[sdiy] discrete ota trimming

[Tony Clark]

   Hi Ryan,

> I just got some samples from intersil of the HFA3096 3NPN, 2PNP array. I 
> wanted to try some experiments for discrete OTAs in VCFs with these. the 
> OTA circuit is basically like the one used in the JH ssm2040 filter. 
> What I am unsure about is the best way to add a trimmer. Here's what I 
> have now that works in simulations ok:
> 
> http://www.sdiy.org/destrukto/schematics/discrete_ota.gif

   Very good circuit, but if you don't want to deal with trimming, you can 
use what's referred to as an "improved Wilson current mirror" (basically 2 
PNP mirrors (Q3-Q4 in above image) stacked vertically but horizontally 
reversed).

   BTW, does anyone know what hFe range works best for the PNP current 
mirror pairs?  I've read somewhere that it should be as high as possible, 
like > 450.  Obviously any matched set works, but I'm just curious what 
everyone's opinions are on the subject.

   Cheers,

   Tony

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