80017a / ir3109 exponential converter

Completed the exponential converter for my open source Juno 106 filter clone.

Mostly based on the great lectures by Aaron Lanterman (ECE4893A: Electronics for Music Synthesis, session 9, Exponential Current Sources). Also based on the SMR-4 filter as designed by mutable instruments. You should really check out Olivier’s work for Mutable instruments, great stuff there and the Shruthi-1 synth is just a fantastic piece of engineering. Very inspiring.

On the the expo converter. Following Aaron’s lecture, the following is the circuit required (in LTSpice):

The Vin is pin 6 on the 80017a chip, the VCF CONT. This input varies between 100mV (FREQ slider @ 0) and -160mV (FREQ slider @ 10). Iout for these values is:

iout = iref * exp((-Vb / Vt)) where Vt = 26mV

Vb = 100mV -> iout = (Vref / Rref) * exp ((- 100mV / 26 mV))

iout = (somewhere around) 0.000015 * exp (-3.8462) = 320.426 nA

Vb = -160mV -> iout = 0.000015 * exp(–16026) = 7.058 mA

which are probably (I did a quick check) fine values to drive the VCF!

Verified the calculations using LTSpice:

only differs a little. I’m not sure why but I guess its our calculation. For the sake of usability we assumed both transistors are ideal and thus have Ic = Ie and Ib = 0.The LTSpice model is probably more¬†sophisticated ;)