DEPARTMENT OF ELECTRICAL AND ELECTRONICS

SEMESTER: III BRANCH: EEE

ELECTRONIC DEVICES AND CIRCUITS (131303)

UNIT - I

Part-A

1. Define valence band, conduction band.

The range of energies possessed by valence electrons is called valence band. The

range of energies possessed by conduction electrons is called conduction band. The free

electrons which are left in the valence band are occupying the conduction band.

2. Define forbidden energy gap.

The energy gap between the valence band and conduction band is defined as

forbidden energy gap. For insulators. it is around 6ev, for semiconductors, its value is

comparatively low. Germanium has energy gap 0.7ev and silicon has 1.1ev . For conductors,

since conduction and valence bands are overlapping the energy gap is zero.

3. What is a semiconductor or intrinsic semiconductor?

Intrinsic semiconductors are pure form of semiconductors. The conductivity of a

semiconductor lies between an insulator and a conductor. As temperature increases, the

conductivity of the semiconductor also increases. Semiconductors have negative temperature

co-efficient of resistance

4. Define extrinsic semiconductor.

The electrical conductivity of pure semiconductor can be increased by adding some

impurity into it. The resulting semiconductor is called extrinsic semiconductor.

5. What is meant by N-type and P-type semiconductor?

When a small amount of impurity (eg. Antimony, Arsenic) is added to a pure

semiconductor crystal the resulting extrinsic semiconductor is N-type semiconductor. If

trivalent impurity (eg. Indium, Gallium ) is added to a pure semiconductor then the resulting

extrinsic semiconductor is known as P-type semiconductor.

6. What is doping?

The process of adding impurity to pure semiconductor is known as doping. As a result

of it the characteristics of semiconductor is changed and hence the conductivity increases.

7. What are donor and acceptor impurities?

Pentavalent impurities (Antimony, Arsenic) have five valence electrons. They can

donate one excess electron to adjacent atoms to complete lattice structure, therefore they are

called donor impurities.

Trivalent impurities (Indium, Gallium) has three valence electrons. They have

tendency to accept one electron from adjacent atoms to complete lattice structure, therefore

they are known as acceptor impurities.

8. Define Fermi level.

The Fermi level is defined as the maximum energy level, which is occupied by

electron at absolute zero temperature. In P- type semiconductor the Fermi level will be above

the top of the valence band. In N-type it lies below the bottom of the conduction band.

9. State mass action law.

Mass action law states that in a semiconductor the product of the number of holes and

the number of electrons is constant and is independent of the amount of donor and acceptor

impurity doping.

np = ni2

where n= free electron concentration

p= hole concentration; nj = intrinsic concentration

10. Define the term the drift current.

If a steady electric field is applied across a semiconductor, it causes the free electrons

to move towards the positive terminal and the holes move towards the negative terminal of

the battery. This combined effect causes a current flow in the semiconductor. The current

produced in this manner is known as drift current.

Drift current density due to electrons

Jn = q n n E

Drift current density due to holes

Jp = q pp E

Jn = Drift current density due to electrons

Jp = Drift current density due to holes

q = Charge of the carrier

n = Mobility of electrons

p = Mobility of holes

E = Applied electric field strength.

11. Define the term diffusion current.

In a semiconductor it is possible to have a non uniform distribution of carriers. A

concentration gradient exists if the number of either holes and electrons is greater in one

region as compared to the rest of the region. The holes and electrons then tend to move from

a region of higher concentration to lower concentration region.

This process is known as diffusion and the electric current produced due this process is

known as diffusion current.

12. What is a PN junction diode?

A PN junction diode is a two terminal device consisting of a PN junction formed

either of Germanium or Silicon crystal. A PN junction is formed by diffusing P type material

to one half side and N type material to other half side.

13. What is Depletion region in a PN junction diode?

In a PN junction diode, the holes and the electrons combine to form electron-hole

pair, leaving the uncovered acceptor and donor ions at the vicinity of the junction. The region

where the charge carriers are depleted and has only immobile charges which are electrically

charged is known as depletion region or space charge region.

14. Define barrier potential.

Potential barrier is defined as an electric potential that is established across the

junction, during the initial diffusion of charge carriers at the junction, which restricts further

movement of charge carriers across the junction.

15. Explain the terms knee voltage and breakdown voltage?

Knee voltage: The forward voltage at which the current through the PN junction starts

increasing rapidly is known as knee voltage. It is also called as cut-in voltage or threshold

voltage.Breakdown voltage: It is the reverse voltage of a PN junction diode at which the

junction breaks down with sudden rise in the reverse current.

16. Write down and explain junction diode equation.

The equation which explains the forward and reverse characteristics of a

semiconductor diode is known diode equation. The diode current is given by

I = I0 (e v/VT -1)

Where Io = reverse saturation current

= 1 for Ge diodes , 2 for silicon diodes

V- External voltage

VT = volt equivalent of temperature. = T/11,600

17. Define and explain peak inverse voltage ( PIV)

Peak inverse voltage is the maximum reverse voltage that can be applied to the PN

junction without damage to the junction. If the reverse voltage across the junction exceeds to

its peak inverse voltage, the junction may be destroyed due to excessive heat.

18. Define recovery time of a diode.

The recovery time is the time difference between the 10 percent point of the diode

voltage and the time when this voltage reaches and remains within 10 percent of its final

value.

19. Differentiate drift and diffusion current

Drift current Diffusion current

1. Developed due to potential gradient Developed due to concentration gradient

2. Phenomenon found both in

semiconductors and metals

Only in semiconductros

3. Jn = qn n E

Jp = q p p E

Jn = q Dn dn / dx

Jp = q Dp dp / dx

20. State the relationship between diode capacitance and the reverse bias voltage.

Transition capacitance CT = A / W

The equation shows that the width of the depletion layer ( W) is inversely proportional to the

capacitance . Since the width increase in the reverse voltage, the width of the depletion region

increases, the capacitance decreases with increase in reverse voltage.

21. Define the term diffusion capacitance or storage capacitance.

The diffusion capacitance effect is found when the diode is forward biased and it is

defined as the rate of change of injected charge with voltage and given by

CD = I / VT

I = diode current, VT = volt equivalent temperature. VT = T /11,600

= constant = 1 for Ge diodes

2 for silicon diodes

= mean life time

22. What is avalanche breakdown in PN junction diode?

The avalanche breakdown takes place when both sides of the junction are lightly

doped and due to this the depletion layer is large. When the reverse bias voltage is increased

the accelerated free electrons collide with the semiconductor atoms in the depletion region

Due to collision, the covalent bonds are broken and electron hole pairs are generated. These

new charge carriers so produce acquire energy from applied potential and in turn produce

additional carriers. This forms a cumulative process called avalanche multiplication ,this

causes the reverse current to increase rapidly . This leads to breakdown of the junction known

as avalanche breakdown.

23. What is forward and reverse recovery time.

Forward recovery time is defined as the time required to reach 10% to 90% of diode

voltage.

Reverse recovery time is defined as the time taken for the minority carrier density to

drop to zero.

24. Define Static resistance and Dynamic resistance?

The resistance offered by the diode to DC operating conditions is called “Static

resistance” and the resistance offered by the diode to AC operating conditions is called

“Dynamic resistance”.

25. List the applications of PN junction diode:

1.used as rectifier in DC power supplies.

2.Used as signal diodes in communication circuits.

3.Used in clipper and clamper circuits.

Part-B

1. Define drift current and diffusion current and derive Equations for it.2. Explain the operation of a PN junction diode in the forward and reverse condition and draw its

characteristics

3. Explain dynamic resistance of a diode using necessary diagrams and derive its relation.

4. Explain intrinsic and extrinsic semiconductor in detail5. Given two different germanium diodes with their data at forward has as follows

6. Explain the effect of temperature on a PN junction diode7. Derive and discuss capacitance effect of a reverse bias PN junction diode and what is its

importance?

8. Write short notes on diffusion capacitance and derive the relation9. Explain PN diode switching times10. A germanium diode has a contact potential of .2volt while the concentration of accepted impurity atoms is 3x1020 /m3. Calculate for a reverse bias of .1 volt, the width of the depletion region. If the reverse bias is increased to 10volt, calculate the new width of the depletion region. Assuming cross sectional area of the junction as 1mm2, calculate the transition capacitance values for both the cases. Assume r=16 for germanium

UNIT II

Part-A1. Why base is made thin in BJT?

Transistor consists of three portions namely emitter, base and collector. Among them

base forms the middle part. It is very thin and lightly doped because it allows most of the

emitter current carriers towards the collector. Since base is acting as an interface it doesn't

need more area.

2. What is meant by biasing a transistor?

Transistor biasing is the process of maintaining proper flow of zero signal collector

current and collector-emitter voltage during the passage of signal. Biasing keeps emitter-base

junction forward biased and collector-base junction reverse biased during the passage of

signal.

3. Define the different operating regions of transistor.

Active region: It is defined in which transistor collector junction is biased in reverse

direction and emitter junction in forward direction.

Cutoff region: The region in which the collector and emitter junctions are both reverse-biased

Saturation region : The region in which both the collector and emitter junctions are forward

biased.

4. Define Base width modulation (Early effect)

In a CB configuration, an increase in collector voltage increases the width of the

depletion region at the output junction diode. This will decrease the effective width of the

base. This is known as early effect. Due to this effect recombination rate reduces at the base

region and charge gradient is increased within the base.

5. Explain the significance of Base width modulation (Early effect)

a) It reduces the charges recombination of electrons with holes in the base region,

hence the current gain increases with the increase in collector -base voltage

b) The charge gradient is increased within base, hence the current due to minority

carriers injected across emitter junction increases.

6. What are the three types of configurations?

Common base configuration

Common emitter configuration

Common collector configuration

7. Among CB, CE, CC which is most important?

The CE configuration is important. The reasons

i) High current gain

ii) Output to input impedance ratio is moderate therefore easy coupling is

possible between various transistor stages

iii) It finds excellent usage in audio frequency applications hence used in

receivers and transmitters

8. Give the advantages of CE configuration.

i. High output impedance

ii. High current gain

iii. High power gain

8. What is thermal runaway?

The reverse saturation current in a semiconductor doubles for every 100 C rise in

temperature I as temperature increases the leakage current increases I and the collector

current also increases. The increase in collector produces an increase in power dissipation at

the collector - base junction. This I in turn further increases the temperature of the collector-

base junction causing the collector current to further increase. This process may become

cumulative and it is possible for the transistor to burn out. This process is known as Thermal

runaway.

9. How thermal runaway can be avoided?

Thermal runaway can be avoided using a stabilization or heat sink with the transistor.

10. What is the use of Heat sink?

Power transistors are temperature dependent devices. As they handle large currents

they can be heated during operation which may lead to self destruction. To avoid this the

transistor is fixed on a metal sheet preferably Aluminum to dissipate heat from the power

transistor .This protection method is known as heat sink protection.

11. How a transistor is used as a switch?

A transistor should be operated in saturation and cutoff regions to use it as a switch I

While operating in saturation region I transistor carry heavy current hence considered as ON

state. In cutoff it doesn't carry current and it is equivalent to open switch.

12. Define alpha and beta of a transistor.

Alpha (a) is the current gain of a common base transistor.

It can be defined as the ratio of change in collector current to the change in the emitter current

at constant collector-base voltage VCB

= IC / IE at constant VCB

Beta () is the current gain of a common emitter transistor. It can be defined as the ratio of

change in collector current to the change in the base current at constant collector-base voltage

VCE

= lC / IB at constant VCE

13. Which configuration is known as emitter follower and why it is named so?

CC configuration is known as emitter follower, whatever may be the signal applied at

the input, may produce same signal at the output. In other words, the gain of the circuit is

unity. So that the common collector circuit - the so called emitter follower is named as

emitter follower. ( output follows the input)

14. Why do the output characteristics of CB transistor have a slight upward slope?

The emitter and collector are forward biased under the saturation region. Hence a

small change in collector voltage causes a significant change in collector current .Therefore a

slight upward slope is found in the output characteristics.

15. Compare the performance of CE, CB, CC

Parameters CB CE CC

Current gain (Ai) Low High High

Voltage gain (Vi) High High Low

Input resistance (Ri) Low Medium High

Output resistance (Ro) High Medium Low

16. What do you understand by h - parameters?

Any linear circuit can be analyzed by four parameters of mixed dimensions( hi measured in

ohms, while ho is measured in mho, parameters like hre and hfe are dimensionless ).Since the

dimensions of the parameters has mixed units they are referred as h- parameters. The h-

parameters are determined by both open circuit and short circuit terminations.

17. Define h-parameters of a CE transistor.

The equations are:

Vbe = hie Ib + hre Vce

Ic = hfe Ib + hoe Vce

The h-parameters of CE transistor are:

(a) Input impedance: It is defined as the ratio between input voltage Vbe and input current

Ib , when output voltage Vee is zero. It's unit is ohms.

hie = Vbe / Ib at Vce = 0

(b) Reverse voltage gain : It is defined as the ratio between input voltage V be and output

voltage Vce ,when input current Ib is zero.

hre = Vbe / Vce at Ib = 0

(c) Output admittance: It is defined as the ratio between output current Ie and input voltage

Vee ,when output current Ib is zero. It's unit is mho.

hoe = Ic / Vce at Ib = 0

(d) Forward current gain: It is defined as the ratio between output current Ie and input current

Ib ,when output voltage Vee is zero.

hfe = Ic / Ib at Vce = 0

18. What is the significance of h-parameters?

Easy to measure

Can be determined from transistor at audio range of frequencies.

h-parameters are real numbers at audio range of frequencies.

Can be employed easily for circuit analysis and design.

19. Define Turn -on time.

The turn-on time is the sum of delay time (td) and rise time (tr)

turn -on time= td + tr

where td Delay time - is the time taken to rise the current to 10% of its maximum

value , Rise time tr -is the time taken for the current to rise from 10% to 90%

of its maximum value.

20. Define Turn -off time.

The turn-off time is the sum of storage time (ts) and fall time (tf)

turn -off time= ts + tf

where ts storage time - is the time taken for the current to drop to 90% of

its maximum value

Fall time tf - is the time taken for the current to fall from 90% to

10% of its maximum value

21. Mention the important features of power transistor.

Transistors handle high voltage and high current rating are known as power transistor.

They are mostly used in switching mode. Power loss in a power transistor is lower than

thyristor.the size of power transistor is large because its collector power dissipation is more.

22. Differentiate small signal model with large signal model?

In a small signal model the circuit response will d\be determined by linear circuit

analysis. Large signal analysis must be performed graphically. Distortion will be high in large

signal model.

23. What are the merits of hybrid model?

i. h-parameters can be easily measured.

ii. They can be calculated from the static characteristics of transistor

iii. h-parameters can be used easily and conveniently in circuit analysis and design

iv. manufacturers provide h-parameters in their data sheets

24. What is maximum voltage rating of a BJT?

There is an upper limit on the maximum allowable collector junction voltage because

at higher voltages, there is a possibility of voltage breakdown in the transistor. This is called

maximum voltage rating of a BJT.

25. What is safe operating area of a BJT?

The safe operating area is the locus of IC-VCE points which defines the boundary

between safe and unsafe operation.

Part-B

1. Explain the construction and operation of BJT in unbiased condition and biased condition?2. Explain the 5 basic techniques for transistor construction

3. Explain the characteristics of a transistor in common base configuration. Also explain the current relations in common base configuration4. Explain the characteristics of a transistor in common emitter configuration. Also explain the current

relations in common emitter configuration5. Explain the characteristics of a transistor in common collector configuration. Also explain the

current relations in common collector configuration

6. Explain the detailed study of currents in a transistor and derive Ebers moll equation7. Explain transistor switching times

8. Explain power transistor construction, characteristics and operation9. Derive the analytical expression for transistor characteristics10.Explain transistor hybrid model for common emitter configuration

UNIT - III

1. What is the disadvantage of FET compared to BJTs?

FET has small gain band width product (GBW).

2. Compare BJT and JFET

BJT JFET

Low input impedance High input impedance

High Output impedance Low output impedance

Bipolar device Unipolar device

Noise is more Less noise

Cheaper Costlier

Gain is more Gain is less

Current controlled device Voltage controlled device

3. Mention the points of superiority of FET's over BJT's?

i) The noise level is very low in FET since there are no junctions.

Ii) FET has very high power gain iii) Offers perfect isolation between input and

output since it has very high input impedence.

iv) FET is a negative temperature coefficient device hence avoids thermal

runaway.

4. What is FET?

A field effect device in which (FET) is a three terminal semiconductor device

in which current conduction is by one type of carriers ( either holes or electrons) and

is controlled by an electric field.

5. Define drain resistance of JFET?

Drain resistance (rd) is defined as the ratio of small change in drain to source

voltage (Vds) to the corresponding change in drain current (~Id) at constant gate to

source voltage (Vgs) .r d = (Vds) / (Id) at constant Vgs

6. Define transconductance of JFET.

Transconductance (gm) is defined as the ratio of small change in drain current

(Id) to the corresponding change to gate source (Vgs) at constant drain to source

voltage (Vds)'

gm = (Id) / (Vgs) at constant Vds

7. Define amplification factor of JFET?

Amplification factor () is defined as the ratio of small change in drain to source

voltage (Vds) to the corresponding change in gate to source voltage (Vgs) at a constant drain

current Id

= (Vds) / (Vgs) at constant Id

8. Write down the relationship between various FET parameters.

Amplification factor ()= Transconductance (gm) x Drain resistance (rd)

9. What are the applications of FET?

FET used as a low noise amplifier, as a buffer amplifier; in phase shift oscillator

10. Give the Shockley's equation for FET.

The Shockley's equation give the relation between drain current (ld) in the pinch of

region and the gate to source voltage Vgs

Id = Idss [ 1 - Vgs / Vp ]

Where Idss = maximum value of drain current when Vgs = 0

Vp = Pinch off voltage.

11. What is meant by Gate -Source threshold voltage and pinch off voltage of a

JFET?

The voltage at which the channel is completely cut-off and the drain current becomes

zero is called as Gate -Source threshold voltage.

In the output characteristics of FET, the drain current rises rapidly with drain source

voltage. After reaching some value, it becomes constant. The drain source voltage above

which drain current becomes constant is known as pinch off voltage. The corresponding Vgs

is called Gate -Source threshold voltage.

12. Why the input impedance in FET is very high in comparison with BJT?

The input impedance of FET is extremely high because reverse bias is applied at input

whereas in BJT , the output impedance is low due to applied forward bias.

13. How is FET used as VVR?

At low voltages, the depletion regions are thin and the drain current increases with

voltages. So in the region where voltage is less then pinch off voltage (Vp) , FET is behaving

as a voltage variable resistor (VVR). That is the drain to source resistance is controlled by

Vgs

14. What is MOSFET?

The MOSFET is an abbreviation of Metal Oxide Semiconductor Field Effect

Transistor. It is a three terminal semiconductor device similar to a FET with gate insulated

from the channel. Therefore it is a also known as insulated Gate (IGFET)

15. Why is the input impedance of a MOSFET higher than that of FET?

The input impedance of a MOSFET is higher than that of FET since the gate is

insulated from the channel by a thin layer of silicon di oxide

16. Depletion MOSFET is commonly known as Normally – ON = MOSFET ?

The depletion MOSFET can conduct even if the gate to source voltage (Vgs) is zero.

Due to this reason depletion MOSFET is commonly known as Normally –ON MOSFET.

17. What is meant by inversion layer in E - MOSFET

In the construction of E- MOSFET, there are two layers of conductor which

sandwiched by Sio2 layer this can act as capacitor. When the gate is positive, it induces

negative charges in the substrate which will form a part of drain current. This is called

inversion layer.

18. Application of MOSFET?

i) It can be used as input amplifiers in oscilloscope, electronic voltmeters.

ii) It is used in computer memories in logic circuits

iii) It is used in phase shift oscillators.

iv) It is used in FM and TV receivers

19. Why E-MOSFET is normally called as OFF – MOSFET

WHEN Vgs = 0, the biasing supply VDD this to force the force the free electron to

move source to drain. But the P substrate has only few generated conduction band electrons.

Aside from this minority carriers some surface leakage, the current between source and drain

is zero hence E- MOSFET is called as OFF MOSFET.

20. Differentiate JFET and MOSFET?

JFET MOSFET

Reverse bias for gate Positive or negative gate voltage

Gate is formed as a diode Gate is made as a capacitor

Operated only in depletion mode Can be operated either in depletion mode or

in enhancement mode

High input impedance Very high input impedance due to capacitive

effect

21. Differentiate Enhancement MOSFET and Depletion MOSFET

Enhancement MOSFET Depletion MOSFET

Positive voltage at the gate Negative voltage at the gate

Inversion layer is made Depletion of majority carries happens

Negative charges are formed Positive charges are formed

22. What are applications of UJT?

1. Phase control

2. Saw – tooth generators

3. Non sinusoidal oscillators

4. Triggering device for SCR and TRIAC

23. What does UJT stand for? Justify the name UJT

UJT stands for Unijunction Transistor. The UJT is a three terminal semiconductor

device having two doped regions. It has one emitter terminal (E) and two base terminals (B1

and B2) It has only One junction, and the outlook of UJT resembles to a transistor and hence

the name unijunction transistor.

24. What is intrinsic stand off ratio of a UJT?

The ratio of voltage between emitter and base 1 to VBB is called as intrinsic stand of

ratio

η = RB1 / (RB1+RB2)

The value lies between 0.51 to 0.82

25. How does UJT differ from a FET?

FET UJT

Gate is much larger Emitter junction area is small

Gate junction reverse biased Emitter junction is forward biased

Amplifying device Switching device

Used in amplifiers, phase shift oscillators Used in Relaxation oscillators

Part-B

1. Explain the voltage variable resister characteristics of JFET with real time example2. Explain the construction and operation of N-channel Depletion type MOSFET with neat diagrams3. Explain the construction and operation of P-channel Depletion type MOSFET with neat diagrams4. I)Obtain the JFET small signal model and explain how to get the small signal parameters

II) an n-channel JFET has a pinchoff voltage of -4.5 v and Idss =9 mA(i) at what value of vgs in the pinchoff region will Id =3mA

what is the value of vds (sat) when id =3Ma

5. With the help of neat diagram and graphs explain construction and operation of an N-channel JFET and show the depletion regions6. Draw a circuit diagram for obtaining drain and transfer characteristics for a P-channel JFET and explain that with neat diagrams and graphs7. (I)Derive An Expression For Pinch Off voltage for JFET

(II) For an N-channel JFET with a=1015 electron/cm3 ,Find (i) pinch off voltage (ii)the channel half with for vgsequal to half Vp and Id =0(€r=12)8. Explain the biasing methods of JFET with neat sketch.9. (a) Derive and explain the analysis of CS amplifier. (8)(b)Design a fixed bias circuit to have operating point of (10V, 3 mA ); The circuit is supplied with 20V and uses a silicon transistor of hfe = 250. (8)10. With neat diagrams explain the analysis and design of cascade amplifiers.11. With neat diagrams explain the analysis and design of Darlington amplifiers.

UNIT-IV

1.What is a differential amplifier? What are its advantages?

An amplifier that has two inputs and produces an output signal that is a function of the

difference between the two inputs.

Advantages :

i) It used no frequency – dependent coupling or by passing capacitors.

ii) It can compare any two signals and detect any difference.

iii) It gives higher gain than two cascaded stages of ordinary direct coupling.

iv) It provides very uniform amplification of signal from dc upto very high

frequencies.

2. What are the applications of differential amplifier?

i) In the medial electronics field

ii) As a input stage in the measuring instruments

iii) In analog computation

iv) In linear integrated circuits

3. What is meant by CMRR of a Differential amplifier?

The common mode rejection ratio (CMRR) serves as a figure of merit of a differential

amplifier and is defined as the ratio of the differential voltage gain (Ad) to the common mode

voltage gain (Acm)

Ad CMRR =

Ac

4. State the different methods of biasing of difference amplifier.

The different methods of biasing of difference amplifier are,

i) Emitter bias ii) Constant current bias

5. Explain the need for constant current source for difference amplifier.

The necessity for constant current source for differential amplifier is to increase the

common mode rejection ratio without changing the quiescent current (operating point) and

without lowering the forward current gain.

6. Why RE is replaced by a constant current bias in a differential amplifier?

The emitter supply VEE used for biasing purpose must become larger as RE is increased in order to maintain the quiescent current at its proper value. If the operating currents of the transistors are allowed to decrease, this will lead to higher H ie values and lower values of Hfe. Both these effects will tend to decrease CMRR. To over come this practical limitations RE is replaced by a constant current bias. 7. What is the input impedance of a differential amplifier with RE at its emitter

junction?

The input impedance of a differential amplifier with RE at its emitter junction is : R i

= 2hie + (1+hfe) RE

8. What should be the value of AC, the common mode gain and Ad the difference mode

gain for an ideal differential amplifier?

The common mode gain of an ideal differential amplifier is 0.

The difference mode gain of an ideal differential amplifier is very high ()

9. Explain why constant current source biasing is preferred for differential amplifier.

The constant current source biasing is preferred for differential amplifier inorder to

increase the input resistance and to make the common mode gain zero.

10. Distinguish between common mod signal and differential mode signal.

If the same input is applied to both the inputs, the operation is called common mode

signal. It is the average value of the input signals.

V1+V2

i.e.vc = 2

If the two opposite polarity input signals are applied, the operation is referred to as

difference mode. The difference between the input voltages in called difference mode

signal.

11. Explain how the differential amplifier can be used as an emitter coupled phase

inverter.

A differential amplifier may be used as an emitter coupled phase inverter. In this

case, the signal is applied at one base only leaving the second base unexcited but with proper

bias. The output voltage is then picked up from the collectors. These two voltages are equal

in magnitude and opposite in phase, Thus the differential amplifier acts as a phase inverter or

phase amplifier.

12. What is the current gain for a darlington pair?

The current gain for a darlington pair is given by, (1+hfe)2

Ai = 1+hoehfeRe

where hfe - forward current gain for common emitter amplifier.

hfc - forward current gain for common collector amplifier.

hoe - output resistance for common emitter amplifier.

13. Mention the effects on bandwidth and output impedance due to various types of feedback.

Types of feedback Bandwidth Output impedance

Voltage series feedback Increases Decreases

Voltage shunt feedback Increases Decreases

Current series feedback Increases Decreases

Current shunt feedback Increases Decreases

14. Negative feedback is preferred to other methods of modifying amplifier

characteristics. Why?

Negative feedback is preferred to other methods of modifying amplifier

characteristics, because it provides very high stability.

15. Distinguish between series and shunt feedback.

Sl.No Series Feedback Shunt Feedback

(i)

(ii)

In series feedback amplifier,

the feedback signal is connected in

series with the input signal.

It increases the input resistance

In shunt feedback amplifiers, the

feedback signal is connected in

parallel with the input signal.

It decreases the input resistance.

16. State the effect of feedback on noise.

When feedback is employed in amplifiers, the noise is reduced.

Let A - Voltage gain without feedback

- Feedback factor

N - Noise without feedback

Nf - Noise with feedback

The noise with feedback is given by the following relation.

N Nf = 1+A

Thus, when feedback is employed, the noise is reduced by a factor (1+A)

17. What does shunt feedback modify ?

Shunt feedback decreases the input resistance.

18. What is the effect of voltage series feedback on input resistance?

The voltage series feedback increases the input resistance.

19.What type of feedback is employed in CE stage with unbypassed emitter resistor?

Current series feedback.

20. What is an oscillator?

An oscillator is an electronic device, which produces an output signal of any desired

frequency without any input singal.

21. What is the difference between an oscillator and amplifier?

Sl.No Series Feedback Shunt Feedback

1.

2.

3.

An amplifier is a circuit used to magnify

the given input signal.

Amplifiers have finite gain.

In an amplifier circuit, the frequency,

waveform and magnitude of ac power

output are controlled by as signal voltage

applied at the input.

An Oscillator is a circuit used to

generate the signals. It has no input.

The gain of the oscillator is infinity.

In an oscillator, the frequency

waveform and magnitude of ac

power generated are controlled by

the circuit itself.

22. What is damped oscillation?

The electrical oscillation in which the amplitude decreases with time are called as

dampled oscillations.

23. What is sustained oscillation?

The electrical oscillations in which amplitude does not change with time are called as

sustained oscillations. It is also called as undamped oscillation.

24. What are the conditions for sustained oscillation? (or) What is barkhausen criterion

for sustained oscillation?

(i) The magnitude of loop gain should be unity i.e. | A | = 1

Where A = open loop gain

= feedback factor

(ii) The net phase shift around the loop should be zero or an integral multiple of 2

25. What are the essential parts of an oscillator?

(i) Tank circuit or oscillatory circuit.

(ii) Amplifier (Transistor amplifier)

(iii) Feedback circuit

26. ---------------------feedback is used in oscillators.

Positive

PART-B

1. Draw the neat ckt diagram of differential amplifier. Explain its working in detail in both common and differential modes.2. i) Draw neat diagram of emitter coupled differential amplifier and explain its operation. Also derive the expression of Ac in terms of Re (9)ii) Explain how the value of CMRR is improved with the use of constant current(7)3. Conduct an AC analysis on the differential amplifier and derive expressions for differential gain, common mode gain and hence CMRR. 4. (i)Write the voltage gain and impedance equations for a differential amplifier ckt (6) ii) The following specifications are given for a dual – input, balanced output differential amplifier Rc = 2.2 KΩ, Re = 4.7 KΩ, Rs1 = Rs2 = 50Ω, Vcc = 10 V and the transistor has a β = 100. Calculate 1) The Icq andVceq value, 2) the voltage gain 3) Determine the input and output resistances. (10)5. i)Draw the ckts of Differential Amplifier in its 4 different configurations(8)ii) Perform D.C. analysis for a dual – input, balanced output differential amplifier, and get equations for Icq &Vcq.(8)6. Draw and describe the four types of topology for feedback of an amplifier. Derive the general expression for gain with feedback. Mention the advantages of negative feedback amplifier.7. Derive the expressions of Avf, Rif, and Rof for a Voltage series feedback and current shunt feedback transistor amplifier.8. i)Discuss the characteristics of negative feedback amplifier (8)

ii) An amplifier with negative feedback has a voltage gain of 100. It is found that without feedback an i/p signal of 50 mv is required to produce a given o/p whereas with f/b the i/p

signal must be 0.6 V for the same o/p. Calculate the value of Av without feedback and . (8)

9. i)Calculate Avf, Rif & Rof of a voltage shunt feedback amplifier having A = 300, Ri = 1.5K, Ro = 50K & = 0 . 066. (8)

ii)Discuss the effect of voltage shunt feedback on i/p and o/p impedances (8)10. i) Compare the 4 different types of negative feedback amplifiers (6)

ii) Identify the type of feedback in given amplifier circuit. Draw the basic amplifier and find its gain expression. Derive for and hence find the amplifier gain with feedback.(10)

UNIT-V

PART_A

1. When the input fed to a differentiating circuit is a triangular wave, the output will be

a _________.

Ans : Square wave

2. What is an integrating circuit? Draw a typical integrating circuit.

A circuit that gives an output voltage directly proportional to the integral of its input

is called as an integrating circuit.

3. The wave shape of output wave obtained from an integrating / differentiating circuit depends upon _____ and _____ of the input wave. Ans : time constant (RC) ; wave shape

4. What is a biased clipper?

A clipper used to remove a small portion of positive or negative half cycle of the

signal voltage is called as biased clipper.

5. What is a combination clipper?

Rs

Rb

Rc

Vcc

Vo

Vin

A clipper which is used to remove a small portion of positive as well as a small

portion of negative half cycle of the signal voltage is called as combination clipper. It is also

called as double diode clipper or two way clipper.

6. What is a negative clamper?

Negative clamper is a circuit that shifts the signal in the negative side in such a way

that the positive peak of the signal falls on the zero level.

7.What is a clamper?

Clamper is a circuit that shifts either positive or negative peak of the signal at a

desired DC level.

8. What is a positive clamper?

A positive clamper is a circuit that shifts the signal in the positive side in such a way

that the negative peak of the signal falls on the zero level.

10. The primary function of a clamper circuits is to introduce a ______ into an ac signal.

Ans : DC Level

11. What is a negative clamper ?

Negative clamper is a circuit that shifts the signal in the negative side in such a way

that the positive peak of the signal falls on the zero level.

12. State two applications of clamper.

i) Clampers are used in TV receivers.

ii) Clampers are used to supply power to high voltage / low current devices like

CRTs used in oscilloscopes and computer displays.

13. What is a multivibrator?

A multivibrator is a two stage amplifier with output of one stage feedback to the input

of the other stage.

14. Name the different classes of multivibrtors.

Astable multivibrator is a two stage amplifier with output of one, stage feedback to

the input of the other stage.

15. Name the different classes of multivibrators.

(i) Astable multivibrator

(ii) Monostable multivibrator

(iii) Bisatable multivibrator.

16. A multivibrator that generates square wave of its own is called ____.

Ans : Astable multivibrator

17. The frequency of oscillation of an astable multivibrator depends on the values of

_____ and ____

Resistance and capacitance

18. An astable multivibrator is also called as ______

Ans : free running multivibrator

19. Give the expression for frequency of oscillation of an astable multivibrator.

1fo =

0.69 (R1C1+R2C2)

1fo = [when R1 = R2 R; C1 = C2 = C]

1.38 RC

20. Why an astable multivibrator is also called a square wave generator?

An astable multivibrator generates square wave of known periodic time. So it is also

called a square wave generator.

21. What is a monostable multivibrator?

A multivibrator which has only one stable stable state is called monostable

multivibrator. In a monostable multivibrator when one transistor is conducting the other

transistor is non – conducting. This is the stable state of the monostable multivibrator.

22. How a monostable multivibrator circuit can be obtained from an astable

multivibrator circuit?

Monostable multivibrator circuit can be obtained from an astable multivibrator circuit

by replacing one R-C timing circuit by DC voltage divider.

23. What is a bistable multivibrator?

A multivibrator which has two stable state is called bistable multivibrator. It can stay

in any one of the states indefinitely.

24. Why is the bistable multivibrator also called as flip – flop multivibrator?

In a bistable multivibrator trigger pulse causes the multivibrator to flip from one state

to the other and the next pulse causes it to flop back to its original state. Therefore it is also

called the flip – flop multivibrator.

25. List the applications of bistable multivibrator.

(i) It can be used as a memory cell (i.e., Flip – flop)

(ii) It is used in binary counting circuits and shift registers.

(iii) It is used in logic circuits.

26. List the applications of Schmitt trigger.

(i) It is extensively used as an amplitude comparator.

(ii) It is used for wave shaping. It can be used as a sine wave to square wave

convertor.

27. Compare Schmitt trigger and bistable multivibrator.

Sl.No Series Feedback Shunt Feedback

1.

2.

3.

The collector output of transistor Q2 is

not coupled to the base of transistor Q1

Here input is a slowly varying AC

voltage.

When the input signal level goes above

the UTP, the output changes its stage.

When the signal level goes below LTP or

the signal is completely removed, the

output again changes its state.

The collector output of each

transistor is coupled to the base of

other transistor.

Here input is a trigger signal or pulse

waveform.

When a trigger pulse is applied, the

output changes from one stable state

to another stable state. The removal

of this trigger pulse will not alter the

output state.

28. Compare Astable, Monostable and Bistable multivibrators.

Sl.No Criterion Astable Multivibrator (AMV)

Monostable Multivibrator (MMV)

Bistable Multivibrator (BMV)

1.

2.

Type of coupling

Stable stages

Both the couplings are capacitive

It has no stable

One coupling the capactive and the other is resisttive (DC) It has one stable

Both the couplings are resistive (DC)

It has two stable states.

3.Trigger input

states. Instead it has two quasistable states. It does not require a trigger input pulse.

state (One stable state and one quasi-stable state)

It requires a trigger pulse to go to the quasistale state.

It requires a trigger pulse to switch from one stable state to another stable state.

PART-B

1. Explain the operation of a RC circuit as an integrator. Also sketch the output w/f of the integrator when the input signal is i) a rectangular wave ii) ramp input pulse2. Explain UJT based saw tooth oscillator with associated waveforms and derive for its frequency of Oscillation.3. Discuss 5 biased clipper ckts when the diode is in series(3ckts) with the load and in shunt(2 ckts) with load.4. Draw the ckts of different clamper ckts and explain their operations.(both biased and simple clamper ckts)5. Explain the operation of a RC circuit as a differentiator. Also sketch the output w/f of a differentiator when the input signal is i) a ramp i/p wave ii) a pulse i/p.6. Calculate the component values of a monostable multivibrator developing an output pulse of 500 μs duration. Assume hfe(min) = 25, Ice(sat) = 5 mA, Vcc = 10 V and VBB = -- 4 V.7. With neat ckt and timing diagrams explain the operation of monostable multivibrator. Also derive the expression for the pulse duration.8. With neat ckt and timing diagrams explain the operation of an astable multivibrator. Derive for the frequency of oscillation.9. Describe the function of Schmitt trigger with its hysterisis characteristics.10. i) With neat ckt and timing diagrams explain the operation of a bistable multivibrator. (8)ii) Determine the value of capacitors to be used in an astable multivibrator to provide a train of pulses 2 μs wide at a repetition rate of 100 Khz if R1 = R2 = 20 KΩ . (8)