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) 0I got this question during an interview.This question is from MOSFET Amplifier with CS Configuration in portion FET Amplifiers of Analog Circuits

If the output impedance of the current source is Ri, what is the outpu

1.	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) 0I got this question during an interview.This question is from MOSFET Amplifier with CS Configuration in portion FET Amplifiers of Analog Circuits
Answer» 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.

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