Question

If channel length modulation is present in M1 but not in M2, what is the voltage gain at node X?

(a) (1/gm2 || ro2) / {1/gm2 + ( 1/gm1 || ro1)}

(b) (1/gm1 || ro1) / (1/gm2 + ro1)

(c) 1/gm1 / (1/gm2 +  1/gm1)

(d) 1/gm1 / {1/gm2 + (1/gm1 || ro1)}

The question was asked in exam.

The above asked question is from MOSFET Amplifier with CD Configuration in portion FET Amplifiers of Analog Circuits

Answer 1

The correct option is (a) (1/gm2 || ro2) / {1/gm2 + ( 1/gm1 || ro1)}

The best I can explain: The voltage gain at node X is due to M1 being used in the CD configuration. Since M1 offers channel length modulation but not M2, we note that the impedance looking into the source of M2 is 1/gm2 but for M1, it’s (1/gm2 || ro2). From the voltage gain of a follower stage, we conclude that the voltage gain is (1/gm2 || ro2) / {1/gm1 + ( 1/gm1 || ro1)}.