# Input Impedance OpAmp Circuit

Discussion in 'General Electronics Chat' started by panfilero, Oct 13, 2010.

1. ### panfilero Thread Starter Member

Mar 12, 2009
13
0
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!

2. ### Audioguru Expert

Dec 20, 2007
11,226
1,333
I answered your question a few minutes ago. Maybe on another website.
It is like an echo, echo, echo, .....

Why do you think that R2 is in series with the input parts??

3. ### panfilero Thread Starter Member

Mar 12, 2009
13
0
Yes you did, thanks! It was on another website

I thought it might be in series because of what I was thinking was a "virtual short" but as I was corrected on the other website, it's a virtual ground not a virtual short.

I was seeing the two inputs at the same voltage potential... and taking them to be electrically the same point. This is where I was getting mixed up.

4. ### Audioguru Expert

Dec 20, 2007
11,226
1,333
The negative feedback makes a DC gain of 1. Which means that the DC output voltage is the same voltage as the (+) input voltage.

If there is a dual polarity supply then the (+) input is biased at 0V so that the output can swing symmetrically up and down equally.
if the supply has a single polarity then the (+) input is usually biased at half the supply voltage so that the output can swing symmetrically up and down equally.

5. ### John P AAC Fanatic!

Oct 14, 2008
1,748
260
I say it's C1, C2 and R3 in series. The op amp simulates a low-impedance node at its negative input terminal, so none of the other components count. At least, that's true as long as the amp remains in its linear region (not exceeding power supply voltage or frequency limits).

6. ### panfilero Thread Starter Member

Mar 12, 2009
13
0
Thanks for the responses, I have a follow up question

why is it that the feedback resistor and the load resistor don't play a role in the input impedance?

7. ### Audioguru Expert

Dec 20, 2007
11,226
1,333
Because the impedance at the (-) input pin of the inverting opamp is zero ohms. Anything that is in parallel with zero ohms still makes zero ohms.