[sdiy] Home made OTA !

[Fredrik Carlqvist]

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


> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl 
> [mailto:owner-synth-diy at dropmix.xs4all.nl] On Behalf Of karl dalen
> Sent: den 4 juni 2005 01:57
> To: Ian Fritz; laxt57 at aol.com; synth-diy at dropmix.xs4all.nl
> Subject: [sdiy] Home made OTA ! ( Was: Hu are not Be Sma).
> 
> Well i think the CA3083 works quite well, obviously bigger
> then the DIP8 of CA3080. The CA3083 are available in SMD
> and are dirt cheap, outperforms the LM3046!
> 
> Q1 and Q2 used for the gillbert mirrors Q3 used for
> sinking current (CV input)Q4 Q5 for current mirrors, 
> NPN tought but anyway, or use Q4 and Q5 for temp
> compensation! 
> 
> My one swedish krona!
> 
> REG
> KD
> 
> --- Ian Fritz <ijfritz at earthlink.net> skrev:
> 
> > At 12:47 PM 6/2/05, laxt57 at aol.com wrote:
> > 
> > >   Well what I am planning is this. A OTA made with SMD that
> > >  will fit the footprint of a ca3080. By input I guess I was not
> > >  asking about the design of the OTA itself, all that is freely
> > >  available on the web. I guess my question is with the available
> > >  smd devices 847s,3904 etc do you think this sdiy do able?
> > 
> > Well, that's the question.  I haven't found any good SMD 
> well-matched, 
> > high-gain transistors.  I've been stocking up on the 
> popular Japanese 
> > matched pairs, some of which are quite good, but all 
> discontinued.  In my 
> > most recent discrete OTA I tried a pair of hand-matched 
> ZTX795A's glued 
> > together, but they didn't work very well.  They had too 
> much LF noise, 
> > presumably due to temperature fluctuations.
> > 
> > Maybe some of the non-monolithic SDMs could be made to 
> work, I dunno.
> > 
> > Another question is whether it is even a good idea to try 
> to reproduce the 
> > 3080.  The Iabc mirror has always been a pain, requiring a 
> mirror in the 
> > expo converter.  Why not work with the simpler discrete 
> designs that don't 
> > have this "feature".
> > 
> >    Ian
> > 
> > 
> 
>