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Measuring transconductance of any circuit

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promach

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May I know if the following transconductance measurement test circuit is correct ?

I suspect that I need to use some derivative function because changing the mosfets width by a large factor does not change the gm result much at all.

However, my test circuit is simulated in AC, not DC, therefore I really confuse if I should use derivative function or not ?

Note: X1 is a CMOS inverter

YumV7ax.png
 

Your transconductance is not going to be linear, a
peak at inverter logic threshold will be found and
gm should be zero outside of the VILmax - VIHmin
range. You would be better off sweeping VIN in DC,
and take gm as its definition of dI/dV. Your Vout also
affects gm and you should step through a few Vout
points to observe this. The load should be a voltage
source for a DC analysis that wants to observe gm
relation to Vout, without a bunch of post-sim
de-embedding of whatever the complex load was up to.
 

You would be better off sweeping VIN in DC, and take gm as its definition of dI/dV.

I will definitely try DC analysis and take derivatives.

The following is self-biasing for cmos inverter input such that both mosfets are in saturation mode. Please comment about this.

B4DW3gC.png
 

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