[sdiy] VCO and expo converter question

[ryan]

hi,

I've been studying expo converters for about 2 or 3 weeks now because I wanted to adapt an active CV summing stage to René Schmitz's 4069 VCO. It also is going to use a PTC tempco resistor instead of the NTC resistors.  What I've come up with seems like it should work, atleast mathematically, but I'm not sure about a few components which I've seen on other expo converters.

here is the schematic for the 4069 VCO with a modified expo converter. Everything after the MAT-02 was copied from the original 4069 VCO
http://www.sdiy.org/destrukto/4069_VCO.png
this seems to be the common way of making an expo converter using the MAT-02 as the differential pair. The part I'm unsure about is the capacitor in the feedback part of the 2nd opamp (the opa2277). I picked a 100pF capacitor here although I have no idea what it should be, or how to calculate what it should be.  I've been told that it's purpose is to prevent the opamp from oscillation but I just put it there because everyone else does...

so, does anyone have any comments, suggestions, explanations about this capacitor? I've seen it in many of your designs although the value seems to range from 1pF to 680pF in different designs.  

one more question. I've chosen 100K pots for all the pots.because I'm unsure how to pick the values of pots in a voltage divider.  Is there some rule of thumb to follow when choosing pot values? It seems like the pot will affect the voltage seen at the input of the opamp so this could cause a problem?

thanks...
ryan
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