[sdiy] Some thoughts about ladder filter circuit

[George Hearn]

I hear the can of worms opening!  Short answer IMO yes.  Modern fabrication
techniques on the LM3046 have improved the matching very much beyond the
original CA3046 and the fact that the transistors are monolithic means they
will be well thermally connected.  You can also use the spare three
transistors on the LM3046 to act as a heater and thermostat thus regulating
the temperature in the expo converter if you wish.  Where the LM3046 falls
behind super matched pairs is in the log conformance (something to do with
the emitter bulk resistance rbe though I'm shaky).  This said I would argue
that you can make very good and useable expo converters with this chip.  Log
errors will most likely be small or similar in magnitude to other non-ideal
circuit effects such as the 'reset' time in sawtooth VCO's amongst many
others.  George

-----Original Message-----
From: David G. Dixon [mailto:dixon at interchange.ubc.ca] 
Sent: 12 December 2008 17:59
To: 'George Hearn'; synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Some thoughts about ladder filter circuit

General question: Are these arrays suitable for VCO expo circuits?  They're
certainly cheaper than super-matched pairs and quads!

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of George Hearn
Sent: Friday, December 12, 2008 7:20 AM
To: synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Some thoughts about ladder filter circuit

The LM3046 from National semiconductor is better than the CA3046 from
Harris/Intersil in terms of transistor matching (Vbe match).  I've carried
out a lot of tests with this chip as it's central to many of my synth
circuits.  They quote a Vbe match of 1mV or better for the LM3046 I think
and 5mV or better for the CA3046.  In the CA3046 only the diff pair is
matched to 1mV Vbe wheras the LM3046 they are all matched to 1mv Vbe.
However, in practice the LM3046 transistors are vbe matched to much better
than 1mV making them really good for all sorts of circuits for synth diyers
ready to get down and dirty with discrete designs!  RS has the SMD part for
20p in 100pcs.  George