[sdiy] current mirror for NPN expo?

[Dave Brown]

The Aries 317 module I just repaired did just this with a current mirror for
two VCOs.  The main VCO generates the triangle wave with the derivative sine
and square waves.  The secondary VCO generates the saw waveform.  They reset
the saw waveform from the square and added the 10R in the second current
mirror to adjust the saw amplitude.  The upside is a great set of waveforms
with no glitches.  The downside is the saw amplitude does vary a bit over
the range.  I have waveform images on my 317 repair page.
http://modularsynthesis.com/aries/AR-317/ar-317.htm

Dave

-----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: Monday, September 19, 2011 11:41 AM
To: 'Harry Bissell'; 'dan snazelle'
Cc: 'synthdiy diy'
Subject: Re: [sdiy] current mirror for NPN expo?

yeah, this "driving two VCOs from one expo" idea is a non-starter, I'd say. 

> -----Original Message-----
> From: Harry Bissell [mailto:harrybissell at wowway.com]
> Sent: Monday, September 19, 2011 9:56 AM
> To: dan snazelle
> Cc: David G Dixon; synthdiy diy
> Subject: Re: [sdiy] current mirror for NPN expo?
> 
> that is a different issue... I'd expect that the additional core must 
> have its timing capacitor closely matched to the first, not at all 
> easy to do...
> 
> H^) harry
> 
> 
> ----- Original Message -----
> From: dan snazelle <subjectivity at hotmail.com>
> To: David G Dixon <dixon at mail.ubc.ca>, synthdiy diy 
> <synth-diy at dropmix.xs4all.nl>
> Sent: Mon, 19 Sep 2011 12:40:12 -0400 (EDT)
> Subject: Re: [sdiy] current mirror for NPN expo?
> 
> that is how i ended up trying it...however the pitch is not exactly 
> the same on the second core...even though the part values are the 
> same...i will keep working at it
> 
> thanks
> 
> On Sep 19, 2011, at 12:15 PM, David G Dixon wrote:
> 
> >> can anyone point me towards info on how to use ONE NPN
> EXPO to drive two vcos?
> > 
> > Can't you just mirror the current-sourcing one?  I.e., tie
> its emitter
> > to the other two, and connect its base to the other
> current-sourcing
> > one?  You should be able to replicate the current as many
> times as you want this way.
> > I did it with five transistors in a four-pole filter once;
> it worked a
> > treat.