[sdiy] voltage followers

[Richard Wentk]

At 12:43 04/11/2004 +0000, Thom Ash wrote:
>hi everyone!
>
>i was thinking of making a cv buffer for my modular, and i can't find a
>schematic to copy on the web, so here come the mundane newbie
>questions... ;)
>
>1. will the standard opamp voltage follower configuration [input signal
>to + input, output linked to - input] be appropriate here?

There are discrete variants, but a single op-amp solution is hard to beat. 
The pedantic may want to use an op-amp with a low offset voltage, but in 
practice you could probably get away with something horrible like a 741 
without too many problems. Key CV sources need to accurate. General 
modulation doesn't.

>2. would it be ok to use one opamp to drive, say, four output jacks, or
>should i have one per output?

Assuming 100k input impedance on each module, four should be no problem. 10 
might start to sag a little.

>3. how would i go about implementing an inverted output [like on the
>ASys RS230 module]?

A -1 gain inverting amp, followed by the same voltage follower. An 
inverting amp with high value (e.g. 1M) resistors is a fairly stiff source, 
so the voltage follower is really belt and braces. But it's good to have a 
circuits with identical properties, and this would match the output 
impedance to a non-inverting follower.

>i was thinking i could use two inverting opamps in
>series, with the feedback loop encompassing the whole thing so it still
>acts as a follower, and tap the inverted signal from in between the two,

That's a little overcomplicated, and I suspect it wouldn't work. :-)

One useful option for both voltage processors and input channels is to have 
a +/- controller, where you include both inverting and non-inverting 
options and fade between them using a pot. You get zero modulation in the 
centre, and + or - at the extremes.

Exrta points for making the pot voltage controlled. :-)

Richard