[sdiy] discrete ota trimming

[Metrophage]

I have been wondering about these HFA3096 units! About two years ago,
just prior to a SDIY hiatus, I bought some CA3096 chips for use in
TomG's update of the ARP Axxe VCO. Word on the street being that the
PNPs are way suckier than the NPNs, I have been wondering about other
PNP/NPN arrays as well. I don't have a datasheet handy for the CA3096,
and I don't understand many of the specs involved. Sounds from your
comments as if the HFA unit has better PNPs.

Probably a very basic question, but what are "Ic" and "hFe"? Sounds
backward, but I started playing with op amps - I am new to transistors!

As for THAT, I have heard some good things about their arrays. Is there
any distributor which vends THAT arrays, or does one buy them from THAT
directly?

CJ

--- Ryan Williams <destrukto at cox.net> wrote:

> 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
> 
> 


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