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

[jhaible]

> For a normal inverting opamp configuration, a small resistor (e.g.
100R->1K) should be >put betwen the opamp output and the output jack, with a
larger resistor (choose size to set >required gain) between the output jack
and inverting input.  Additionally put a small >capacitor between the opamp
output and inverting input for extra stability.

"extra stability": Actually, ringing from capacitive loading comes from the
internal
(open loop) output resistance of the opamp. Adding an external resistor
without the
capacitor can make the situation *worse*.  This might still be good enough
for
some applications (i.e. no big capacitive load), but without a carefully
chosen capacitor
the external resistor will just create the same additional pole as before at
even lower
capacitive loading.


> 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.

JH.