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? enter image description here


1 Answer 1


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


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.

  • \$\begingroup\$ 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? \$\endgroup\$
    – user94942
    Dec 21, 2015 at 1:18
  • \$\begingroup\$ 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. \$\endgroup\$
    – The Photon
    Dec 21, 2015 at 1:21

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.