[sdiy] Thought experiment...

[D A F]

Perhaps I did a crappy job explaining clearly what I was trying to
accomplish.  When I mentioned a 'positive-going' and 'negative going'
version of the CV input, what I meant that the CV coming in would be first
put through a "phase inverter" (to borrow an audio amplifier term) such that
when you feed a CV into the input, you make two CVs - one "in phase with"
and one "180 degrees out of phase with" the input.  This could for example
be accomplished by feeding the CV into two op-amps, one set up as an
inverting amp, the other non-inverting, but both stages having the same
(abosolute value) gain.  The outputs would then be a -pair- of CVs that
track one another, one going up as the other went down, etc..  This pair of
'inverted phase' signals drive the two pairs of transistors, so that when
you feed the outputs from the two log amp stages into the dif amp at the
end, you do _not_ cancel the CV (since itself has been made differential
prior to the transistor stages), but _do_ cancel the temperature effects
from the transistors themselves (which, given matched devices and good
thermal coupling, would track one another).

Then again, perhaps I should just build one and see if it works..  :-)


Dave



----- Original Message -----
From: "harrybissell" <harrybissell at prodigy.net>
To: "D A F" <polaris30 at ncx.com>
Cc: "Synth-Diy" <synth-diy at dropmix.xs4all.nl>
Sent: Tuesday, March 22, 2005 2:47 AM
Subject: Re: [sdiy] Thought experiment...


> It would also reject the CV itself...   :^(
>
> H^) harry
>
> D A F wrote:
>
> > Hi all, I was taking a look at the schematic for the music from outer
> > space VCO board, when I got an idea concerning a possible way to
> > elliminate tempcos from VCO's.  I'm sure someone has probably thought
> > of this (like 40 years ago) but in any case, here goes...   If you had
> > a quad of matched transistors on a single substrate (such as in the
> > new THAT 300 series), or at least two thermally coupled pairs,
> > couldn't you send one pair a positive-going CV and the other pair a
> > negative-going CV and then send the two outputs to a differential
> > amplifier, such that only the difference signal would be amplified.
> > This would elliminate the change in current with temperature from the
> > final output, because it would be equal in both pairs, assuming
> > perfectly matched and thermally-coupled devices, and the diff amp
> > would 'ignore' it.  Any thoughts?  Dave
>