Analog Electronics Test - 2 - PDF Flipbook
Analog Electronics Test - 2
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Analog
Electronics
Test-02Solutions
ANALOG ELECTRONICS
1. The amplifier circuit shown below uses a composite transistor of
a MOSFET and BIPOLAR in cascade. All capacitance is large.
gm of the MOSFET = 2 mA/V, and ℎ of the BIPOLAR = 99.
The overall Transconductance gm of the composite Transistor is
a) 198 mA/V
b) 9.9 mA/V
c) 4.95 mA/V
d) 1.98 mA/V
Answer: (d)
Solution:
=
= � + 1�
1
= � + 1�
= �19090� MOSFET
= �19090� 2m
= 1.98 mA/V
2. Two silicon diodes, with a forward voltage drop of 0.7 V, are
used in the circuit shown in the figure. The range of input
voltage Vi for which the output voltage V0: Vi, is
a) – 0.3V < V < 1.3V
b) – 0.3V < Vi < 2V
c) – 1.0 V < Vi < 2.0 V
d) – 1 .7 V < Vi < 2.7 V
Answer: (d)
Solution:
D2 is FB when Vi > 0.7 + 2 = 2.7 V i.e.
D2 is RB when Vi > 2.7V
Similarly
D1 is FB when Vi > – 1 – 0.7 = –1.7 V
i.e. D2 is RB when Vi > 1.7 V
2
⸫V0 = Vin when both diodes are RB
i.e. – 1.7 < Vi < 2.7 V
V0 = Vin both diodes are RB
3. Which of the following components are chosen to construct a
d.c. power supply to supply 6 V DC voltage from 230 V a.c. to
operate a tape recorder?
1. Step down transformer
2. Diodes
3. Resistors and capacitors
4. Three-pin voltage stabilizer
Select the correct answer using the code given below:
a) 1, 2 and 3 only
b) 1 and 4 only
c) 3 and 4 only
d) 1, 2, 3 and 4
Answer: (d)
Solution:
Components are used as given below to construct a 6V DC from
230 V AC:
i) Step down transformer for high AC to low AC.
ii) Diode as rectifiers to convert AC to DC.
iii) Resistors and capacitors to filter the output.
iv) 3-pin voltage stabilizer to stabilize the signal.
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4. Assertion (A): In a transistor switching circuit, it is desirable
that the transistor should not be driven into hard saturation for
fast switching applications.
Reason (R): When a transistor is under saturation on state, both
its emitter-base and collector-base junctions remain under
forward bias.
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is NOT the correct explanation
of A
c) A is true but R is false
d) A is false but R is true
Answer: (b)
5. Why npn-transistors are preferred over pnp transistors?
a) Leakage current in npn-transistors is less than pnp-transistors
b) Mobility of majority carrier in npn-transistors is greater than
the mobility of majority carrier in pnp-transistors
c) Bias voltage required in npn is less than in pnp-transistors
d) Bias voltage required in npn is greater than in pnp-transistors
Answer: (b)
Solution:
Mobility of electron is higher than that of hole.
6. The action of a JFET in its equivalent circuit can be best
represented as a
a) current controlled current source
b) current controlled voltage source
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c) voltage controlled voltage source
d) voltage controlled current source
Answer: (d)
Solution:
The action of a JFET in its equivalent circuit can be represented
as “voltage controlled current source”.
According to the channel length modulation if voltage (VGS) is
applied at gate the channel length increases (or) decreases,
depending on the applied voltage. So that current (Id) will vary.
Hence JFET is a voltage controlled current source.
7. Thermal runaway in a transistor biased in the active region is
due to
1. heating of the transistor.
2. changes in β due to increase in temperature.
3. change in reverse collector saturation current due to rise in
temperature.
4. base emitter voltage VBE, which decreases with rise in
temperature.
Which of the above statements is/are correct?
a) 1 and 2
b) 2 and 3
c) 3 only
d) 4 only
Answer: (c)
5
Solution:
8. For the circuit shown below if gm = 3 × 10– 3 and RS = 3000 Ω,
then what is the value of R0?
a) 3000 Ω
b) 1000/3 Ω
c) 300 Ω
d) 100 Ω
Answer: (c)
Solution:
0 = 1 1 = 1 1 ⟹ 0 = 300 Ω
3000
+ 3×10−3 +
9. The noise figure of a lossy network at room temperature equals
the
a) Gain of the network
b) Attenuation factor of the network
c) Equivalent noise temperature of the network
d) input noise power
Answer: (b)
6
10. A small signal voltage amplifier in common emitter
configuration was working satisfactorily. Suddenly its emitter-
bypass capacitor (CE) got disconnected. Its:
1. Voltage gain will decrease
2. Voltage gain will increase
3. Bandwidth will decrease
4. Bandwidth will increase
Which of these statements are correct?
a) 1 and 4 only
b) 2 and 3 only
c) 3 and 4 only
d) 1, 2, 3 and 4
Answer: (a)
Solution:
If bypass capacitor is disconnected, then negative feedback will
come into picture and gain will be reduced. As gain bandwidth
product is constant so bandwidth will be increased.
11. The Bandwidth of an n-stage tuned Amplifier, with each stage
having a bandwidth of B, is given by
a) B/n
b) B/√n
c) B√21/n - 1
d) B/√21/n - 1
Answer: (c)
7
12. In a BJT amplifier with the introduction of feedback, the input
impedance is reduced, output impedance is increased, band
width is increased and distortion is reduced. The feedback is
a) Voltage series
b) Current series
c) Voltage shunt
d) Current shunt
Answer: (d)
Solution:
Feedback Input output
Topology impedance impedance
Voltage series increases decreases
Voltage shunt decreases decreases
Current series increases increases
Current shunt decreases increases
13. Assertion (A): Miller sweep circuit producing saw tooth
waveform is a relaxation oscillator.
Reason (R): The active device alternately supplies power to the
load and relaxes when it is cut off.
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is NOT the correct explanation
of A
c) A is true but R is false
d) A is false but R is true
Answer: (d)
8
14. Assertion (A): The frequency stability of an oscillator
improves as increases, where refers to the phase angle of
the loop gain.
Reason (R): For sustained oscillation to occur in an oscillator
circuit, the loop shift should be 0o or 2nπ where n is an integer.
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is NOT the correct explanation
of A
c) A is true but R is false
d) A is false but R is true
Answer: (b)
15. Choose the correct match for input resistance of various
Amplifier Configuration shown in below.
Configuration
CB: Common Base
CC: Common Collector
CE: Common Emitter
Input Resistance
LO: Low
MO: Moderate
HI: High
a) CB-LO, CC-MO, CE-HI
b) CB-HI, CC-LO, CE-MO
c) CB-LO, CC-HI, CE-MO
d) CB-MO, CC-HI, CE-LO
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Answer: (c)
Solution:
In common base input resistance is very low. In common
collector input resistance is high.
In common Emitter input resistance is moderate.
16. Assertion (A): An operational amplifier should have a low
input offset current.
Reason (R): Input impedance of op-amp should be very high.
a) Both A and R are true and R is the correct explanation of A
b) Both A and R are true but R is NOT the correct explanation
of A
c) A is true but R is false
d) A is false but R is true
Answer: (a)
17. Which one of the following causes phase shift through an Op-
Amp?
a) Internal RC circuits
b) External RC circuits
c) Gain roll off of the internal transistor
d) Negative feedback
Answer: (c)
18. Consider the following statements:
1. A differential amplifier is used at the input stage of an
operational amplifier.
2. Differential amplifiers have very high CMRR.
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Which of these statements are correct?
a) Both 1 and 2
b) Neither 1 nor 2
c) 1 only
d) 2 only
Answer: (a)
Solution:
Block diagram of op-amp is
• When the same voltage is applied at both the input terminals,
then the differential amplifier is said to be in common mode
configuration.
• The advantage of common-inputs is that noise at the inputs
gets cancelled.
CMRR = =
⟹ = ( 1 − 2)
=
1− 2
= ∞ at 1 = 2
⸫CMRR = ∞
Ideally CMRR should be infinite or practically very high.
11
19. If the emitter resistance in a common emitter voltage amplifier
is not bypassed, it will
a) reduce both the voltage gain and the input impedance.
b) reduce the voltage gain and increase the input impedance-
c) increase the voltage gain and reduce the input impedance.
d) increase both the voltage gains and the input impedance.
Answer: (b)
Solution:
Common emitter amplifier with emitter resistor unbiased
Proof:
AC picture of the amplifier:
= �−ℎ �
= ℎ + �1 + ℎ �
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Voltage gain,
= = ℎ −ℎ
+ �1+ℎ �
(1 + ℎ ) >> ℎ
⸫ = −ℎ ≃ − ……… (1)
ℎ + �1+ℎ �
Input resistance
= = ℎ + �1+ℎ �
Input Resistance = ℎ + �1 + ℎ � ………. (2)
From equation 1 and equation 2 it is understood that increasing
the emitter resistor RE will reduce the voltage gain and increase
the input resistance.
20. The unit of a thermal resistance of a semiconductor device is
a) Ohms
b) Ohms/0C
c) 0C/Ohm
d) 0C/Watt
Answer: (d)
21. Assertion (A): Many power devices utilize the case itself as
one of the principal current-carrying terminals.
Reason (R): Such a connection maximizes the thermal
resistance between the working semiconductors and the
transistor case.
a) Both A and R are true and R is the correct explanation of A
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b) Both A and R are true but R is NOT the correct explanation
of A
c) A is true but R is false
d) A is false but R is true
Answer: (c)
22. In the differential amplifier shown in the figure, the
magnitudes of the common mode and differential-mode gains
are , and Ad, respectively. If the resistance RE is increased,
then
a) increases.
b) common-mode rejection ratio increases.
c) Ad increases.
d) common-mode rejection ratio decreases.
Answer: (b)
Solution:
If RE increases in a differential amplifier CMRR increases as
decreases
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| | = � �
= � �− = 2
−
2
⸫More RE means less and more CMRR
23. For the reduced incidence matrix given below, which is the set
of branches forming a tree?
a) 1, 2, 3
b) 2, 4, 6
c) 2, 3, 5
d) 1, 4, 6
Answer: (a)
Solution:
Complete incidence matrix branch:
Nodes:
Node(4) can be found such that ∑( ℎ) =0
Graph:
15
A tree can be formed with branch 1, 2, 3 only.
24. In a transistor biased in the active region, thermal runaway is
due to
a) Base emitter voltage VBE which decreases with rise in
temperature.
b) Change in reverse collector saturation current due to rise in
temperature.
c) Heating of the transistor.
d) Changes in B which increases with temperature.
Answer: (b)
Solution:
When the temperature increases and use to operation, junction
temperature increases and thus reverse collector saturation
current increases which causes collector current to increase by
equation.
= + (1 + )
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25. Generally, the gain of a transistor Amplifier falls at high
frequency due to the
a) Internal capacitance of the device
b) Coupling Capacitor at the input
c) Skin Effect
d) Coupling Capacitor at the output
Answer: (a)
Solution:
The gain of a transistor amplifier falls at high frequencies due to
internal parasitic capacitance of the device.
26. Match List-I (Circuit Name) with List-II (Characteristics)
and select the correct answer using the codes given below the
lists:
List-I
A. Schmitt trigger
B. Monostable Multivibrator
C. A stable Multivibrator
D. Blocking oscillator
List-II
1. It needs a pulse transformer
2. It is used to generate gating pulse whose width can be
controlled
3. It is a bistable circuit
4. It has no stable state
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Codes:
A BC D
a) 3 2 4 1
b) 2 3 1 4
c) 3 2 1 4
d) 2 3 4 1
Answer: a
27. A 1 µs pulse can be converted into a 1 ms pulse by using
a) A monostable Multivibrator
b) An a stable Multivibrator
c) A bistable Multivibrator
d) A J – K flip-flop
Answer: (a)
Solution:
Monostable Multivibrator works as a pulse stretcher.
28. The advantage of using a function diode as temperature
stabilizing element in transistor circuit is that
a) its resistance temperature coefficient is negative
b) it is made of the same material as that of the transistor
c) its reverse current is low
d) its resistance changes with change in temperature
Answer: (b)
29. The first dominant pole encountered in the frequency response
of a compensated op amp is approximately at
a) 5Hz
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b) 10KHz
c) 1 MHz
d) 100 MHz
Answer: (a)
Solution:
The 5 Hz is near to the origin. So, it is a most dominant pole.
30. In an ideal differential amplifier shown in the figure, a large
value of (RE).
a) increases both the differential and common-mode gain.
b) increases the common-mode gain only.
c) decreases the differential-mode gain only.
d) decreases the common-mode gain only.
Answer: (d)
Solution:
The common both gain depends on RE and differential gain is
independent of RE. So, for large value of RE the common mode
gain decreases.
In differential Amplifier, a change in the value of the emitter
resistance (Re) affects the common mode gain AC.
i.e; ACM = −
2
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