Plotting G(t) of an oscillator in Cadence

Question

I have a cross-coupled NMOS-only oscillator that has its bias voltage changing so its transconductance is also changing over time. I want to plot its transconductance (G(t)=G0-gm/2) over time in Cadence. G0 is the tank loss and -gm/2 is the negative transconductance introduced by the cross-coupled NMOS transistors. How can this be done in Cadence? I tried to plot only gm of transistors using oppoint in transient analysis. but it's showing a plot that's always positive which is wrong. any comments would be appreciated.

Answer 1

The gm is a small signal parameter which is only calculated and available after a DC operating point simulation. Note that an AC analysis first does this DC operating point calculation as it needs that gm because an AC analysis uses the small signal model for the transistors.

Your transient simulation uses the large signal behavior of the NMOS so gm isn't even calculated as it is not needed.

As gm = dId/dVgs you could pick a point on the transient curves of Id and Vgs, clip the curves to you only get a small part and then determine the derivative.

However that will include some currents you might not want to see, for example the current through Cgd and Cdg which aren't really part of gm.