Filter Impedance – How to Compute Input Impedance


I use a Sallen-Key filter for a project at university and I need to know its input impedance. Is there a way to compute it theoretically ?

Here is my circuit:

Sallen-Key filter

Best Answer

Yes, this is a standard circuit analysis problem.

Perform the analysis in the frequency domain (R and Xc) and connect a 1A ac current source at the input. Solve for the input voltage as a function of frequency and that expression is the impedance.

I suggest using nodal analysis to perform the analysis.

Assume that the op amp is ideal and so the current into the +/- terminals is zero and the voltage at these terminals are equal.

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