Problem with diff Op-Amp designed with CMFB

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Problem with this CMFB

Hi, All

I designed a full diff op amp with the simple CMFB, but I still can not make the output common mode level equal to supply/2. So, what is the trick for this problem? thanks
 

Re: Problem with this CMFB

Your cmfb will make your output node stable. In your simple CMFB, the output voltage will depend on you adjustment on the size of transistor. It may get the Vdd/2, but it needs time and patience.

A better way to get a certain output common voltage, you'd better use a feedback network to amplifer the difference between your current Voc and the Voc you need and then feedback to the gate of current mirror. Just as reference book tell us to do.

Then you will get a specific Voc voltage.
 

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