GATE 2024 IN – Question 23
Consider the circuit shown. The CMOS digital logic has infinite input impedance and the opamp is ideal. A 1.8 V Zener diode requires a minimum Zener current of 2 mA. The maximum series resistance $R_Z$ is ______ kΩ (one decimal place).
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Correct answer: 1.6 to 1.6
Explanation
The comparator high level is 5 V and the Zener clamps the logic input at 1.8 V. Infinite CMOS impedance means all series-resistor current flows through the Zener.
Hence $R_Z\le(5-1.8)/(2\times10^{-3})=1600$ Ω. Maximum resistance: **1.6 kΩ**.