scientificstick
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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
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