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What is the input impedance of the following CG stage?

(a) 1/gm

(b) 2* Rs || 1/gm

(c) Rs || 1/gm

(d) Rs || 2/gm

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This question is from MOSFET Amplifier with CG Configuration in chapter FET Amplifiers of Analog Circuits

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Correct answer is (c) Rs || 1/gm

For explanation: The input impedance can be calculated by performing a small signal analysis at the input node. We need to set Vg and Vdd to 0V and apply a small input voltage at the source. Now, the impedance looking into the source is 1/gm and Rs is parallel to this impedance. Hence, the total input impedance is Rs || 1/gm.

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