ISRO EC 2025 Q.74: solution and answer

Official answer: CSource: ISRO's final answer key of 15 May 2026

1. A Schmitt trigger is a comparator with positive feedback. It has two different switching thresholds: an upper threshold $V_{UT}$ and a lower threshold $V_{LT}$.

2. The output switches high only when the input rises above $V_{UT}$, and switches low only when the input falls below $V_{LT}$. The gap $V_H=V_{UT}-V_{LT}$ is the hysteresis.

3. For an input between $V_{LT}$ and $V_{UT}$ the output keeps its previous state, so it is not a function of the present input value alone. The output is therefore governed by the hysteresis in the threshold voltages, which also makes the circuit immune to noise near the switching point.

4. Option (a) is wrong: because of hysteresis, the same input value can give either output level depending on the earlier state.

5. Option (b) is wrong: a Schmitt trigger is an asynchronous, level-sensitive circuit with no clock.

6. Option (d) is wrong: fan-in is the number of inputs a gate has. It does not set the switching behaviour of a Schmitt trigger.

7. Hence the answer is option (c).