Question

What is the total output impedance at node x?

(a) R1 || ro1 || 2 * ro2 || ro2 || 2*ro4  … || ron

(b) R1 || 2 * ( ro1 || ro2 … || ron)

(c) R1 || ro1 || ro2 … || ron

(d) R1 + (ro1 || ro2 … || ron)

This question was posed to me in quiz.

My question is based upon MOSFET Amplifier with CG Configuration topic in portion FET Amplifiers of Analog Circuits

Answer 1

Right answer is (c) R1 || ro1 || ro2 … || ron

The best explanation: We note that all the MOSFET’s are in parallel. The output impedance of the i^th MOSFET is given by roi. Hence, the total impedance at node x is simply R1 || ro1 || ro2…|| ron. Note that R1 is parallel to each MOSFET. This is because, while dining the output impedance, we set Vin and Vcc to zero. Note that R1 is connected from node X to ground, all the MOSFET’s are connected form node x to ground.