# Electronic Devices and Circuits Questions and Answers – Circuit Models for Amplifier

This set of Electronic Devices and Circuits Multiple Choice Questions & Answers (MCQs) focuses on “Circuit Models for Amplifier”.

1. Buffer amplifier needs to have
a) Low input resistance and low output resistance
b) High Input resistance and high output resistance
c) Low input resistance and high output resistance
d) High input resistance and low output resistance

Explanation: Buffer amplifiers are used to connect high input resistance source to a low output resistance load.

2. The ideal values for the input resistance (Ri) and the output resistance (Ro) of a transconductance amplifier are
a) Ri = 0 and Ro = 0
b) Ri = ∞ and R0 = ∞
c) Ri = 0 and R0 = ∞
d) Ri = ∞ and Ri = 0

Explanation: It is a desired characteristics of transconductance amplifier ideally.

3. An amplifier has a voltage gain of 40db. The value of AVO is
a) 10
b) 100
c) 20
d) 200

Explanation: The expression is given by 10 log AVO = 40. Solving for Avo gives 100 as the answer.

4. The output voltage of a voltage amplifier has been found to decrease by 20% when a load resistance of 1 kΩ is connected. What is the value of the amplifier output resistance?
a) 50Ω
b) 200Ω
c) 250Ω
d) 350Ω

Explanation: 250 / (1000 + 250) X 100% = 20%. Hence the output resistance is 250 ohm.

5. Which of the following is a transresistance amplifier?
a)
b)
c)
d)

Explanation: Figure d is the correct representation rest are voltage, current, transconductance amplifiers.

6. The signal to be amplified is current signal and the output desired is a voltage signal. Which of the following amplifier can perform this task?
a) Voltage amplifier
b) Current amplifier
c) Transconductance amplifier
d) Transresistance amplifier

Explanation: It is a characteristic of transconductance amplifier.

7. You are given two amplifiers, A and B, to connect in cascade between a 10-mV, 100-kΩ source (S) and a 100-Ω load (L). The amplifiers have voltage gain, input resistance, and output resistance as follows: for A, 100 V/V, 10 kΩ, 10 kΩ, respectively; for B, 1 V/V, 100 kΩ, 100 Ω, respectively. Your problem is to decide how the amplifiers should be connected so that the voltage gain is maximum.
a) SABL
b) SBAL
c) Both have the same voltage gain
d) None as neither combination is able to amplify the input signal

Explanation: None

8. A transconductance amplifier with Ri = 2 kΩ, Gm = 40 mA/V, and Ro = 20 kΩ is fed with a voltage source having a source resistance of 2 kΩ and is loaded with a 1-kΩ resistance. Find the voltage gain realized.
a) 18.05 V/V
b) 19.05 V/V
c) 20.05 V/V
d) 21.05V/V

Explanation:

9. The ratio of the short circuit current gain of a current amplifier (Ai) to the open circuit voltage gain of a voltage amplifier (AV), given that both amplifiers have the same value of the input resistance (Ri) and output resistance (R0), is
a) Ri
b) Ro
c) Ri / R0
d) Ro / Ri

Explanation: It is a standard mathematical relation.

10. The ratio of the open circuit voltage of a voltage amplifier (AV) to the short circuit transconductance of a (Gm) of a transconductance amplifier, given that both have the same value of the internal resistance (Ri) and the output resistance (R0), is
a) Ri
b) R0
c) 1/Ri
d) 1/R0

Explanation: It is a standard mathematical relation.

Sanfoundry Global Education & Learning Series – Electronic Devices and Circuits.

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