[sdiy] Home made OTA !

[Yves Usson]

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



>
> 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