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What is the input impedance of the following C.S. stage?

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What is the input impedance of the following C.S. stage?

(a) R2 || R3

(b) R1 || (R2 + R3)

(c) (R1 || R2) + R3

(d) R1 + (R2 || R3)

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Question is taken from MOSFET Amplifier with CS Configuration in division FET Amplifiers of Analog Circuits

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The correct choice is (a) R2 || R3

To explain: The input impedance can be calculated by performing a small signal analysis at the input side ie the Gate of M1. We need to set Vcc and V1 to 0 and place a voltage source at the node after R1. Henceforth we find that R2 and R3 are simply connected from the same node to ground so they are parallel to each other. Note that by input impedance of the C.S. stage, we refer to the impedance seen by the signal after it crosses R1.

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