[sdiy] OTA question

[Ian Fritz]

Hello --

I'm working on a Tri core VCO based on a CA3080 OTA.  It's similar to the 
EN thru-zero VCO, which is up at Bernie's site.  Currently, its output is 
swinging 0 to 6 V.

When I drive the OTA bias current from a pot across the PS and a 22k series 
resistor it works fine.  But when I drive it with a linear V-I converter 
the waveform at low frequencies is distorted.  The distortion makes the 
waveform dome shaped, ie, it is always curved downward.  It appears that 
there must be some kind of leakage which increases with increasing output 
signal.  But where exactly is it and why would it only show with the active 
drive?

I've tried a couple of versions of the linear V-I converter, both of which 
are common and include a PNP transistor driving the control input pin from 
its collector.

Any thoughts?

   Ian