I am currently trying to build a DIY analog synthesizer. I've looked at a lot of different designs and I am quite stuck on understanding how some exponential amplifier designs work.
In analog synthesizers an exponential amplifier based on a BJT transistor differential pair is used most commonly. The problem is it's thermal stability. While most of the effects cancel by using a differential pair, an effect related to the thermal dependence in the Ebers-Moll equation will be still present. A solution that I understand completely is using a tempco resistor, just as shown on the first image.
However, I've seen also an another solution (in Arturia minibrute, for example) that I do not understand at all. It is shown on the second image - the exponential amp without the tempco resistor next to R5 is on the left and a weird "thermal stabilisation" part is on the right.
The question is: how does it work? I don't see any connections with the left part at all beside the fact that all of the transistors shown are on a same CA3046 chip.
And which solution will be more stable?