Question about the size of a capacitor in an op amp circuit

[dougman0988]

Hello everyone!  This is probably an easy question to answer, but I have a typical op amp circuit shown here.  I've noticed that if the cap that's circled isn't large enough, the gain of the op amp is unaffected when the resistance in the feedback loop changes.  I came across this problem when I used a cap that was roughly in the 1nF range, and when I swapped it out for something in the 2.2uF range, the gain responded as usual.  What is it about the size of this cap that prevents gain?  I know that it blocks DC gain, but does it prevent AC gain as well under certain circumstances?  Thanks!

[PRR]

> blocks DC gain, but does it prevent AC gain

Where do you draw the line between "DC" and "AC"?

In hi-fi we think of it as the "lower bass limit". Gain should be held from high audio frequency down to below 20Hz, then allowed to fall to a lower level, ultimately to zero for DC.

And this bass-corner, where gain changes, is related to the C and the R in the circuit.

> roughly in the 1nF range, ... something in the 2.2uF range

But what is the R?

In this case, the R next to the C is the main factor.

Let us pretend you used 1K here.

With 1nFd cap, the corner is 16KHz. Your gain is affected above 16KHz, and that's the far end of audio (higher than anything you want to hear from a guitar). Anything lower is "DC", or getting toward the ultimate DC gain (unity for this plan). So most of the audio band is unity gain, whatever you do to the "gain".

With 2.2uFd the corner is 72Hz. Guitar starts at 82Hz so this affects the "whole guitar band". The bass player may notice that his lowest note (41Hz) is not much affected by "gain", though all his higher notes and overtones are. 

[BAARON]

Here's something that may help you.  Capacitors offer different impedance to different frequencies.  This calculator should give you an idea what those impedances are like, and you'll see why pairing a very small cap with various small resistors gives you very little change from resistor to resistor when they're in series.

http://www.electronics2000.co.uk/calc/reactance-calculator.php