Homework Help: Non-Inverting Non-Ideal OpAmp Gain

1. Sep 15, 2010

jnava

The first part of this problem was to find the gain using an ideal OpAmp.
I am on the second half of this homework problem, where we find Vout/Vin, aka Gain, using a non ideal OpAmp. The attachment is the picture of the circuit.

I know that since the OpAmp is no longer ideal A no longer goes to infinity, which provides a small input on the negative terminal. So, this is what I have right now...but its not getting me to the correct answer....

Vout*(R2 / [R1 + R2]) = Vin + Vout/A

Is this how I would start to find the gain? Once i get Vout/Vin, I could take the lim as A goes to infinity to verify that, the gain equals the ideal OpAmp.
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

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2. Sep 17, 2010

gabbagabbahey

An Op-Amp multiples the difference between the voltage into the positive input and the voltage into the the negative input by its open-loop gain $A$. In other words,

$$V_{\text{out}}=A(V_{+}-V_{-})$$

What are $V_{\pm}$ for the circuit in your diagram?