[sdiy] voltage controlled slew with transistors

[Chris McDowell]

Howdy list, 

I'm working on an envelope design, and currently using an LM13700 for the "make ramps" part, but wondering if anyone knows of some resources or reasonable strategies for replacing that with a few transistors? I've been playing in LTSpice trying to see how a discrete OTA arrangement can do it (and I know it can) but I don't understand it enough to make it a 1 pole filter cell instead of a VCA. I've got a long tailed pair with tail current control, and I understand how we can take the difference of the collector voltages to get a VCA, but not how to keep it a current output for charging a cap. obviously have some kind of hole in my understanding! :) any pointers would be much appreciated. 

Cheers, 
Chris