Feedback Amplifier MCQ

1. Applications of negative feedback to a certain amplifier reduced its gain from 200 to 100. If the gain with the same feedback is to be raised to 150, in the case of another such appliance, the gain of the amplifier without feedback must have been

  1. 400
  2. 450
  3. 500
  4. 600
Answer. d

2. Consider the following circuits:

  1. Oscillator
  2. Emitter follower
  3. Power amplifier

Which of the above circuits employ feedback?

  1. i and ii only
  2. ii and iii only
  3. i and iii only
  4. i, ii and iii
Answer. a

3. In an amplifier with a gin of 1000 without feedback and cut-off frequencies at 2 kHz and 20 kHz, negative feedback of 1% is employed. The cut-off frequencies with feedback would be

  1. 220 Hz and 22 kHz
  2. 182 Hz and 220 kHz
  3. 220 Hz and 220 kHz
  4. 182 Hz and 22 kHz
Answer. b

4. An amplifier circuit has an overall current gain of -100 and an input resistance of 10 kΩ with a load resistance of 1 kΩ. The overall voltage gain of the amplifier is

  1. 5 dB
  2. 10 dB
  3. 20 dB
  4. 40 dB
Answer. c

5. The effect of current shunt feedback in an amplifier is to

  1. increase the input resistance and decrease the output resistance
  2. increase both input and output resistance
  3. decrease both input and output resistance
  4. decrease the input resistance and increase the output resistance
Answer. d

6. In any function where microphone, amplifier and speakers are used often one would hear a humming sound, which increases in volume gradually. This is due to

  1. positive feedback between micro-phone and speaker
  2. negative feedback between micro-phone and speaker
  3. inadequate frequency response of amplifier
  4. noise pickup (50Hz) from power supply
Answer. a

7. A signal Vmsin(ωt+Φ) is applied to an amplifier whose gain A is independent of frequency. The amplifier will preserve the form of the input signal (though with a delay) if the phase shift Φ is

  1. constant
  2. inversely proportional to the frequency
  3. proportional to frequency
  4. proportional to the square of the frequency
Answer. c

8. A feedback amplifier is designed with an amplifier gain of -1000 and feedback of β = -0.1. If the amplifier has a gain change of 20% due to temperature, the change in gain of the feedback amplifier is

  1. 10%
  2. 5%
  3. 0.2%
  4. 0.01%
Answer. c

9. Three identical amplifiers, each having a gain of (Ao/2)∠60° are connected in cascade. The positive feedback loop has a gain of 0.008. The value of Ao that will render the cascaded system oscillatory is

  1. 10
  2. -10
  3. 250/3
  4. 250
Answer. b

10. For a transconductance amplifier, input and output resistances are respectively

  1. ∞ and 0
  2. 0 and ∞
  3. 0 and 0
  4. ∞ and ∞
Answer. d

11. An amplifier without feedback, when fed with a 1V, 50 Hz input signal gives an output of 30V, 50 Hz with a 5% 2nd order distortion. When 10% of the output is feedback, what is the 2nd order distortion?

  1. 0.375 V
  2. 1.3 V
  3. 0.75 V
  4. 3 V
Answer. a

12. Negative feedback in an amplifier leads to which one of the following?

  1. decrease in bandwidth
  2. increase in current gain
  3. increase in voltage gain
  4. decrease in voltage gain
Answer. d

13. The open-loop voltage gain of an amplifier is 240. The noise level in the output without feedback is 100 mV. If negative feedback with β = 1/60 is used, the noise level in the output will be

  1. 1.66 mV
  2. 2.4 mV
  3. 4.0 mV
  4. 20 mV
Answer. d

14. In a voltage-series feedback amplifier with open-loop gain Av and the feedback factor β, the input resistance becomes

  1. \frac{R_i}{1+\beta A_v}
  2. R_i \sqrt{1+\beta A_v}
  3. R_i (1+\beta A_v)
  4. \frac{R_i}{\sqrt{1+\beta A_v}}
Answer. c

15. A negative feedback amplifier with open-loop gain \frac{-A_o}{1+j\frac{\omega}{\omega_o}}

Ao > 0 and feedback factor β (>0) will have a 3 dB cut-off at what frequency?

  1. ωoAoβ
  2. ωo(1+Aoβ)
  3. ωo/(1+Aoβ)
  4. ωo(1-Aoβ)
Answer. b
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