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GATE 2022 EE – Question 36

Analog and Digital Electronics · Operational amplifiers: characteristics and applications · 2 marks · Multiple choice

The output impedance of a non-ideal operational amplifier is denoted by $Z_{out}$. The variation in the magnitude of $Z_{out}$ with increasing frequency, $f$, in the circuit shown below, is best represented by

Diagram for GATE 2022 EE question 36
  1. Plot (A)
  2. Plot (B)
  3. Plot (C)
  4. Plot (D)

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Show answer and explanation

Correct answer: (C) Plot (C)

Explanation

In the voltage follower the closed-loop output impedance is $\frac{Z_o}{1+A(s)}$. At low frequency the open-loop gain $A$ is large, so the impedance is very small. As $f$ increases $A(s)$ falls, so $|Z_{out}|$ rises, and once $A$ has dropped to about 1 it flattens at the open-loop output impedance $Z_o$. This low, rising, then flat shape is plot (C).