Answer to Question #85274 in Electric Circuits for Anand

Question

A high frequency signal source with large output resistance is to be connected to the
input of an op-amp inverting amplifier. Which characteristics of the op-amp will be
significant in handling such signal? Justify your answer.

Answer 1

The amplifiers are created to amplify signals within the given bandwidth.

At high frequencies the capacitor' reactance decreases according to

$$X_C=\frac{1}{2\pi fC},$$

so it may seem that we should use higher $R_{in}$. But the signal source already has high resistance, so it will cover low value of $R_{in}$. Moreover, as the frequency increases, the closed-loop gain begins to drop, so to avoid it according to its definition,

$$G_{CL} \approx -\frac{R_f}{R_{in}},$$

take low $R_{in}$.

What about R feedback? It shouldn't be too high because at very high values (megaohms) the bandwidth drops off thus limiting the frequency response and making noise go up.