vivkr
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Hi Sheldon,
Thanks for the tips. Let me try these different methods. Unfortunately, I cannot post the schematic. It is just a simple two-stage amplifier, where the compensation is at the output (output load >>>> any possible on-chip cap by about 4-5 orders of magnitude). The first stage is a simple diff. gain stage, the feedback path has an R-divider. I was experimenting with an additional level shifting stage between the first and the second stages.
There are several capacitive paths which might act as parallel paths at high-f, such as gate cap etc. So, I placed the probe where I was sure that there were no signal paths in parallel to it.
The only possible reason I see is that the supply is not ideal (source impedance of ~ 2 Ohms), but I would not have expected the kind of variation that I saw just because of this (~ 5 degree variation in PM).
But let me check the methods you describe.
Regards Vivek
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