GATE EC 2018 Set 1 — Question 12
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Analog Circuits → Op-Amp Circuits → Differentiator & Integrator
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Question
In the circuit shown below, the op-amp is ideal and Zener voltage of the diode is 2.5 volts. At the input, unit step voltage is applied, i.e. volts. Also, at , the voltage across each of the capacitors is zero.
The time , in milliseconds, at which the output voltage crosses V is

Correct answer
(C) 7.5
Solution
1.The input current is .
2.This current flows through the first capacitor . The voltage at the intermediate node is .
3.The current also flows through the second capacitor until the Zener diode (in parallel with ) breaks down.
4.The voltage across is . Before breakdown, . Thus .
5.The Zener breaks down when , which occurs at . At this time, .
6.For , is clamped at . Therefore, .
7.To find when : .
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