I am trying to understand how to calculate the gain when the input is AC voltage. I am familiar with ideal Op Amp calculations, figuring out the voltage will be the same at both input! terminals. But how does this change when the voltage is AC?!
Answer
A simple procedure when calculating AC versus DC gain is to consider all the capacitors open for the DC case and shorts for the AC case. Then you can look more closely and see what resistance are accross or in series with the capacitors and use the formula:
F = 1 / (2πRC)
to see at what frequency the high pass or low pass rolloff is at. When R is in Ohms, C in Farads, then F is in Hertz.
From the above, it should be pretty obvious that the DC gain is 10, and the AC gain at high frequencies is 1. Now, can you tell us what the rolloff frequency of this low pass filter is?
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