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GATE 2022 EC – Question 41

Analog Circuits · Op-amp Circuits · 2 marks · Multiple choice

For the following circuit with an ideal OPAMP, the difference between the maximum and the minimum values of the capacitor voltage ($V_C$) is ________.

Diagram for GATE 2022 EC question 41
  1. 15 V
  2. 27 V
  3. 13 V
  4. 14 V

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Show answer and explanation

Correct answer: (C) 13 V

Explanation

This is a relaxation oscillator whose output saturates at $+15$ V or $-12$ V, and $V_C$ swings between the two switching thresholds at the non-inverting input. The diodes (ideal, $0$ V drop when on) change the feedback divider. With $V_o=+15$ V the right diode is off and the left one on, so the divider is $R$ to ground and $2R$ to the output: $V_+=15\cdot\frac{R}{3R}=+5$ V. With $V_o=-12$ V the right diode is on and the left one off, so the divider is $2R$ to ground and $R$ to the output: $V_+=-12\cdot\frac{2R}{3R}=-8$ V. The capacitor voltage therefore swings between $-8$ V and $+5$ V, a difference of 13 V.