Input Impedance OpAmp Circuit

[panfilero]

Hello,

I'm trying to figure out what the input impedance to this op-amp circuit is. I have 2 ways I've been thinking about this, and I'm not sure which one is right, or if they're both wrong. I'm looking for the input impedance as seen by the AC voltage source, V1.




Way 1: It's the impedances of C1, C2, and R3 in series. Done

Way 2: Thinking about a virtual short, it's C1, C2, R3, and R2 in series. Done

Basically, I'm wondering if R2 plays a role in the input impedance (as defined above) in this circuit.

much thanks!

 

[crutschow]

The virtual ground appears at the op amp (-) input, which is due to the negative feedback through R4. It's a virtual ground not a virtual short. Thus the impedance at the (+) input (R1 in parallel with R2) has no effect on that.

So the correct answer is Way 1.

 

[audioguru]

R2 has nothing to do with the input. It forms a voltage divider with R1 to bias the (+) input of the opamp.
So Way 1 is correct.