What are the input and output impedances of a voltage-series feedback amplifier?
Given:
Input impedance without feedback: Ri = 5 kΩ
Output impedance without feedback: Ro = 10 kΩ
Feedback factor: Aβ = 19

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Question

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RRB JE ECE 22 Apr 2025 Shift 1 CBT 2 Official Paper

Answer 1

Concept:

In a voltage-series feedback amplifier, the input impedance increases and the output impedance decreases due to the feedback network.

Input impedance with feedback is given by: \( R_{if} = R_i (1 + A\beta) \)

Output impedance with feedback is given by: \( R_{of} = \frac{R_o}{(1 + A\beta)} \)

Given:

\(R_i = 5~k\Omega, R_o = 10~k\Omega, A\beta = 19 \Rightarrow (1 + A\beta) = 20\)

Calculation:

Input impedance with feedback: \( R_{if} = 5 \times 20 = 100~k\Omega \)

Output impedance with feedback: \( R_{of} = \frac{10}{20} = 0.5~k\Omega \)

Conclusion:

The input impedance increases to 100 kΩ and the output impedance decreases to 0.5 kΩ due to voltage-series feedback.

Correct Answer: Option 4) 100 kΩ, 0.5 kΩ

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