what is the phase margin in this case?

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princerock

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Hi,

I designed a fully diff op amp with cmfb. And when I did the ac sweep to the cmfb network I got the following result. I am puzzled by the phase response. Is it oscillating? If not, what is the phase margin?

Thank you very much for your help!

princerock
 

It is oscillating. The phase margin = 180-(the changing range of phase till gain at 0dB).
 

why is this phase curve climbing so high? r there so many zeros in the transfer func?
 

From the gain curve it seems you have one zero and two pole before unit gain point. And your zero is in right half plane. So it's unstable.
 

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