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If g is the transconductance, r is the resistance due to channel length modulation and if M2 and M1 has channel length modulation but M3 doesn’t, what is the total resistance at the drain of M2?(a) {(1 + gr)* R + R} || 2R(b) {(1 + gr)* 3R + 2R} || R(c) {(1 + gr)* (R || 1/g || r) + (R || 1/g || r)} || R(d) InfiniteThe question was posed to me at a job interview.The doubt is from MOSFET Amplifier with CD Configuration topic in chapter FET Amplifiers of Analog Circuits

Answer»

Right choice is (c) {(1 + gr)* (R || 1/g || r) + (R || 1/g || r)} || R

Best explanation: The DRAIN of M2 offers a resistance of {(1 + gr)* (R || 1/g || r) + (R || 1/g || r) due to channel length modulation and the degeneration resistance R || 1/g || r connected to the drain of M2. This comes in parallel to R connected to the SOURCE of M2. Hence the OVERALL resistance becomes {(1 + gr)* (R || 1/g || r) + (R || 1/g || r).


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