GATE EC 2024 Set 1 — Question 17
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Analog Circuits → BJT & MOSFET Amplifiers → Small-Signal & Hybrid-Pi Model
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Question
In the circuit below, assume that the long channel NMOS transistor is biased in saturation. The small signal trans-conductance of the transistor is . Neglect body effect, channel length modulation and intrinsic device capacitances. The small signal input impedance is _______.
Correct answer
(A) [figure]
Solution
To find the input impedance , we use the small-signal model of the NMOS transistor in saturation.
This matches option (A).
1.Let be the gate voltage and be the source voltage. The drain is connected to ground.
2.The input current flows through the capacitor :
3.At the source node, applying KCL:
4.From step 2, we have . Substituting this into the KCL equation:
5.The gate voltage is:
6.The input impedance is:
This matches option (A).
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