thank you for your reply.
actually, i am reading an example in "johns & martin" analog design book. there, to calculate the
linear settling time and to determine the
unity gain frequency of the opamp, the circuit is considered as a feedback network.
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thats why i need to know the feedback factor.
i think the feedback factor (β) is C2. ie the only feedback element in the circuit is capacitor C2. but in the book the feedback factor is said to be C2/(C1+C2).
BTW it is welcome if you have an alternative approach to solve the problem:
the problem is: what is the open-loop unity gain frequency of opamp if a settling time of 0.1µs is required.