# Input resistance of an inverter op-amp circuit

I need to find the input resistance of this circuit. There are two parts of this exercise:

• The first one is to find the input resistance of the circuit without the capacitor.
• The second is to the find the input resistance of the circuit with the capacitor ( C = 10 nF.)

It is not mentioned if the op-amp is ideal or not.

Do i make it by finding the equivalent impedance of R3, R4, C and R5?

I need to find the input resistance of this circuit. Do I make it by finding the equivalent impedance of R3, R4, C and R5?

It's slightly simpler than that. Because the -Vin input is a virtual ground, you can regard it as a short-circuit to ground and hence, R5 can be ignored. This then makes the input impedance: -

$$Z_{IN}\hspace{1cm}=\hspace{1cm}R_3 + X_C||R_4$$

This answer assumes an ideal op-amp.

Can you take it from here?

• I didn't understand why the -Vin input is a virtual ground. isn't it supposed to have a voltage because of vs? Mar 21, 2023 at 11:21
• It's voltage will be the same as +Vin and.... because +Vin is connected to ground via R1 and R2, -Vin also will be at ground. If you haven't learnt about the term "virtual ground" then maybe you should look it up. I'll try and leave a link @Aleat Mar 21, 2023 at 11:28
• Mar 21, 2023 at 11:29
• Mar 21, 2023 at 11:35
• Do you understand my answer; did you follow the concept of the virtual ground? Apr 9, 2023 at 10:10