# Homework Help: Thevenin resistance in an opamp RC circuit

1. Jan 10, 2017

### Abdulwahab Hajar

1. The problem statement, all variables and given/known data
In the figure given what is the Thevenin resistance between the nodes of the capacitor??

2. Relevant equations
The Thevenin resistance is the open node resistance between two points.

3. The attempt at a solution
The way I see it, R2, Rf and R1 are in series and that combination is in parallel with R3...
However the book says that Rf, and R1 don't count because the output resistance of the op-amp is 0 since it's ideal....
Is that correct?? and what is output resistance to begin with?

Thank you

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2. Jan 10, 2017

### Staff: Mentor

Yes it's correct. The output resistance is the effective resistance that a load "sees" when driven by some circuit, typically an amplifier circuit. Circuit stages are typically characterized by input resistance and output resistance (or more generally, input impedance and output impedance).

At its output an ideal op-amp looks like an ideal controlled voltage source. As such it has zero internal resistance.

3. Jan 10, 2017

### Abdulwahab Hajar

Thank you sir noted and appreciated