[sdiy] buffer opamp configuration [was: 1/8th inch questions]

[jhaible]

> jhaible wrote:
>
> > > Unity gain (non-inverting) buffers can be problematic since opamps are
> > less stable at unity gain, especially when driving capacitive loads.
There
> > seem to be two common configurations to give stability:
> > > (a) Put a capacitor and resistor in series between the opamp inputs,
and a
> > larger resistor in series with the input signal
> > > (b) Put a capacitor and resistor in series between the opamp inputs,
and a
> > larger resistor in the feedback loop.
> > > Note that both these ways increase the AC noise gain, and decrease the
> > buffer bandwidth.
> >
> > Method (b) is normally used to compensate a pole that comes from the
opamp's
> > *input* capacitance. It's always recommended if you have large feedback
> > resistors.
> > I'm not sure if it really helps against capacitive loading on the
output.
>
> Hmm.  I'm pretty sure configuration (a) helps against capacitive loading
on the output (see http://www.national.com/ms/LB/LB-42.pdf ).
>
> But configuration (b) is better because if you use 2 resistors in the
feedback loop you can >take the output jack from the node between the
resistors, thus incorporating current >limiting, but maintaining an accurate
voltage.

This was nonsense from my side. I didn't read your (b) point carefully, and
didn't have
a picture in front of me either. I read "large resistor in the feedback
loop" and thought
about input pole compensation. My fault.


I've read RAP's app note now, and apparently he concentrates on _one_ method
to increase stability: increasing noise gain, with various circuits. But
this also
means more noise - sometimes more noise than necessary.
I normally use the other method, with resistor at the opamp output, and two
feedback paths, as described in previous mails in this thread.
(But that hint of mine on input cap compensation was totally misleading,
sorry.)

JH.