[sdiy] OTA diodes

[Rutger Vlek]

Hi guys,

> A few years ago I put some effort into making the best discrete OTA I could.  My goal was to  have performance as good as the CA3280.  It took some care, but it worked fine.  However, it was necessary to use a fully symmetric structure, with a four transistor current mirror -- using MAT transistors -- and a fully differential I/V converter.  Without all this, the offset variation with Iabc was excessive.  Keeping the C-B voltages small reduces problems with the Early effect.  I believe this may be what Don is referring to, although there may be other benefits.
> 
> But a two or three transistor mirror did not work well enough for what I was trying to achieve, even with the reduced voltage at the top of the mirror.


Hmmm... sounds very similar to what I'm trying to do here and the results I'm getting. Nothing radically new as I've seen by now, but interesting experience in itself. Although I've figured out the noise-problem with my OTA and came to the conclusion that there's only so much "quality" you can squeeze out of a certain topology and it's up to you how to decide where to put that "quality". Either you go for low noise and low distortion, and you get high CV-bleed and offset problems, or you go for a bit more noise and distortion and gain better CV-bleed and offset specs. I've decided to proceed with the design, knowing the limitations, because I believe it can still hold great musical value. I'm currently going for a second prototype VCA in Eurorack format. In terms of "clean" and "high fidelity" it will never be able to compete with something like a 2164, and then I'm not even thinking about the digital domain.

Best,

Rutger