Question

If the output impedance of the current source is Ri, what is the output impedance of the CS stage shown below, if channel length modulation is neglected?

(a) (1 + gm * (R1 || R2)) * Ri + (R1 || R2)

(b) {R1 * (R2 + ro)} || Ri

(c) R1 || R2

(d) 0

I got this question during an interview.

This question is from MOSFET Amplifier with CS Configuration in portion FET Amplifiers of Analog Circuits

Answer 1

The correct option is (a) (1 + gm * (R1 || R2)) * Ri + (R1 || R2)

For explanation: We calculate the output impedance by shorting the two voltage sources to ground. Thereafter, as we apply a simple step input at the output node, i.e. the collector node, we’ll find that the total impedance at connected to the drain of M1 is nothing but (1 + gm * (R1 || R2)) * Ri + (R1 || R2) where gm is the transconductance of M1, R1 || R2 is the total resistance connected at the drain and Ri is the total resistance connected at the source. The output impedance would’ve been R1 || R2 if the current source was absent.