The GATE Grind

GATE 2020 EC – Question 18

Analog Circuits · Op-amp Circuits · 1 mark · Multiple choice

The components in the circuit shown below are ideal. If the op-amp is in positive feedback and the input voltage $V_i$ is a sine wave of amplitude 1 V, the output voltage $V_o$ is

Diagram for GATE 2020 EC question 18
  1. a non-inverted sine wave of 2 V amplitude.
  2. an inverted sine wave of 1 V amplitude.
  3. a square wave of 5 V amplitude.
  4. a constant of either $+5$ V or $-5$ V.

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Correct answer: (D) a constant of either $+5$ V or $-5$ V.

Explanation

The non-inverting input sees $V_+=\frac{V_i+V_o}{2}$ and the inverting input is grounded, so the output switches only when $V_i=-V_o$. With $V_o=\pm5$ V the input would have to reach $\mp5$ V, but its amplitude is only 1 V. So the output never switches and stays at a constant $+5$ V or $-5$ V.