66490652-Edc-2-Marks-Qb

ELECTRONIC DEVICES AND CIRCUITS

KONGUNADU COLLEGE OF ENGINEERING &TECHNOLOGY Page 1

DEPARTMENT OF EEE.

UNIT – 1

SEMICONDUCTOR DIODE AND BJT

1. How a PN junction can be formed?

The P-type semiconductor and N-type semiconductor are chemically combined

together with a special fabrication technique to form a P-N junction.

2.List out the common diode applications? (or)State the applications of PN

Junction diode.

i.Rectifiers

ii.Power supplies

iii. clippers and Clampers

iv.Oscillators

3. What is diffusion capacitance?

In forward biased condition, width of the depletion region decreases and holes from

p side get diffused in n side while electrons from n side move into the p-side.As the

applied voltage increases,consentration of injected charged particles increases. This

rate of change of the injected charge with applied voltage is defined as a capacitance

called diffusion capacitance.

4.Define the term transition capacitance?

When a PN junction is reverse biased, the depletion layer acts like a dielectric

material while P and N –type regions on either side which has low resistance act as

the plates. In this way a reverse biased PN junction may be regarded as parallel plate

capacitor and thus the capacitance across this set up is called as the transition

capacitance.

C T = A / W

Where

CT - transition capacitance

A - Cross section area of the junction

W – Width of the depletion region

5. what is recovery time? Give its types.

When a diode has its state changed from one type of bias to other a transient

accompanies the diode response, i.e., the diode reaches steady state only after an interval



DEPARTMENT OF EEE.

of time “ t r” called as recoverytime. The recovery time can be divided in to two types

such as

(i) forward recovery time

(ii) reverse recovery time

What is meant by forward recovery time?

The forward recovery time may be defined as the time interval from the instant of 10%

diode voltage to the instant this voltage reaches 90% of the final value. It is represented

as t f r.

6. What is meant by reverse recovery time?

The reverse recovery time can be defined as the time required for injected or the

excess minority carrier density reduced to zero , when external voltage is suddenly

reversed.

7. Define storage time.

The interval time for the stored minority charge to become zero is called storage

time. It is represented as t s.

8. Define transition time.

The time when the diode has normally recovered and the diode reverse current

reaches reverse saturaton current I0 is called as transition time. It is represented as tt.

9. What is break down? What are its types?

When the reverse voltage across the pn junction is increased rapidly at a voltage

the junction breaksdown leading to a current flow across the device. This

phenomenon is called as break down and thevoltage is break down voltage. The

types of break down are

i) zener break down

ii)Avalanche breakdown

10. What is zener breakdown?

Zener break down takes place when both sides of the junction are very heavily

doped and Consequently the depletion layer is thin and consequently the depletion layer

is tin. When a small value of reverse bias voltage is applied, a very strong electric field is

set up across the thin depletion layer. This electric field is enough to break the covalent



DEPARTMENT OF EEE.

bonds. Now extremely large number of free charge carriers are produced which constitute

the zener current. This process is known as zener break down.

11. What is a varactor diode?

A diode which is based on the voltage variable capacitance of the reverse biased

p-n junction is said to be varactor diode. It has other names such as varicaps, voltacaps.

12. What is a tunnel diode?

The tunnel diode is a pn junction diode in which the impurity concentration is

greatly increased about 1000 times higher than a conventional PN junction diode thus

yielding a very thin depletion layer. This diode utilizes a phenomenon called tunneling

and hence the diode is referred as tunnel diode.

13. What is tunneling phenomenon?

The phenomenon of penetration of the charge carriers directly though the potential barrier

instead of climbing over it is called as tunneling.

14. What is Transistor?

Transistor is a three terminal device whose output current, voltage and /or power

is controlled by input current. Three terminals: emitter, base, collector

15. What is biasing?

Inorder to operate transistor in the desired region we have to apply external d.c.

voltages of correct polarity and magnitude to the two junctions of the transistor. This is

nothing but the biasing of the transistor.

16. When the transistor operates as a switch ?

Transistor operates as a switch when it is operated in the cutoff region or

saturation region.

17. Define stability factor.

Stability factor is defined as the rate of change of collector current with respect to

the rate of change of reverse saturation current

18.What do you meant by thermal runway?

Due to the self heating at the collector junction, the collector current rises. This

causes damage to the device. This phenomenon is called thermal runway.

19. Why is the transistor called a current controlled device?



DEPARTMENT OF EEE.

The output characteristics of the transistor depend on the input current. So the transistor

is called a current controlled device.

20. Define operating point(Q point).

The Q point or Quiescent point or operating point is the point where d.c.load line

intersects proper base current curve. The co-ordinates of Q point decide the zero values

of Ic and Vce in a common Emitter transistor.

21. Why do we choose q point at the center of the load line?

The operating point of a transistor is kept fixed usually at the center of the active

region in order that the input signal is well amplified. If the point is fixed in the saturation

region or the cut off region the positive and negative half cycle gets clipped off

respectively.

22. What is reverse saturation current?

The thermally generated charge carriers cross the junction & giving rise to the

current called reverse saturation current.

23.State the applications of Zener diode

1. To regulate the voltage against the variations in supply voltage or load

current

2. In linear wave shaping circuits

3. In non-linear wave shaping circuits like clipping & clamping circuits

24. What is Diffusion Current?

The movement of charge carriers from a region of higher concentration to that

of lower concentration of the same type of charge carriers in semiconductors is known as

diffusion current

25. What is Continuity equation?

The fundamental law governing the flow of charge is called continuity

equation. The equation is based on the fact “ Energy can neither be created nor

destroyed”.

26. Define Depletion region & space region.

The region containing the uncovered acceptor & donor ions, in the vicinity of

the junction is called depletion region. Since this region has immobile ions, which are

electrically charged, the depletion region is also known as space charge region.

27. What is Cut-in or threshold voltage?

It is the level below which the current is very small & at the cut-in voltage the

potential barrier is overcome and the current through the junction starts to increase

rapidly.

28. Define Potential Barrier(or)Barrier potential.



DEPARTMENT OF EEE.

The additional holes trying to diffuse to the N-region are repelled by the

uncovered positive charge of the donor ions. Similarly the electrons trying to diffuse into

the p-region are repelled by the uncovered negative charge of the acceptor ions. Thus the

barrier is set up near the junction, which prevents the further movement of charge

carriers. This is called as potential barrier or junction barrier.

29. What do you mean by d.c load line?

The load line represents the relationship between current and voltage in the linear

part of the circuit. The load line is drawn between the saturation point in the collector

current axis and the cutoff point in the collector voltage axis. If the input signal is

assumed to be zero,then the load line is called as d.c load line.

30. What are the current components in a diode?

On P side, I=Ipp(x) +Inp(x)

Where, Ipp(x)-current due to holes on p side.

Inp(x)-current due to electrons on p side.

On N side, I=Inn(x)+ Ipn (x)

Where, Inn(x)-current due to electrons on n side.

Ipn(x)-current due to holes on n side.

UNIT – 2

FET, UJT AND SCR

1. What is JFET? And What are the terminals and types in JFET?

JFET- Junction Field Effect Transistor. And the terminals are Gate, Drain and Source

and the types are N- Channel JFET and P- Channel JFET .

2. List out the biasing schemes available to achieve the required bias in a FET.

Voltage divider bias, Base bias, Emitter feedback bias, Collector feedback bias,

Emitter bias.

3.Mention the parameters of JFET.

i) a.c. drain resistance

ii) Transconductance

iii) Amplification factor

4.What is transconductance in JFET?

IC

(mA)

VCE

VCE(off)

= VCC

IC(sat)

= VCC

/(RC+R

E)

DC Load Line



DEPARTMENT OF EEE.

It is the ratio of small change in drain current to the corresponding change in drain

to source voltage.

5. What is amplification factor in JFET?

It is the ratio of small change in drain to source voltage to the corresponding change

in Gate to source voltage.

6. Why do we choose q point at the center of the load line?

The operating point of a transistor is kept fixed usually at the center of the active

region in order that the input signal is well amplified. If the point is fixed in the saturation

region or the cut off region the positive and negative half cycle gets clipped off

respectively.

7. Define MOSFET and what are all the terminals?

Metal oxide semiconductor field effect transistor. The terminals are gate, Drain

and source.

8. What are all the types of MOSFET?

i) Enhancement type

ii) Depletion type

9. Differentiate Enhancement and Depletion MOSFET.

Enhancement MOSFET Depletion MOSFET

1.Positive voltage at the gate

2.Inversion layer is made

3. Negative charges are formed

1. Negative voltage at the gate

2. Depletion of majority carriers happens

3.Positive charges are formed

10. Compare P channel with N channel MOSFET.

P – Channel N-Channel

1. Channel is made up of P-Type material

2. Can be operated both in enhancement

and depletion mode.

3. Drain current ID will be less

4. Transconductance (µ) will be less

5. ID is contributed due to holes

1. Channel is made up of N-type material

2. Can be operated both in enhancement

and depletion mode.

3. Drain current ID will be more

4. Comparatively more

5. ID is contributed due to electrons



DEPARTMENT OF EEE.

11. Mention few applications of UJT?

1.Phase control

2. Saw – tooth generators

3. Non-sinusoidal oscillators

4. Triggering device for SCR and DIAC.

12. Give some applications of DIAC.

1.To trigger TRIAC

2. Motor speed control

3. Heat control

4. Light dimmer circuits.

13. Give typical applications of TRIAC.

1.Heater control

2. Motor speed control

3. Phase control

4. Static switches.

14.Define holding current.

It is that value of current below which the SCR switches from the

conduction state (ON state) to forward blocking state.It is denoted as IH.

15. Define Latching current.

It is the minimum current flowing from anode to cathode when SCR goes from OFF to

ON state even after gate bias is removed. It is greater than, but very close to holding

current.

16. Define pinch off voltage.

The drain source voltage(VDS) at which the drain current (ID )reaches to its

constant saturation level is called as “pinch off voltage,VP”

VP = (q ND a2)/2є

17. Why thermal runaway not present in FET?

Thermal runaway does not exist in JFET, because drain resistance rd increases

with the temperature, which reduces ID.Thus with the reduction of ID the temperature of

the device is reduced.

18. Define intrinsic standoff ratio of UJT.



DEPARTMENT OF EEE.

It is defined as the ratio between the internal resistance of a base (RB1) to the

interbase resistance(RB1+RB2),with IE=0(i.e)emitter diode is not conducting.

Ƞ=RB1/(RB1+RB2) IE=0.

19. What is meant by unipolar device? Give an example.

In a device, if current is carried by only one type of charged particles ,i.e electrons

or holes. It is called as unipolar device. eg.FET.

20. What is TRIAC?

21. What is DIAC?

22. What is UJT?

UNIT – 3

AMPLIFIERS

1. How the transistors work as an amplifier?

The transistor can be work as an amplifier when it is operated in the active region. In

active region the base emitter junction is forwars biased and collector emitter junction

will be reverse biased.

2. What is an amplifier?

An amplifier is a device which produces a large electrical output of similar

Characteristics to that of the input parameters.

3. How are amplifiers classified according to the input?

1. Small – signal amplifier 2. Large – signal amplifier

4. How are amplifiers classified according to the transistor configuration?

1. Common emitter amplifier 2. Common base amplifier 3. Common collector

Amplifier.

5. What is the different analysis available to analyze a transistor?

1. AC analysis 2. DC analysis

6. What is the necessary of the coupling capacitor?

It is used to block the c signal to the transistor amplifier. It allows a c &blocks the d c.

7.Why bypass and coupling capacitor are used in amplifier circuits?

i. Bypass capacitor CE :

The capacitor connected in parallel with the emitter resistor RE is called



DEPARTMENT OF EEE.

as the emitter bypass capacitor. This capacitor offers a low reactance to the amplified ac

signal.Therefore the emitter resistor RE gets bypassed through CE for only the ac signals.

This will increase the voltage gain of the amplifier. Moreover as CE acts as an

open circuit for dc voltages, it does not bypass RE for dc conditions. Thus, presence of

CE does not alter the dc biasing conditions.

ii. Input coupling capacitor :

The input coupling capacitor is used for coupling the ac input voltage Vi to

the base of the transistor.As capacitors block dc, this capacitor helps to block any dc

component present in Vi and couples only the ac component of the input signal.

iii. Out put coupling capacitor :

This capacitor couples the amplifier out put to the load resistance or to

the next stage of the amplifier. It is used for blocking the dc part and

passing the ac part of the amplified signal to the load.

8. Define the term feedback of an amplifier.

It is the process of injecting some energy from the output and then returns it back

to the input. Amplifiers which uses feed back principle is called as feed back amplifiers.

9. What is meant by h-parameter?

In transistor analysis Input impedance and output impedance and forward current

transfer, fraction of output voltage at input are calculated. They are mixture of different

units and hence referred to as hybrid parameters(h-parameters).

10. Mention the benefits of h-parameters.

i) real numbers at audio frequency

ii) Easy to measure

iii) Obtained from the transistor static characteristic curves

iv) Convenient to use in circuit analysis.

11. Define class A amplifier and mention its types.

The power amplifier is said to be class B amplifier if the Q point and the input signal

are selected such that the output signal is obtained for a full input cycle For this the Q

point is approximately on the reference axis.



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For this the Q point is approximately at the midpoint of the load line. The class A

amplifier are further classified into series fed directly couple and Transformer coupled

amplifier.

12. Define class B amplifier.

The power amplifier is said to be class A amplifier if the Q point and the input signal

are selected such that the output signal is obtained only for one half cycle for a full input

cycle.

13. Define class C amplifier and mention its types.

The power amplifier is said to be class B amplifier if the Q point and the input

signal are selected such that the output signal is obtained for less than a half cycle, for a

full input cycle. For this the Q point is approximately on the reference axis.

14. Define class AB operation of a Power amplifier.

The power amplifier is said to be class AB amplifier if the Q point and the

input signal are selected such that the output signal is obtained for more than 180˚ but

less than 360˚ for a full input cycle. For this the Q point is above X axis but below the

midpoint of a load line.

15. Compare Class A, Class B, Class C and Class AB amplifier.

Class A B C AB

Operating cycle 360˚ 180˚ less than 180˚ 180˚ to 360˚

Position of Q

point

Centre of load

line

On X axis Below X axis Above X axis

but below the

centre of load

line

Efficiency Poor,25% to

50%

Better, 78.5% High Higher than A

but less than B

50 % to 78.5%

Distortion Absent PRESENT.

More than class

A

Highest Present



DEPARTMENT OF EEE.

16. What are the advantages and disadvantages of push-pull amplifier?

Advantages:

i) The efficiency is much higher than the class A operation

ii) When there is no input signal, the power dissipation is zero

iii) The even harmonics get cancelled.

iv) Due to transformer impedance matching is possible

Disadvantages:

i) Two center tap transformers are necessary

ii) The transformers make the circuit bulky and hence costlier.

iii) Frequency response is poor.

17. What are the advantages and disadvantages of Complementry symmentry class

B amplifier?

Advantages:

i) As the circuit is transformerless,its weight,size and cost are less

ii) Due to common collector configuration,impedance matching is possible.

iii) Frequency response improves.

Disadvantages:

i) the circuit needs two separate voltage supplies.

ii) The output is distorted to cross over distortion.

18. What is meant by Complementary symmetry Class B amplifier?

The push pull amplifier is constructed using two transistors of same type but the

Complementary symmetry Class B amplifier is making use of one n-p-n and one p-n-p

type transistor. The common collector configuration has lowest output impedance and

hence the impedance matching is possible.

19. What is cross over distortion?

The distortion in the output voltage waveform due to the cut-in voltage of the

transistor is called as cross-over distortion.

20. What is emitter follower?

The common collector amplifier circuit is called as emitter follower. Since, in the

common collector circuit, the emitter terminal follows the signal voltage applied to the

base.



DEPARTMENT OF EEE.

UNIT-IV

FEEDBACK AMPLIFERS AND OSCILLATORS

1) What is feed back?

To improve the performance of the amplifier, a part of the output signal is

sampled and fed back to the input of the amplifier that is called feedback.

2) What are the types of feed back?

i) Positive feedback

ii) Negative feedback

3) What is negative feedback?

If the input signal and part of the output signal is sampled and fed back to the

input are in out of phase then that is called positive feedback.which is used in amplifiers

4) What is positive feedback?

If the input signal and part of the output signal is sampled and fed back to the

input are in phase then that is called positive feedback.it is used in oscillators.

5) Classify the feedback amplifiers.

Based on the magnitudes of the input and output impedances of an amplifier

relative to the source and load impedances respectively. The amplifiers can be broadly

classified into four categories:

a. Voltage Amplifier

b.Current amplifier

c. Transconductance amplifier

d.Tran resistance amplifier

6) Compare the negative feedback on feedback amplifier.(OR)What are the

advantages and disadvantages of negative feedback.

Parameter Voltage

series

Current

series

Current

shunt

Voltage

shunt

Gain with

feedback

Decreases Decreases Decreases Decreases

Stability Improves Improves Improves Improves

Frequency Improves Improves Improves Improves



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response

Frequency

Distortion

Reduces Reduces Reduces Reduces

Noise and

Nonlinear

distortion

Reduces Reduces Reduces Reduces

Input

resistace

Increases Increases Increases Increases

Output

resistace

Decreases Increases Increases Decreases

7) Define Oscillators.

An oscillator is a circuit which basically acts as a generator, generating the output

signal which oscillates with constant amplitude and constant desired frequency.

8) State Barkhausen criterion(or)State the conditions for oscillation.

i) The total phase shift around a loop is 360˚or 0˚

ii) The magnitude of the product of the open loop gain of the amplifier (A) and

the feedback factor (ß) is unity.

9) Give the classification of oscillators.

*Based on the output waveform

i) Sinusoidal oscillator

ii) Non sinusoidal oscillator

* Based on the circuit components

i) RC oscillators

ii) LC oscillators

* Based on the range of output frequency

i) Audio frequency oscillator (frequency lies between 20 Hz to 200 KHz)

ii) Radio frequency oscillator (frequency lies in GHz)

10) Write the frequency of oscillation for RC phase shift oscillator.



DEPARTMENT OF EEE.

F = 1

______________

2∏ RC √ (4 K + 6)

11) Compare RC phase shift and wien bridge oscillator

S.No RC phase shift oscillator wien bridge oscillator

1. It is a phase shift oscillator used for

low frequency range.

It is a phase shift oscillator used for low

frequency range.

2. The feedback network is RC

network with three RC sections

The feedback network is lead lag

network which is called wien bridge

circuit.

3. The feedback network introduces

180˚ phase shift

The feedback network does not

introduces any phase shift

4. Op-amp is used in an inverting

mode

Op-amp is used in not inverting mode

5. Op-amp circuit introduces 180˚

phase shift

Op-amp circuit does not introduces any

phase shift

6.

7.

The frequency of oscillation is

F = 1

_________

2∏ RC √ 6

The frequency variation is difficult

The frequency of oscillation is

F = 1

____

2∏ RC

Mounting the two capacitors on common

shaft and varying their values, frequency

can be varied

12) What is meant by Tank circuit?



DEPARTMENT OF EEE.

The oscillators which use the elements L and C to produce the oscillations are

called LC oscillators. The circuit using elements L and C is called Tank circuit or

oscillatory circuit. This circuit is also called the resonating circuit.

13) Define Hartley oscillator.

A LC oscillator which uses two inductive reactance and one capacitive reactance

in its feedback network is called Hartley Oscillator.

14) Define Colpitts oscillator.

A LC oscillator which uses two capacitive reactance and one inductive reactance

in its feedback network is called Hartley Oscillator.

15) What are the main advantages of crystal oscillator?

The main advantages of crystal oscillator are frequency accuracy, stability

and low power consumption.

UNIT - V

PULSE CIRCUITS AND POWER SUPPLY

1.What do you mean by clipping/clipper/limiter circuits?

The clipper circuits are used to remove the certain portions of the

waveform,above or below the certain levels,as per the requirements.thus the circuits

which are used to clip off unwanted portions of the waveform without distoring the

remaining part of the waveform are called clipper circuits or clippers.

2. write the types of clipper.

i)series clipper

* Series positive clipper

* Series negative clipper

ii) Parallel clipper

* Parallel positive clipper

* Parallel Negative clipper

3.What do you mean by clamping circuits and mention its types?

The circuits which are used to add a dc level as per the requirements to the a.c.

output signal are called clamper circuits.

i) Negative clampers

ii) Positive clampers



DEPARTMENT OF EEE.

4. What do you mean by positive and negative clipper circuits?

The positive clipper is used to clip off the positive portion of the input across the

output. And the negative clipper is used to clip off the positive portion of the input across

the output.

5. What do you mean by Multivibrators and mention its types?

The multivibrators are used to produce the non – sinusoidal input signal.

Types:

* Astable multivibrators

* Monostable multivibrators

* Bistable multivibrators

6. Mention the application of schimitt trigger circuit.

An important application of schimitt trigger circuit is its use as an amplitude

comparator. It identifies the moment at which any arbitary waveform attains a particular

reference level(LTP & UTP).That Is Any Arbitrary Input (Unshaped) can be converted to

a square wave.

7. What is rectifier?

Rectifiers is a device which is used to convert Ac voltage to pulsating Dc.

8. What are all the types of rectifiers?

*Uncontrolled rectifier (making use of diodes)

Single phase half wave rectifier

Single phase full wave rectifier

Three phase half wave rectifier

Three phase full wave rectifier

* Controlled Rectifier (making use of Thyristors)

Single phase half wave rectifier

Single phase full wave rectifier

Three phase half wave rectifier

Three phase full wave rectifier

9. What is voltage regulator?

The voltage regulator circuits keeps the output voltage constant inspite of changes

in load current or input voltage.



DEPARTMENT OF EEE.

10. Mention the types of voltage regulators.

*Shunt voltage regulators

* Series voltage regulators

11. What is the need of Switched mode power supply (SMPS)?

To overcome the following demerits present in linear regulators the SMPS is

used.

The required input step down transformer is bulky and expensive

Due to low line frequency large values of filter capacitors are required.

The efficiency is very low

Input must be greater than he output voltage

As large is the difference between input and output voltage, more is the power

dissipation in the series pass transistor

For higher input voltages, efficiency decreases

The need for dual supply, is not economical and feasible to achieve with the

the help of linear regulators.

12.Define PIV.

When diode is reverse biased then full transformer secondary voltage is impressed

across it.Thus,peak value of inverse voltage to which the diode get subjected is called as

peak inverse voltage.

13.What is meant by monostable,bistable,astable multivibrator?

Bistable multivibrator-It has two stable states.The multivibrator can exist indefinitely in

either of the two stable states .It requires an external triggering pulse to change from one

state to another.

Monostable Multivibrator:It has one stable state and one quasi state.The multivibrator

remains in a stable state and when external triggering is applied,then multivibrator goes

to quasi state .After some time interval,the circuit automatically returns to normal state.

Astable Multivibrator-The astable multivibrator has both the states as the quasi stable

states .None of the state is stable.Due to this,the multivibrator automatically makes the

successive transition from one quasi stable state to other ,without any triggering pulse.

14.What is the need for filter circuit?



DEPARTMENT OF EEE.

The output of the rectifier is not pure dc ,it contains fluctuations or ripple ,which

is undesired.To minimize the ripple content in the output the filter circuit is used.These

circuits are connected between the filter and the load circuit.

15. Applications of Monostable,bistable,astable multivibrator.

Refer book.

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