[sdiy] discrete ota trimming

[Ryan Williams]

thanks, I don't mind a trimmer on each OTA but I may try it that way 
also.  Incase someone was interested, I measured some HFA3096s I got 
today and the NPNs were within 1mV and the PNPs were about 3mV. I tried 
that on 2 different HFA3096 with Ic of 60uA and 260uA on each. maybe I 
got lucky with these but that seems to be up with the specs for the 
HFA3046's NPN pair.

hfe for these PNPs is only 60, but I'll try it anyway. I was thinking of 
also trying some THAT340 for this. the PNP hfe on those are also only 75 
although both the NPNs and PNPs are typically matched to .5mV according 
to the datasheet.

-ryan

Tony Clark wrote:
>    Hi Ryan,
> 
>    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