[sdiy] Op Amp Buffer stability

[jhaible at debitel.net]

Most amplifiers have a range of capacitance where they are unstable /
osillating. So you can either stay below that critical capacitance, or go above
it. For instance, a specific unity gain opamp buffer may be stabe with 10pF on
its output and with 10uF, too - but not with 500pF.

The bigger caps help to keep impedance low at higher frequencies, which often
results in lower noise / better decoupling of what you've connected to that output.

All small signal stuff, so far.

Looking at large signal aspects: Big caps store a big amount of charge, which
may or may not find its way through your opamp *output* when the power of your
circuit is switched off, and the cap-buffered opamp output suddenly sees a
higher voltage that its supply pins. Diodes between output and supply pins will
help. But a lot of opamps probably can live with 10uF, without external diodes.
But then again, some might not.
Power-up isn't that critical, at least not as long as the opam has a current
limit function implemented.

JH.



> Hello,
> 
> I'm on a bit of a power supply stability mission at the moment.
> 
> I'm using an Op Amp buffer/voltage follower to supply VCC/2 (10K Resistor
> from VCC and 10K Resistor from Ground - both to Non Inverting Input).
> 
> I've noticed that putting the positive side of a 10uF electrolytic cap on the
> Inverting input and running the Negative side of the cap to Ground has
> drastically smoothed the output of the buffer.
> 
> Is there a reason why I *shouldn't* be doing this? I can't find an example of
> anyone using this on a buffer anywhere - so I'm a little cautious.
> 
> Oh - any other low component count, power supply stability tips would be most
> welcome.
> 
> FWIW VCC is 9 Volt.
> 
> Thanks all,
> 
> Justin
> 
> 
> 
> 
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