Only in the amplifer phase, the loop is close loop. then a feedback factor is meaning. other time , the feedback factor is no meaning due to open loop.
while the loop is close, open output node of feedback capacitor, then insert a voltage souce, then derive the loop gain G(s), after this, we know the forward gain of op is A(s) , then we can get the feedback factor is G(s)/A(s).
Based on the principle, the feedback factor is Cf/(Cf+Cs).
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