# How to find thevenin resistance of a capacitor in an op amp circuit?

for the following circuit could someone explain how to find the thevenin resistance across the capacitor? Most answers use the 10K resistance in parallel as the thevenin resistance but I don't understand why. Why do they neglect all the other branches, resistors and op amp?

If you're doing a static (DC) analysis to find the operating point of the circuit, then the capacitor acts as an open circuit. So the dc feedback resistance is in fact 10 kOhms, the resistance of the resistor in parallel.

If you're analyzing the AC behavior of the circuit, you should consider the impedance of the capacitor

$$Z_C=\frac{1}{j\omega{}C}$$

But we don't call that the Thevenin equivalent resistance of the capacitor, we call it the impedance of the capacitor.

Why do they neglect all the other branches, resistors and op amp?

Because what you're trying to find is the resistance in the feedback path. Later you'll consider how the feedback resistor works in combination with the 20 kOhm resistor and the op-amp itself to determine the closed-loop gain of the circuit.

• Sorry the aim of the question was to actually find the step response voltage across the capacitor and not to find the gain. So that being said, why do they neglect all other branches, resistors and op amps? Dec 21 '15 at 1:18
• Probably because that's the subcircuit they're actually interested in. Without more context there's no way for me to read the mind of whoever wrote whatever you read. Dec 21 '15 at 1:21