input signal,thus providing an output which is a replica of the input. Figure 3.4 is an example of a class A

amplifier. Although the output from this amplifier is 180 degrees out of phase with the input, theoutput current still flows for the complete duration of the input.

The operation of the class "A" amplification

The signalless condition operating point of the class "A" amplification is in the center of the loadline. So, both on the side of the positive of the input signal and the side of the negative voltagechanges can be amplified.In the small part of VCE, the base electric current and VCE aren't proportional. So, in case of the big

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input signal, the distortion occurs to the output.In the figure, the output signal is smaller than the input signal and can be seen but the collectorelectric current is the mA and the base electric current of the input is the A.

The class A operated amplifier is used as an audio- and radio-frequency amplifier in radio, radar,

and sound systems, just to mention a few examples.

Class AB Amplifier Operation

Class AB Amplifier Operation Amplifiers designed for class AB operation are biased so thatcollector current is zero (cutoff) for a portion of one alternation of the input signal. This isaccomplished by making the forward-bias voltage less than the peak value of the input signal. Bydoing this, the base-emitter junction will be reverse biased during one alternation for the amountof time that the input signal voltage opposes and exceeds the value of forward-bias voltage.Therefore, collector current will flow for more than 180 degrees but less than 360 degrees of theinput signal, as shown in figure 3.4 view B. As compared to the class A amplifier, the dc operatingpoint for the class AB amplifier is closer to cutoff.

The class AB operated amplifier is commonly used as a push-pull amplifier to overcome a sideeffect of class B operation called crossover distortion.

Class B Amplifier Operation

Amplifiers biased so that collector current is cut off during one-half of the input signal areclassified class B. The dc operating point for this class of amplifier is set up so that base currentis zero with no input signal. When a signal is applied, one half cycle will forward bias the base-emitter junction and IC will flow. The other half cycle will reverse bias the base-emitter junctionand IC will be cut off. Thus, for class B operation, collector current will flow for approximately 180

degrees (half) of the input signal, as shown in figure 3.4 view C.

The operation of the class "B" amplification

As for the class "B" amplification, it establishes the signalless condition operating point near thecondition which the collector electric current(Ic) doesn't flow through. So, only the half of the inputsignal is amplified.

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In the class "B" amplification, the two transistors combine and are used.The characteristic of the direct current is opposite about the NPN and the PNP transistor. The halfof the input signal can be amplified when combining these.

The class B operated amplifier is used extensively for audio amplifiers that require high-poweroutputs. It is also used as the driver- and power-amplifier stages of transmitters.

Class C Amplifier Operation

In class C operation, collector current flows for less than one half cycle of the input signal, asshown in figure 3.4 view D. The class C operation is achieved by reverse biasing the emitter-base

junction, which sets the dc operating point below cutoff and allows only the portion of the inputsignal that overcomes the reverse bias to cause collector current flow.

The operation of the class "C" amplification

The operating point of the class "C" amplification is not on the load line.The part of the input signal is amplified.So, it isn't possible to use for the amplification such as the sound.It is used for the high frequency multiplying circuit and so on.

The class C operated amplifier is used as a radio-frequency amplifier in transmitters.

For a comparison of output signals for the different amplifier classes of operation, refer to figurebelow during the following discussion.

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Figure 3.4 A comparison of output signals for the different amplifier classes of operation.

From the previous discussion, you can conclude that two primary items determine the class ofoperation of an amplifier - (1) the amount of bias and (2) the amplitude of the input signal. With agiven input signal and bias level, you can change the operation of an amplifier from class A toclass B just by removing forward bias. Also, a class A amplifier can be changed to class AB byincreasing the input signal amplitude. However, if an input signal amplitude is increased to thepoint that the transistor goes into saturation and cutoff, it is then called an OVERDRIVENamplifier.

You should be familiar with two terms used in conjunction with amplifiers - FIDELITY andEFFICIENCY. Fidelity is the faithful reproduction of a signal. In other words, if the output of anamplifier is just like the input except in amplitude, the amplifier has a high degree of fidelity. Theopposite of fidelity is a term we mentioned earlier - distortion. Therefore, a circuit that has highfidelity has low distortion. In conclusion, a class A amplifier has a high degree of fidelity. A class

AB amplifier has less fidelity, and class B and class C amplifiers have low or "poor" fidelity.

The efficiency of an amplifier refers to the ratio of output-signal power compared to the total inputpower. An amplifier has two input power sources: one from the signal, and one from the powersupply. Since every device takes power to operate, an amplifier that operates for 360 degrees ofthe input signal uses more power than if operated for 180 degrees of the input signal. By usingmore power, an amplifier has less power available for the output signal; thus the efficiency of theamplifier is low. This is the case with the class A amplifier. It operates for 360 degrees of the inputsignal and requires a relatively large input from the power supply. Even with no input signal, theclass A amplifier still uses power from the power supply. Therefore, the output from the class Aamplifier is relatively small compared to the total input power. This results in low efficiency, whichis acceptable in class A amplifiers because they are used where efficiency is not as important asfidelity.

Class AB amplifiers are biased so that collector current is cut off for a portion of one alternation ofthe input, which results in less total input power than the class A amplifier. This leads to betterefficiency.

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Class B amplifiers are biased with little or no collector current at the dc operating point. With noinput signal, there is little wasted power. Therefore, the efficiency of class B amplifiers is higherstill.

The efficiency of class C is the highest of the four classes of amplifier operations.

Amplifiers in classes A, B, and AB operate their output3.5 Other biasing techniques of common emitter transistor configuration

a) Base biased with emitter feedback technique

Inserting a resistor RE in the emitter circuit as in Figure below causes degeneration, also knownas negative feedback. This opposes a change in emitter current IE due to temperature changes,resistor tolerances, beta variation, or power supply tolerance. Typical tolerances are as follows:resistor 5%, beta 100-300, power supply 5%. Why might the emitter resistor stabilize achange in current? The polarity of the voltage drop across RE is due to the collector battery VCC.The end of the resistor closest to the (-) battery terminal is (-), the end closest to the (+) terminal it

(+). Note that the (-) end of RE is connected via VBB battery and RB to the base. Any increase incurrent flow through RE will increase the magnitude of negative voltage applied to the base circuit,decreasing the base current, decreasing the emitter current. This decreasing emitter currentpartially compensates the original increase.

Emitter-bias

Note that base-bias battery VBB is used instead of VCC to bias the base in Figure above. Later wewill show that the emitter-bias is more effective with a lower base bias battery. Meanwhile, wewrite the KVL equation for the loop through the base-emitter circuit, paying attention to thepolarities on the components. We substitute IBIE/ and solve for emitter current IE. This equationcan be solved for RB , equation: RB emitter-bias, Figure above.

Before applying the equations: RB emitter-bias and IE emitter-bias, Figure above, we need tochoose values for RC and RE . RC is related to the collector supply VCC and the desired collector

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current IC which we assume is approximately the emitter current IE. Normally the bias point for VCis set to half of VCC. Though, it could be set higher to compensate for the voltage drop across theemitter resistor RE. The collector current is whatever we require or choose. It could range frommicro-Amps to Amps depending on the application and transistor rating. We choose IC = 1mA,typical of a small-signal transistor circuit. We calculate a value for RC and choose a closestandard value. An emitter resistor which is 10-50% of the collector load resistor usually workswell.

Our first example sets the base-bias supply to high at VBB = VCC = 10V to show why a lowervoltage is desirable. Determine the required value of base-bias resistor RB. Choose a standardvalue resistor. Calculate the emitter current for =100 and =300. Compare the stabilization ofthe current to prior bias circuits.

An 883k resistor was calculated for RB, an 870k chosen. At =100, IE is 1.01mA.

For =300 the emitter currents are shown in Table below.

Emitter current comparison for =100, =300.

Bias circuit IC =100 IC =300

base-bias 1.02mA 3.07mA

collector feedback bias 0.989mA 1.48mA

emitter-bias, VBB=10V 1.01mA 2.76mA

Table above shows that for VBB = 10V, emitter-bias does not do a very good job of stabilizing theemitter current. The emitter-bias example is better than the previous base-bias example, but, notby much. The key to effective emitter bias is lowering the base supply VBB nearer to the amount ofemitter bias.

How much emitter bias do we Have? Rounding, that is emitter current times emitter resistor: IERE= (1mA)(470) = 0.47V. In addition, we need to overcome the VBE = 0.7V. Thus, we need a VBB>(0.47 + 0.7)V or >1.17V. If emitter current deviates, this number will change compared with the

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fixed base supply VBB,causing a correction to base current IB and emitter current IE. A good valuefor VB >1.17V is 2V.

The calculated base resistor of 83k is much lower than the previous 883k. We choose 82k

from the list of standard values. The emitter currents with the 82k RB for =100 and

=300 are:

Comparing the emitter currents for emitter-bias with VBB = 2V at =100 and =300 to the previousbias circuit examples in Table below, we see considerable improvement at 1.75mA, though, notas good as the 1.48mA of collector feedback.







How can we improve the performance of emitter-bias? Either increase the emitter resistor RB ordecrease the base-bias supply VBB or both. As an example, we double the emitter resistor to thenearest standard value of 910.

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The calculated RB = 39k is a standard value resistor. No need to recalculate IE for = 100. For = 300, it is:

The performance of the emitter-bias circuit with a 910 emitter resistor is much improved. SeeTable below.






emitter-bias, VBB=2V, RB=470 1.01mA 1.75mA


As an exercise, rework the emitter-bias example with the base resistor reverted back to 470,and the base-bias supply reduced to 1.5V.

The 33k base resistor is a standard value, emitter current at = 100 is OK. The emitter current at = 300 is:

Table below compares the exercise results 1mA and 1.38mA to the previous examples.







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emitter-bias, VBB=1.5V, RB=470 1.00mA 1.38mA

The emitter-bias equations have been repeated in Figure below with the internal emitterresistance included for better accuracy. The internal emitter resistance is the resistance in theemitter circuit contained within the transistor package. This internal resistance REE is significantwhen the (external) emitter resistor RE is small, or even zero.

Bypass Capacitor for RE

One problem with emitter bias is that a considerable part of the output signal is dropped acrossthe emitter resistor RE (Figure below). This voltage drop across the emitter resistor is in serieswith the base and of opposite polarity compared with the input signal. (This is similar to acommon collector configuration having

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Voltage Divider Biasing circuit.

The voltage divider is formed using external resistors R1 and R2. The voltage across R2 forwardbiases the emitter junction. By proper selection of resistors R1 and R2, the operating point of thetransistor can be made independent of . In this circuit, the voltage divider holds the base voltage

fixed independent of base current provided the divider current is large compared to the basecurrent. However, even with a fixed base voltage, collector current varies with temperature (forexample) so an emitter resistor is added to stabilize the Q-point, similar to the above circuits withemitter resistor.

Merits:

Unlike above circuits, only one dc supply is necessary. Operating point is almost independent of variation. Operating point stabilized against shift in temperature.

Demerits:

In this circuit, to keep IC independent of the following condition must be met:

which is approximately the case if where R1 || R2 denotes the equivalent resistance

of R1 and R2 connected in parallel. As -value is fixed for a given transistor, this relation can be satisfied either by keeping

RE fairly large, or making R1||R2 very low.

If RE is of large value, high VCC is necessary. This increases cost as well as precautionsnecessary while handling.

If R1 || R2 is low, either R1 is low, or R2 is low, or both are low. A low R1 raises VB closer toVC, reducing the available swing in collector voltage, and limiting how large RL can bemade without driving the transistor out of active mode. A low R2 lowers Vbe, reducing the

allowed collector current. Lowering both resistor values draws more current from thepower supply and lowers the input resistance of the amplifier as seen from the base.

AC as well as DC feedback is caused by RE, which reduces the AC voltage gain of theamplifier. A method to avoid AC feedback while retaining DC feedback is discussedbelow.

Usage:

The circuit's stability and merits as above make it widely used for linear circuits.

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Voltage divider with capacitor

The standard voltage divider circuit discussed above faces a drawback - AC feedback caused byresistor RE reduces the gain. This can be avoided by placing a capacitor (C2) in parallel with RE,as shown in circuit diagram.

This capacitor is usually chosen to have a low enough reactance at the signal frequencies of

interest such that RE is essentially shorted at AC, thus grounding the emitter. Feedback istherefore only present at DC to stabilize the operating point, in which case any AC advantagesof feedback are lost.

Of course, this idea can be used to shunt only a portion of RE, thereby retaining some ACfeedback.
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