I have a small question.  

Imagine a 2-stage op-amp, with a miller compensation cap between the first and second (output) stage.

Everybody knows how to check the loop gain.  In Cadence, with an stb analysis, you simply add an iprobe (to break the AC loop) and plot the loop-gain and phase margin.  When I see this done in books, typically only the external loop (op-amp output to op-amp input) is broken.  But the miller cap and the second stage makes an internal feedback loop as well.  Why don't we check this?  For example, why not break the loop at the output of the second stage BEFORE the miller capacitor connection?

I tried this, but then I measured much lower phase margins than when I took the conventional approach of only breaking the external loop.  Intuitively, I think this is because the compensation capacitor doesn't see the Miller amplification in my case.  Correct? But how much PM is sufficient for this case?  After all, I understand EVERY loop must be stable, no?

So my question is - why isn't it common to break both loops?  Especially for "indirect compensation", such as described in Baker, where the compensation capacitor goes to the cascode source, the compensation loop could also have more than 1 pole.

Any insight would be useful.  Thanks

Thank you very much for your thoughtful reply.

Yes I agree with what you said.  For a simple miller compensation, there is little chance of instability around the internal loop itself.  After all, it doesn't have enough poles.

My question is - if I break the loop at the output of the second stage (before the miller cap connection), then I am breaking both loops simultaneously.  Perhaps this is what you meant by "intersection".  In the stability analysis I see a drastically reduced phase margin.  How can I interpret the meaning of this?

For example, if I compensate the op-amp like normal (breaking on the external loop), I might design it for a PM of 75 degrees.  But then, if I break it as I suggested above, that same op-amp might show only 25 degrees - which makes me nervous.  How can I explain it.

Thanks again for your reply.

this has been discussed here many times before, suggest you search the site.