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

[René Schmitz]

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.

>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. 

Cheers,
 René
-------------- next part --------------
A non-text attachment was scrubbed...
Name: unibuf.gif
Type: image/gif
Size: 5230 bytes
Desc: not available
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20021025/63fd8e34/attachment.gif>
-------------- next part --------------

--=20
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159

=20