AC analysis in hspice

Hello All,

I was wondering if there was a specific method of doing .AC analysis at a specific moment in time. I.e you are doing a transient analysis from

0 to 2us and you need to AC analysis at the 1us mark because that's when a tranistion occurs and you wish to know the stability at that point. I ask because my circuit has dynamic biasing points and thus the gain is different during transitions vs the steady state value but I can't find the change.

Usually when you do the AC analysis it's at time=0 and not helpful for me. Is it possible to do a .SAVE at the time I am interested and then do a LOAD and then start an AC analysis? How would hspice know to use the conditions specified in the LOAD file?

Variations in gain affects my phase margin which is causing ripples and I can't find out without doing an AC at that point.

Thanks again for any help

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Even if you can do this, I'd suggest testing the loop with a step response. Find a spot to inject a current step in the loop and watch the loop settle. Use current since a current source is infinite impedance and won't effect the time constants of the circuit. Use all 4 parameters on the .tran line, i.e force a time step.

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