Input Impedance of a MOSFET circuit

CoolDude420 · Nov 15, 2018

Homework Statement

Circuit schematic of a common-source MOSFET stage with input capacitors C1 and C2 in series from the input node Zin, a feedback path from the node between them to the gate of M1, a current source I1 from VDD to the drain, lambda = 0.

(From Razavi's Design of Analog CMOS Integrated Circuits - Chapter 6)

Homework Equations

The Attempt at a Solution

Not too sure about calculating the input impedance of this. Initially I thought that it would be the series combination of C1 and C2, since we are neglecting Cgs, so

$Equation for Z_in as a fraction: numerator is (1/(jwC1))(1/(jwC2)), denominator is 1/(jwC1) + 1/(jwC2).$

But after I actually did it on paper, I got something like this,

Handwritten small-signal derivation of input impedance for a MOSFET amplifier: a top schematic with C1, C2, a current source I1 and NMOS n1 with lambda = 0, then two equivalent circuits with Vx, Ix, V1, gmV1 and I1 - gmV1, ending with Zin = Vx/Ix.

Usually, I would try and use superposition for this but I don't see how that would work since Vx would dissapear. Honestly, I have no idea what to do now?

CoolDude420 · Nov 15, 2018

EDIT: I just realized my mistake. When calculating input/output resistances, all independent sources have to be zeroed just like in Thevenin's Theorem, so that current source there shouldn't be there in my small-signal model.
From that it's just one KCL and job done.

rude man · Nov 24, 2018

So what is your answer?

Input Impedance of a MOSFET circuit

Homework Statement

Homework Equations

The Attempt at a Solution

Discussion

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