Tlaloc
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Posts: 81
Dallas, TX
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I believe you might have misunderstood what subgold was saying. When you have two cmfb circuits in this case, you do not use the same outputs to regulate both stages. You should use the outputs of the first stage to regulate the first stage and the outputs of the second stage to regulate the second stage. On nice thing about this is that you can have two CM levels for your two stages because in general, maximum signal swing is not reached when both the input and the output of an amp stage are at the same CM level. Doing it this way, you will not have any +ve feedback at DC. Of course, you have to check the stability at high freq.
Another way to do it is to use a CMFB circuit that has both a +ve tap and a -ve tap. That way, you drive the output stage with the +ve tap (since there is another inversion from either the tail or the load) and the input stage with the -ve tap (since there will be two inversions in that path). Most continuous time CMFB circuits give both taps.
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