[sdiy] Quick CV output buffer query

[Steve Lenham]

On 13/10/2016 12:17, Tom Wiltshire wrote:

> I'm planning a inverting op-amp mixer mixing some CVs in a module.
> This is then fed to the standard CV output like Olivier has done here
> (page 2):
>
> http://mutable-instruments.net/static/schematics/Yarns-v03.pdf
>
> If my mixer uses 100K 0.1% resistors, I can be sure what I get out is
> the same (but upside down) as what I put in. No problem.
>
> How do I arrange that with the output buffer? If I use 100K for the
> feedback resistor, and 1K for the output protection, do I need a 101K
> input resistor? How does that work?

No, you just need a 100K feedback resistor. Feedback works so as to 
regulate the point from which it is taken. More often than not, this 
point IS the output of an opamp, but it doesn't have to be. In this 
case, feedback is taken from the far side of the protection resistor and 
that resistor is therefore inside the feedback loop.

The feedback mechanism will work so as to make sure that you have the 
correct voltage at the node from which feedback is taken. If the load is 
drawing current then the output of the opamp will increase to compensate 
for the voltage drop across the protection resistor and keep that node 
at the correct voltage. This works right up until the point where so 
much current is being drawn that the opamp cannot deliver any higher 
voltage, and this is the point where the resistor is protecting the 
opamp by limiting the current and dissipating the excess power.

Note that the rolloff capacitor IS connected directly to the opamp 
output - this ensures stability with a capacitive load, which would 
otherwise combine with the protection resistor to introduce problematic 
phase shifts.

Hope that helps!

Cheers,

Steve L.
Benden Sound Technology