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

[Seb Francis]

Hi René,

René Schmitz wrote:

> Hi Seb et al,
>
> >Now here's a point which I haven't quite worked out yet .. how to get the required current limiting / short circuit protection when using a unity gain buffer.  It seems to me that by using configuration (b) and connecting the output jack to the inverting input of the opamp, then the feedback resistor will act as a current limiter.  However, this sounds a little wrong to me (running an output directly from an input - static discharge could damage input, other reasons I can't think of?) .. probably it's better to use 2 resistors in the feedback loop and connect the output jack to the node between the resistors?  I'd welcome any comments on this ...
>
> What I do in this case is add a resistor in series with the output, take the feedback to the
> - input via another resistor at the far end of the limiting resistor, and add a capacitor directly
> from the opamp output to the - input. (100R series, 1k for feedback and a 330p cap.)

What you describe here is the configuration for stabilising a normal inverting opamp configuration.  Do you think it works also for a unity gain buffer?  It's quite different to what Bob Pease says in this document: http://www.national.com/ms/LB/LB-42.pdf  .. but then his configuration does not give any current limiting on the output.

Seb