[sdiy] NPN pair VCF?

[tarcan]

The Pin 2(7)of CA3280 may also be used for source current when when pin
1,(8) not used.
Best regards

-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of David G. Dixon
Sent: 21 Nisan 2009 Saly 22:09
To: 'Dan Snazelle'; 'sdiy'
Subject: RE: [sdiy] NPN pair VCF?


Dan,

Generally, VCFs use OTAs as variable resistors in active LPF blocks.
Unfortunately, OTAs can only sink current into their Iabc gain control
inputs.  This means that the expo trannie must source current, and this
requires a pnp.  However, I know that Ray Wilson has rigged up his latest
VCFs to use an npn expo pair which sinks current from the bases of pnp's in
parallel sourcing the OTA currents.  My simulations suggest that this may be
a bit dicey, but I'm not sure.  It would be more conventional to use a pnp
pair, and just duplicate the current sourcing transistor of the pair with
parallel pnp's with common bases and emitters.  One thing about Ray's
circuit is that the reference voltage-generating opamp (which feeds current
to the transistor) does not have to source as much current.  If you have all
pnp's feeding, say, four OTAs in a four-pole filter, then the maximum
current each one is going to sink is probably about 0.6 to 0.7 mA.  This
means that the opamp will have to source about 2.5 to 3 mA max.  A TL07x
sources up to 10 mA, so this is not a problem, and this level of current
would only be at the highest cutoff frequency, which one never really runs a
filter at anyway, so ohmic heating probably wouldn't be too much of an
issue.  Of course, the max current can be decreased by increasing the
resistor between the opamp output and the expo trannies, but then the
resistor pairs on the OTAs (input and feedback) must be increased
accordingly to ensure adequate bandwidth in the filter.  One could probably
design the filter to run with 0.2 to 0.3 mA to each OTA, but I haven't
looked into it in that much detail yet.

Cheers,
Dave

David G. Dixon
Professor
Department of Materials Engineering
University of British Columbia
309-6350 Stores Road
Vancouver, B.C. V6T 1Z4
Canada

Tel 1-604-822-3679
Fax 1-604-822-3619

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> -----Original Message-----
> From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-
> bounces at dropmix.xs4all.nl] On Behalf Of Dan Snazelle
> Sent: Tuesday, April 21, 2009 7:23 AM
> To: sdiy
> Subject: [sdiy] NPN pair VCF?
>
>
> been noticing lately that just about every VCF out there uses a PNP pair.
> is it possible to use an npn pair in a vcf?
>
> it seems the cheapest option is the SSM2220 which is about 5 bucks or more
> at digikey
>
> thanks
>
>
>
>
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