[sdiy] How to add stability cap to voltage follower?

[jhaible]

> The circuit in question is a state variable filter and I want all the
> outputs to be non-inverting. I started out with a simple inverting
> amplifier, gain of -1, with Rin=100K, Rfeedback = 100K, and 1K
> hanging off the output. To this I added a 22 pF cap in parallel with
> Rfeedback. But now I realize that I need a non-inverting amp. OK, a
> voltage follower is simple enough to build. But I'm not sure how to
> hook up a stabilizing cap to that. I can put Rfeedback back in and
> add the cap in parallel with that as I had with the inverting
> amplifier. It doesn't seem to hurt anything, but is it doing good? Or
> should I not worry about adding the cap?

It's easier than you might think. (Not that compensation is easy at
all, but *that* part of compensation _is_ easy.)

If you have no resistor, you don't need a capacitor. (no resistor, no
pole from input capacitance to compensate ...)
If, for some reason, you need a resistor even for a unity gain noninverting
buffer (opamps like the LM318 require this, if memory serves,
or you might want some protection, of matching impedance to
keep offset current low etc.), the add the capacitor as you would
do it for a noninverting amplifier.
Basic rule: the opamp "doesn't know" if it's inverting or not.

JH.