# Analog Circuits Questions and Answers – Cascade and Darlington Amplifier

This set of Analog Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Cascade and Darlington Amplifier”.

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

Explanation: 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 hfe=50.

Calculate the input resistance of the network.
a) 255 kΩ
b) 13 MΩ
c) 5 MΩ
d) 250 kΩ

Explanation: The load for the first transistor in the figure is the input resistance of the second.
RE1 = (1+hfe)5k = 255kΩ
Net input resistance, RI = (1+hfe)RE1=(1+hfe)25k = 13005k = 13MΩ.

3. Given the following circuit.

It is given that hfe=55, hie=1kΩ, hoe=25μΩ-1. Calculate the net current gain and the voltage gain of the network.
a) AI=192.6, Av=220
b) AI=1, AV=220
c) AI=192.6, AV=1
d) AI=192.6, AV=55

Explanation: AI=A1xA2
AI = [1+hfe/1+hoehfeRE]x[1+hfe].
AI = 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

Explanation: IB = 20 μA
IC = β.IB = 15000 x 20μ = 300 mA
IC1 = β1.IB = 100.20μ = 2mA
IC2 = 300 – 2 = 298mA.

5. Darlington amplifier is an emitter follower.
a) True
b) False

Explanation: 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

Explanation: 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 hfe = 50 and emitter resistance = 10kΩ.
a) 141 kΩ
b) 15 MΩ
c) 20 MΩ
d) 200 kΩ

Explanation: R’ = 220k||400k = 142 kΩ
RI = (1+hfe)2RE = 26MΩ
RI’ = 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

Explanation: 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 gm1 = 30mΩ-1 and gm2 = 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

Explanation: The above circuit is a cascade pair. For this circuit, the overall transconductance is
gm = α1gm2
gm = 1.1 gm2 = 55mΩ-1.

10. Find the transconductance of the network given below, provided that gm1 = 30mΩ-1. VT = 25mV, VBias > 4V.

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

a) RI=956Ω, RO=1.6 kΩ
b) RI=956 kΩ, RO=2 kΩ
c) RI=956 Ω, RO=2 kΩ
d) RI=900Ω, RO=10 kΩ

Explanation: RO = RC = 2kΩ
Input resistance = hie||50k||40k = 0.956 kΩ.

Sanfoundry Global Education & Learning Series – Analog Circuits.

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