GATE EC 2025 Set 1 — Question 43
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Analog Circuits → Current Mirrors & Diff Amps → Common & Differential Mode Gain
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Question
The identical MOSFETs and in the circuit given below are ideal and biased in the saturation region. and have a transconductance of 5 mS.
The input signals (in Volts) are:
The output signal (in Volts) is ________.
The input signals (in Volts) are:
The output signal (in Volts) is ________.
Correct answer
(D) 4 - 0.05 sin ω t
Solution
The given circuit is a differential amplifier. The input signals are and .
The drain resistor for is .
The DC voltage at the drain of (which is ) is .
Given and .
.
.The final answer is
1.Determine DC operating point for :
The total current through the common source resistor is 2 mA. Since and are identical and the circuit is symmetric, the DC current through each MOSFET () is .The drain resistor for is .
The DC voltage at the drain of (which is ) is .
2.Determine differential input voltage:
The differential input voltage .3.Determine small-signal output voltage :
For a differential amplifier, the small-signal voltage at the drain of due to a differential input is given by:Given and .
.
4.Combine DC and AC components:
The total output voltage is the sum of its DC and small-signal AC components:.The final answer is
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