Stb analysis like Cadence Virtouso


Please add stb analysis for Loop stability analysis.


It might get added at some future time, since I know the mathematics behind Michael Tian’s admirable method, even though that method doesn’t give the closed loop behavior in general, only whether or not the linearized small signal circuit is stable. It’s considerably elegant in that it considers feedback in both directions in one fell swoop, but the stability criteria turns out to give the same answer as Middlebrook’s original 1975 method, if that method is applied in both directions.

But, without a doubt, the best stability analysis already there: A .tran analysis. Not everything is unstable because of a feedback loop and normal stability analysis/Nyquist stability criterion misses it. That’s just for a fully analog circuit. For general circuits with extensive digital logic, only a .tran can tell you whether or not your circuit oscillates.

The only general answer to whether a circuit is stable or not is whether it is stable or not, and only a full, non linear transient analysis can tell you that.


Hi Mike,
As you know, everybody wants a loop gain plot, as an engineer I may OK with transient analysis, but reviewers still needs a bode plot :slight_smile:

I used to do with LTspice, thats the reason I asked for it.

Have you tried the .bode directive in QSPICE yet?

The .bode directive in QSPICE calculates voltage loop gain, whereas the stb analysis in Cadence Spectre uses Tian’s Method. Both give similar results only under certain conditions.

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