[sdiy] Output buffer question

[Neil Johnson]

Hi Tom,

> I'm working on a glide circuit, and the first attempt used a simple
> voltage follower as a buffer. This didn't work that well, because
> the current limiting resistor messes things up (thumbs down).

Under what circumstances does it "mess up"?  If you only have a DVM on 
the output (i.e., practically zero load) then I would suggest the 
construction has messed up, so check wiring.

> The second attempt (thumbs up) puts the resistor inside the feedback
> loop. This works much better - in fact, it works. However, most of
> the buffers I've seen in this style use a feedback resistor, a
> stability capacitor, and are inverting. So it's had a few
> "modifications".
>
> http://www.electricdruid.net/ThumbsUp.GIF

You can do a non-inverting buffer in the same way, with a capacitor 
directly across the output-to-neg.input, then the main feedback path 
with a suitable resistor.

> What I'd like to know is whether there are any problems with this.
> If I can't modify the standard CV output buffer like this, I'd like
> to learn why not.

To understand what can go wrong you first need to draw out the complete 
circuit.  Your diagram is only half the story.  You need to add the 
load, especially the parasitic capacitance.  Only then will it be 
obvious *why* it can go wrong and oscillate wildly.

Cheers
Neil
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