[sdiy] Dual Transistor Expo-Converter Question

[Nils Pipenbrinck]

Hi folks,

I have a question about a little detail in dual transistor expo converters.

Often you'll see an opamp working as a servo to keep one collector at a 
fixed voltage. For example the OpAmp A4 in René Schmitz MS20 VCF schematic:

   http://www.schmitzbits.de/rs20.png

You'll see a capacitor from A4's output to the negative input terminal.

Q: What is the purpose of this capacitor and how should I dimension it? 
I've seen anything from nanofarads down to few picofarads used. What do 
I trade off when I make it larger or smaller?


Best,
   Nils