250+ TOP MCQs on Cascade and Darlington Amplifier and Answers

Analog Circuits Multiple Choice Questions on “Cascade and Darlington Amplifier”.

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1. Which of these are incorrect about Darlington amplifier?
A. It has a high input resistance
B. The output resistance is low
C. It has a unity voltage gain
D. It is a current buffer

Answer: D
Clarification: A Darlington amplifier has a very high input resistance, low output resistance, unity voltage gain and a high current gain. It is a voltage buffer, not a current buffer.

2. Consider the circuit shown below where h_fe=50.

Calculate the input resistance of the network.
A. 255 kΩ
B. 13 MΩ
C. 5 MΩ
D. 250 kΩ

Answer: B
Clarification: The load for the first transistor in the figure is the input resistance of the second.
R_E1 = (1+h_fe)5k = 255kΩ
Net input resistance, R_I = (1+h_fe)R_E1=(1+hfe)²5k = 13005k = 13MΩ.

3. Given the following circuit.

It is given that h_fe=55, h_ie=1kΩ, h_oe=25μΩ^-1. Calculate the net current gain and the voltage gain of the network.
A. A_I=192.6, A_v=220
B. A_I=1, A_V=220
C. A_I=192.6, A_V=1
D. A_I=192.6, A_V=55

Answer: C
Clarification: A_I=A1xA2
A_I = [1+h_fe/1+h_oeh_feR_E]x[1+h_fe].
A_I = 51×51/(1+25x50x10x10^-3) = 192.6.

4. In a Darlington pair, the overall β=15000.β1=100. Calculate the collector current for Q2 given base current for Q1 is 20 μA.

A. 300 mA
B. 298 mA
C. 2 mA
D. 200mA

Answer: B
Clarification: I_B = 20 μA
I_C = β.I_B = 15000 x 20μ = 300 mA
I_C1 = β1.I_B = 100.20μ = 2mA
I_C2 = 300 – 2 = 298mA.

5. Darlington amplifier is an emitter follower.
A. True
B. False

Answer: A
Clarification: Darlington pair is an emitter follower circuit, in which a darling pair is used in place of a single
transistor. It also provides a large β as per requirements.

6. What is the need for bootstrap biasing?
A. To prevent a decrease in the gain of network
B. To prevent an increase in the input resistance due to the biasing network
C. To prevent a decrease in the input resistance due to the presence of multiple BJT amplifiers
D. To prevent a decrease in the input resistance due to the biasing network

Answer: B
Clarification: A bootstrap biasing network is a special biasing circuit used in the Darlington amplifier to prevent the decrease in input resistance due to the biasing network being used. Capacitors and resistors are added to the circuit to prevent it from happening.

7. Consider a Darlington amplifier. In the self bias network, the biasing resistances are 220kΩ and 400 kΩ. What can be the correct value of input resistance if h_fe = 50 and emitter resistance = 10kΩ.
A. 141 kΩ
B. 15 MΩ
C. 20 MΩ
D. 200 kΩ

Answer: A
Clarification: R’ = 220k||400k = 142 kΩ
R_I = (1+h_fe)²R_E = 26MΩ
R_I’ = 26M||142k = 141.22 K.

8. What is a cascade amplifier?
A. A cascade of two CE amplifiers
B. A cascade of two CB amplifiers
C. A cascade of CE and CB amplifiers
D. A cascade of CB and CC amplifiers

Answer: C
Clarification: A cascade amplifier is a cascade network of CE and CB amplifiers, or CS and CG amplifiers.
It is used as a wide-band amplifier.

9. Consider the figure shown.

Given that g_m1 = 30mΩ^-1 and g_m2 = 50mΩ^-1, α₁ = 1.1, α₂ = 1.5 what is the transconductance of the entire network?
A. 80 mΩ^-1
B. 75 mΩ^-1
C. 33 mΩ^-1
D. 55 mΩ^-1

Answer: d
Clarification: The above circuit is a cascade pair. For this circuit, the overall transconductance is
g_m = α₁g_m2
g_m = 1.1 g_m2 = 55mΩ^-1.

10. Find the transconductance of the network given below, provided that g_m1 = 30mΩ^-1. V_T = 25mV, V_Bias > 4V.

A. 30mΩ^-1
B. 10mΩ^-1
C. 1mΩ^-1
D. 20mΩ^-1

Answer: A
Clarification: For a MOSFET cascode amplifier, the net transconductance in the above network shown is equal to the transconductance of MOSFET M1 that is equal to 30mΩ^-1.

11. In the given circuit, h_fe = 50 and h_ie = 1000Ω, find overall input and output resistance.

A. R_I=956Ω, R_O=1.6 kΩ
B. R_I=956 kΩ, R_O=2 kΩ
C. R_I=956 Ω, R_O=2 kΩ
D. R_I=900Ω, R_O=10 kΩ

Answer: C
Clarification: R_O = R_C = 2kΩ
Input resistance = h_ie||50k||40k = 0.956 kΩ.