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

[Seb Francis]

Thanks JH & René for the feedback - I'll update the summary later and re-post it in case someone wants a complete and corrected version.  Hell, I might even get round to putting it on a webserver one day ;-)

Regarding this point ...

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.

I wonder if a combination of the 2 would work - put a resistor in series with the input signal into the + input, and a resistor in the feedback loop.  There's something like this in FIGURE 5 of the above PDF .. it seems to imply that the value of the input and feedback resistor should be the same.  Probably this is more critical with opamps with larger input currents.

BTW, in my application the input voltage source is a charged capacitor (S&H).  Perhaps this also affects the choice of configuration - certainly any DC resistive path from input to ouput would be a bad thing and cause droop.

Seb