[sdiy] Home made OTA !

[Roy J. Tellason]

On Thursday 16 June 2005 09:21 am, Yves Usson wrote:
> Hi Fredrik
>
> I suggest that you have a look to the discrte VCA developped
> by Roman Sowa who actually did all that some years ago. It will save you
> times.
> http://www.sowa.synth.net/modular/m_xvca.gif
>
> He uses very similar SMD transistors.
>
> Cheers
>
> Yves

Interesting circuit,  but as for SMD I'm not going there.  Is this do-able 
with thru-hole parts?  Anybody care to recommend some other transistor 
numbers?

> > I was thinking of using the Philips SMT transistor pairs BC857BS and
> > BC847BS to build a 3080 replacement. I have been reading
> > http://www.intersil.com/data/an/an6668.pdf for implementation info. Now
> > I have some questions:
> >
> > 1.
> > I would try the simplest variant with just three 2-transistor current
> > mirrors and a differential amplifier, as in figure 2 in an6668. Just to
> > see if it works at all. Is this a good idea?
> >
> > 2.
> > Am I correct in assuming only the transistors within each mirror needs
> > to be matched, but not across different mirrors?
> >
> > 3.
> > Then I would like to try the improved 3-transistor current mirrors (fig
> > 3B). Which (if not all three) transistors need to be matched?
> >
> > 4.
> > How about the darlington versions used in the top of figure 4? They
> > seem
> > to imply that only the two transistors with connected bases in the
> > 3-transistor mirror needs to be matched.
> >
> > 5.
> > What the hell are the diodes D2 and D4 doing? Are they just there to
> > switch off reverse biased transistors faster?
> >
> >
> > Thanks,
> > Fredrik C
>
> Yves Usson
> email : yves.usson at imag.fr
> http://www-timc.imag.fr/Yves.Usson/personnel/SDIY