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

[Seb Francis]

René Schmitz wrote:

> At 23:46 24.10.02 +0100, you wrote:
> >jhaible wrote:
> >
> >> 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.
> >
> >That's the most common method I've seen for driving capacitive loads - a cap and series resistors in parallel in the feedback loop to get a gain roll off at higher frequencies, with the output taken from the junction of the series resistors.  It would appear that this works for both inverting and non-inverting (gain<1) configurations, but *not* for unity-gain non-inverting - in this case a capacitor parallel with the feedback resistor would have no effect on the gain at higher frequencies .. it's always 1 (I think!).  Hence the alternative technique of increasing the noise gain to increase stability becomes necessary I guess.
>
> That isn't quite what was meant by two feedback paths. See the attachment.
>

That's what I described (but not very well).  Thing's are so much easier with pictures ;)  I wasn't thinking about the capacitive load properly - hence I was considering the series 2 resistors in the feedback loop as 1 resistor.

>
> >It seems unity-gain buffers are tricky, but I can't see any other way with a multi output MIDI2CV - I don't want to add trimmers for each output.
>
> You need to imagine the loading cap also, then you see how it works:
> The oscillation occurs because the finite open loop impedance together with a capacitive load forms a lowpass which shifts phase and consumes up the phase margin. (I.e. the feedback is taken at the output of this RC-combination.) For low frequencies we can neglect the effects of the cap, and see that the gain is really unity. For higher frequencies the second feedback path takes over. With the feedback taken directly at the opamp output the phase shift doesn't take place. The input structure doesn't play a role here.
>

Ok, I'm starting to understand (thanks to your help).  I've attached a pic of my own to make things easier (the cap on the front is just the S&H).  Also in my pic I have shown the load resistance, but I assume this is high enough to be ignored when thinking about oscillation.

So you are saying that oscillation occurs because at some frequency the phase of the feedback signal becomes enough out-of-phase to cause the feedback to be positive, not negative.  Without the 100R resistor, the phase shift is only due to the lowpass filter formed by combination of the internal output resistance of the opamp and the capacitive load, and is not too bad for medium-speed opamps where the gain rolls off at higher frequencies.  But adding the 100R resistor as short circuit protection changes the nature of the RC filter, causing the frequency at which the phase shift becomes a problem to fall - causing even a slower opamp to oscillate.  The 330pF cap in the inner feedback loop stops this oscillation because it provides a more direct feedback path for higher frequencies.

Sorry if I'm being slow to understand here .. all this stuff is very new to me.

Seb

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