Anybody solve this mosfet transconductance problem

[munna007]

anybody please solve this mosfet transconductance problem ...

hello friends...i can't understand about this question totally...can anybody help me regarding this...its answer is option (C).

.i have attached image for this question ,question no. 48...

thanks...

 

[uart]

The definition of "saturation mode" in a mosfet is that Id is independent of Vds and only dependant on (Vgs-VT). It follows that as long as the bias voltage at the gate of M2 is such that M1 remains in the saturation region then Id1 (and hence Id2) will only depend upon Vgs1. In other words, the characteristics from Vg1 to Id2 of the pair is essentually the same as that of M1 alone.

BTW. This is known as "Cascode" configuration and an amplifier stage constructed from such a pair actually corresponds to a common source - common gate multistage (common emitter - common base pair in the case of BJT's replacing the mosfets).

Since the gain of this pair is equal to that of just the CS stage (or CE in the BJT case) alone then you may wonder what is the purpose of introducing the second transistor. It turns out that the difference is at high frequency operation. Here the (AC) grounded gate of M2 breaks the capacitive feedback from output to input which can drastically improve the high frequency performance.