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 pp 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 Only in semiconductros
semiconductors and metals
3. Jn = qn n E
Jn = q Dn dn / dx
Jp = q p p E
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 detail
5. Given two different germanium diodes with their data at forward has as follows
6. Explain the effect of temperature on a PN junction diode
7. 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 relation
9. Explain PN diode switching times
10. 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-A
1.
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 collectorbase 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 hparameters 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 configuration
4. Explain the characteristics of a transistor in common emitter configuration. Also explain the current
relations in common emitter configuration
5. 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 equation
7. Explain transistor switching times
8. Explain power transistor construction, characteristics and operation
9. Derive the analytical expression for transistor characteristics
10.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.
19.
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
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 example
2. Explain the construction and operation of N-channel Depletion type MOSFET with neat
diagrams
3. Explain the construction and operation of P-channel Depletion type MOSFET with neat
diagrams
4. 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 regions
6. Draw a circuit diagram for obtaining drain and transfer characteristics for a P-channel JFET
and explain that with neat diagrams and graphs
7. (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 Hie 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 : Ri
= 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
(i)
Series Feedback
In shunt feedback amplifiers, the
In series feedback amplifier,
the feedback signal is connected
(ii)
Shunt Feedback
in feedback signal is connected in
series with the input signal.
parallel with the input signal.
It increases the input resistance
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
1.
Series Feedback
Shunt Feedback
An amplifier is a circuit used to magnify An Oscillator is a circuit used to
the given input signal.
generate the signals. It has no input.
The gain of the oscillator is infinity.
2.
Amplifiers have finite gain.
In
an
oscillator,
the
frequency
waveform and magnitude of ac
3.
In an amplifier circuit, the frequency, power generated are controlled by
waveform and magnitude of ac power the circuit itself.
output are controlled by as signal voltage
applied at the input.
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 and
Vceq 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
Rb
Vo
Rc
Vin
Rs
Vcc
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?
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.
1
fo
=
0.69 (R1C1+R2C2)
1
fo
=
[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
1.
Series Feedback
Shunt Feedback
The collector output of transistor Q2 is The
not coupled to the base of transistor Q1
collector
output
of
each
transistor is coupled to the base of
Here input is a slowly varying AC other transistor.
2.
voltage.
Here input is a trigger signal or pulse
When the input signal level goes above waveform.
3.
the UTP, the output changes its stage. When a trigger pulse is applied, the
When the signal level goes below LTP or output changes from one stable state
the signal is completely removed, the to another stable state. The removal
output again changes its state.
of this trigger pulse will not alter the
output state.
28. Compare Astable, Monostable and Bistable multivibrators.
Sl.No
Criterion
1.
Type of coupling
Astable
Multivibrator
(AMV)
Both the
couplings are
capacitive
Stable stages
2.
It has no stable
Monostable
Multivibrator
(MMV)
One coupling the
capactive and the
other is resisttive
(DC)
It has one stable
Bistable
Multivibrator
(BMV)
Both the couplings
are resistive (DC)
It has two stable
states.
Trigger input
3.
states. Instead it
has two
quasistable states.
It does not require
a trigger input
pulse.
state (One stable
state and one quasistable 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 pulse
2. 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)