[sdiy] Some thoughts about ladder filter circuit

[George Hearn]

Every exponential converter based on the classic diff pair will have the
same 1/Vt dependence on temperature and this cannot really be called small.
Whether super-matched like the ssm2210 or simply well-matched like the
LM3046 they will all suffer the same problem.  The heater/thermostat method
is just one way of reducing this dependence by keeping the transistor
temperature constant.  Another method is of course to use a tempco and let
the transistor temperature vary and the tempco compensate.  If you will
'heat' the transistors a good temperature is one just above the largest
ambient temperature likely to be reached, maybe 50-60C?  George

-----Original Message-----
From: David G. Dixon [mailto:dixon at interchange.ubc.ca] 
Sent: 12 December 2008 19:27
To: 'mark verbos'
Cc: 'George Hearn'; synth-diy at dropmix.xs4all.nl
Subject: RE: [sdiy] Some thoughts about ladder filter circuit

I'm (more or less) a complete noobie when it comes to circuit design
(although I think I'm learning fast), but I'm assuming that Mark's
"transistor oven system" is the same as George's "transistor heater and
thermostat".  This all sounds a bit excessive, but nonetheless: what sort of
currents are drawn by such systems, and what is a typical target
temperature?

Also, I don't get the impression that this is done when super-matched pairs
are used in expos, but wouldn't they be succeptible to the same temperature
effects?  After all, using a matched pair (or even an unmatched pair)
cancels out the biggest temperature effect (I_s), and the small effect (V_T)
is typically compensated (if at all) with a tempco resistor.  Why would a
general-purpose transistor array be any more susceptible to temperature
effects than a super-matched pair?  (Maybe I'm reading too much into your
responses.)  Or, does the oven/heater simply obviate the need for the
tempco?

Dave

-----Original Message-----
From: mark verbos [mailto:mverbos at earthlink.net] 
Sent: Friday, December 12, 2008 11:12 AM
To: David G. Dixon
Cc: 'George Hearn'; synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] Some thoughts about ladder filter circuit

They're good enough for the Serge PCO and NTO. in that case (as well  
as many others) they're used in a transistor oven system.

Mark

On Dec 12, 2008, at 12:59 PM, David G. Dixon wrote:

> 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