[sdiy] Thought experiment...

[Harry Bissell Jr]

The two log amp stages would have to be NPN and
PNP in that case, wouldn't they ?

Matched transistors are probably more expensive than
the tempcos are.  You double a $3 component (not to
mention the extra opamps, precision resistors) to save
a $1 component.

Jim Patchell's circuit will get rid of the tempco, and
iirc Ian Fritz has the "Dial-a-Tempco" which allows
you
to adjust for oscillator temperature drift as well...

I would buy the tempcos and be done with it... Its not
a bad idea to actively compensate, but the cheap
first-order approximation only uses maybe 0.1 sq"

H^) harry



--- D A F <polaris30 at ncx.com> wrote:
> 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
> >
> 
>