[sdiy] Using BC847S in exponential converter

[George Hearn]

I haven't used the 847s in question but I do use a lot of matched pairs in
my SMT discrete stuff.  The DMMT3904 and DMMT3906 are available from diodes
inc.  They are tiny (SOT363), cheap and really very good for discrete OTAs,
filters, buffers and simple exponential converters.  I would use them for an
expo for an LFO for example, or to get the reverse log type response for VC
resonance on a filter, or a current mirror to name a few.  
  When it comes to a VCO exponential converter, the LM3046 is a bit better
and a bit more expensive but has Vbe matching of usually better than 500uV.
The expensive SSM2210s are really good and have a better log conformance
than the 3046 allowing un-trimmed operation over a wider range (maybe
another octave?).
  If you have auto-tune, and this is not hard to implement if you use a
microcontroller to generate Midi-to-CV, then you can slack a bit on the
exponential converter.  George

A discrete State variable VCF using the DMMT3904s and 3906s extensively is
at 
http://sites.google.com/site/hearnmorley/synth-diy-resources/GH_SVF2A.pdf?at
tredirects=0

A voltage controlled delay line with a simple pseudo-exponential current
source using the DMMT3904 is at
http://sites.google.com/site/hearnmorley/synth-diy-resources/GHSYN2bWeb.pdf?
attredirects=0

Also my modules such as the HM2040 use them for matched pairs in some places
http://sites.google.com/site/hearnmorley/products/hm2040

Thanks, George

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Barry Klein
Sent: 23 August 2009 04:54
To: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Using BC847S in exponential converter

Hey, these look interesting.  Please expand on what George has done 
with them.  Google comes up with nada...

Barry

============================================

Another option found looking at some of George Hearn's work:

http://www.diodes.com/products/catalog/list.php?parent-id=28

-Dave

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