[sdiy] Lowest distortion neede for VCA - linearizing the LM13600/13700 & other dual VCAs

[Ian Fritz]

Hmmm ... Lots of handwaving here.  Deriving the xfer function is actually a 
doodle:

Assume no input current to the OTAs. Then the output current of the first 
OTA is minus the input current. The first OTAs equation is:
I_out1 = -I_in = -g . V_in . I_abc1, where Iabc is the bias current (R2).

The second OTA has the same V_in as the first (wired together). Its 
equation is:
I_out2 = - g . V_in . I_abc2, where I_abc2 is the VCA control current (Q1).

Ratio these equations and get:
I_out2 = I_in (I_abc2/I_abc1)

So the first OTA  simply acts as a predistortion for the second.  I'm not 
sure what advantage adding the on-chip predistortion diodes would provide 
-- predistort the predistortion?  :-)

Ian