sheldon
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ee484,
Quoting from "Striving for Small-Signal Stability", M. Tian, et al., Circuits and Devices Magazine, Jan. 2001, page 31-41
The return-loop model used in Middlebrook’s approach implicitly assumes that signals flow through the feedback loop unilaterally. This is a reasonable assumption for most low-frequency applications.
The issue is that at high frequencies the loading and the reverse return ratio may effect the results. If you look at section 3.4.1.4, Feedback Parameters of Real Circuits, in Ken's book, it discusses using four parameter characterization of blocks[z-parameter, g-parameter, ...] to isolate the contribution to the loop gain from the op-amp from the contribution to the loop gain due to loading, ... This approach correctly accounts for high frequency effects and should produce results consistent with the results from the approach implemented in the Spectre's stability analysis.
Best Regards,
Sheldon
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