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Actually, there is a way to simulate the CMFB circuit for determining LOOP-GAIN using a simple SPICE AC analysis (no need for a SpectreRF PAC).
P.J. Hurst, S.H. Lewis, "Determination of stability using return ratios in balanced fully differential feedback circuits", Circuits and Systems II: Analog and Digital Signal Processing, IEEE Transactions on [see also Circuits and Systems II: Express Briefs, IEEE Transactions on] Volume 42, Issue 12, Dec. 1995 Page(s):805 - 817
However, note that this is an IDEALIZED simulation and you need to model the cap loadings correctly to getter a better estimate of your stability.
I do NOT use this to check stability of my CMFB, but use it for defining a DC operating point for my Op Amp for AC sweep analysis. I prefer using transient simulations for testing stability of my CMFB. Anyway, finally a transient simulation is the true check for stability.
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