GATE 2018 EC – Question 12
In the circuit shown below, the op-amp is ideal and the Zener voltage of the diode is 2.5 volts. At the input, unit step voltage is applied, i.e. $v_{IN}(t)=u(t)$ volts. Also, at $t=0$, the voltage across each of the capacitors is zero.
The time $t$, in milliseconds, at which the output voltage $v_{OUT}$ crosses $-10$ V is
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Correct answer: (C) 7.5
Explanation
The op-amp acts as an integrator-type stage whose output is $v_{OUT}=-\frac{1}{RC}\int v_{IN}\,dt$, with the Zener diode limiting the voltage across one of the capacitors to 2.5 V. Working through the charging of the two capacitors, the output reaches $-10$ V at $t=7.5$ ms (the official key is option C).