[sdiy] current mirror for NPN expo?

[Harry Bissell]

Its hard to get capacitor matched to better than 2% typically... you can sometimes find
1% parts but they would need to have good temperature stability (well you need than anyway for a VCO...)

You might have to add 'calibration' caps in parallel with the smaller to the two VCO caps to pad the value
up. Then you can adjust the expo converter for both.

Its also possible to inject a small error term into one of the current mirrors. Aries (modular) expo
converters do this. They add a 10 ohm in series with one transistor and then force a tiny voltage offset.
I think the schematics should be online (somewhere). If not I could e-mail what I think is in the converter...

H^) harry


----- Original Message -----
From: dan snazelle <subjectivity at hotmail.com>
To: Harry Bissell <harrybissell at wowway.com>
Cc: David G Dixon <dixon at mail.ubc.ca>, synthdiy diy <synth-diy at dropmix.xs4all.nl>
Sent: Mon, 19 Sep 2011 13:09:03 -0400 (EDT)
Subject: Re: [sdiy] current mirror for NPN expo?

ok

so its not as easy as just picking the same cap value?  (i suppose the caps have a +/- tolerance )


On Sep 19, 2011, at 12:55 PM, Harry Bissell wrote:

> 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.
>> 
>> 
> 
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> -- 
> Harry Bissell & Nora Abdullah 4eva
> 


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Harry Bissell & Nora Abdullah 4eva