Please see attached image. I've become confused if the answer to this question, which uses Miller's Theorem is correct as it should not be a minus, rather it should be a plus. Please see my working out.
Electrical Engineering Stack Exchange is a question and answer site for electronics and electrical engineering professionals, students, and enthusiasts. It only takes a minute to sign up.Sign up to join this community
In general case the situations look like this
Now let as try to find a input resistance.
Rin = Vin/Iin
In = (Vin - Vout)/R = (Vin - A*Vin)/R = Vin * (1 - A)/R
Rin = Vin/Iin = R/(1 - A)
As you can see we have a "minus" sign. We get the "plus" sign only when our amplifier gain is negative (inverting amplify)
Rin = R/(1 - (-A)) = R/(1+|A|)