Can't figure out the purpose of this cap I always see in circuits.

[Keller]

Hi, I have a very basic question about a capacitor that shows up in many circuits. It seems like there is always a cap after the output of an op amp. It also seems like the cap. is polarized. Is this like the output capacitor you usually see on a circuit, like an IC output cap? Does it have to be polarized? What effect will changing the value have and what values are acceptable?  Hope some one can help... Thanks.

[snoof]

kinda hard to tell without a schematic, but it's prolly a coupling cap.  used to stop DC and control freq response.  doesn't have to be electro,  and changing values will alter freq response.

[petemoore]

  Great reads on this and...everything else is available at top left of DIYStompboxes screen.

[StickMan]

Generally, when running off a single pole power supply, the input signal to an op amp is biased with a DC signal 1/2 the battery voltage.  The resultant output signal will also oscillate around the same reference voltage. 

The large capacitor after the op amp will filter out the DC component of the output signal, and leave a signal that oscillates around 0V, which is what the remaining parts of the circuit usually expect.