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What is the input impedance of the following circuit, if channel length modulation is present?

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What is the input impedance of the following circuit, if channel length modulation is present?

(a) gm || 1/ro

(b) gm * ro

(c) gm || 2/ro

(d) gm

I got this question in an interview for internship.

Origin of the question is MOSFET Amplifier with CG Configuration in chapter FET Amplifiers of Analog Circuits

1 Answer

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Right answer is (a) gm || 1/ro

To explain: The input impedance can be found by performing a small signal analysis at the input node i.e. source of the MOSFET. Due to channel length modulation, the effect of ro would make the input impedance gm || 1/ro.

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