[sdiy] feedback cap formula?

[TIm Daugard]

> what is the formula for the size of the cap  in the feedback loop of a
> typical inverting opamp situation ? is it 2*pi*Rf*Cf ?  is the cap's value
> gain-dependent? in this instance, i am just trying to limit a TL072 to the
> audio frequency range, say 20 or 40kHz or whatever.

I use a 51 pf ceramic cap with that IC. Measured at max gain and lower gain the
response roll off ranges from 20K Hz to 50K Hz. I would have to pull my notes
out to verify, but if I remember right the 20K Hz was at high gain and the
response opened up as you lowered the gain.

Your formula is good if you have a parallel resistor in the feedback loop. I was
going to use 50 pf, but Mouser had 51 pf at like 2 cents each.


Tim Daugard