Tlaloc
Community Member
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Posts: 81
Dallas, TX
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Realize that there are two Acm's; one through the input diff pair, which ideally is zero, and one through your CMFB circuit. The CMRR is Adm/Acm where Acm is the common mode gain through your diff pair, but that is not the gain that you get for your CMFB. That path generally controls the tail source or the loads, which is a different path through your input devices. Since your CMFB shares many of the same transistors as your diff path, it can be tricky to set a vastly different CMFB gain and phase response than the main amp. Also, because of the different path, the CMFB gain is unrelated to CMRR. The CMFB gain determines the common mode voltage error at the output, which normally is not critical.
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