I am trying to understand the circuit in the figure here Circuit:

I hope I analyzed correctly its gain (for small AC signal). My small-AC-signal-circuit is attached in the bottom of this post). However, I don't understand its purpose. What is the advantage of concatenating two mosfets this way? i.e.

  1. Does it allow us to raise (or attenuate) the gain without effecting Rin and Rout?
  2. Does it allow us to reduce (or raise) VDD without effecting the resistances or currents or gain?
  3. Does it allow us to change the values of Rs1, Rs2?
  4. Also, I hope I get correct Rin and Rout. Can I say that Rin=(R1||R2) and Rout=(r_ds2||R2)?

I tried to run these scenarios in my head but I didnt manage to come with any satisfying explanation to any of the scenarios.


My small signal circuit

The gain


1 Answer 1


It raises the gain of the overall circuit as often one MOSFET can't increase the gain dramatically. Also taking a quick look at your drawing did you forget the source resistors? It'll separate the FETs from ground and alter your resistances. For example the vgs will be nonzero so the dependent current sources will have to be taken into consideration.

Google common source with source degeneration for worked out examples to find the gain.

EDIT: when I say the current source needs to be taken into consideration I mean the equation for output resistance will in some way include the transconductance.


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