[sdiy] interesting expo design

[patchell]

    No, that is why I use the two OTA's, they not only linearize the OTA's, but also provides
temperature compensation for the OTA...I have tested the circuit, and it does work...I hope.

    By the way, that OTA circuit was posted here by another Synth-DIYer (I don't remember who now) as
a low noise, low distortion VCA...when I saw it, I realized it was the answer I needed to simplify the
temperature compensation circuit that I was using at the time, which consisted of a CA3046 and 1/2 of
an LM13700....using just an LM13700 is simpler, and a lot more linear.

    I ran the VCO on a bread board for a month, going through a lot of temperature
extremes...depending on which thermometer I used (I had two in the room), tempcos ranged from
300->500ppM.

Scott Bernardi wrote:

> I've been looking at your circuit, and one thing puzzles me about using a derived temperature
> dependent current to multiply by the control voltages - doesn't the OTA itself have a temperature
> dependence in the gm term?  Iout = Vin * 2q/kT * Iabc (the 2q/kT term turns into the familiar 19.2
> value at 25deg C). Q16C's collector is running at 100 times Q16A's, so you get a delta Vbe of
> kT/q*ln(100) = 4.6*kT/q. This voltage is amplified by U43A by 5K/620, so you have 37.1*kT/q at the
> output of U43A (about .965v). This is converted into a current by U45A to drive the input OTA.
> Ic17 = 37.1/10K*kT/q or 3.71e-3*kT/q.
>
> Iout = Vin * 2q/kT * (3.71e-3 * kT/q) ---> the kT/q terms cancel, so instead of a temperature
> dependent term to multiply the input control voltage by, you have canceled it!  Then you run it
> through a second OTA, U44B running at a constant current (adjusted by the Scale control) - and it
> introduces a kT/q term in its gm, but its a different chip at a different temperature.
> Am I missing something here?
>
> patchell wrote:
>
> >     Been there...Done that...sorta...
> >
> >     I saw that article some months back, and the solution I came up with is here:
> >
> > http://www.silcom.com/~patchell/seqproj/1001013.pdf
> >
> >     On page 10.  I used a single LM13700 (although both OTAs are used) as the VCA.  The boards
> > that this is on will be in my greasy little hands in a couple of days.  Earlier experments say
> > this should work really good.  I will keep you posted as to the results.
> >
> > Michael Ruberto wrote:
> >
> > > this is interesting, check it out:
> > >
> > > http://www.e-insite.net/ednmag/index.asp?layout=article&articleid=CA178103&industryid=2281
> > >
> > > the problem I see with this approach is adding in external CVs. I suppose it
> > > could be done using a very precise VCA. the external CV's can be summed into
> > > the VCA's CV input and the reference from IC3 connected to IC1's + input
> > > through the VCA? hmmm I imagine there would have to be some offset
> > > correction too.
> > >
> > > may have to experiment with this...
> > >
> > > ~M
> > >
> > > _________________________________________________________________
> > > Chat with friends online, try MSN Messenger: http://messenger.msn.com
> >
> > --
> >  -Jim
> > ------------------------------------------------
> > * Visit:http://www.silcom.com/~patchell/
> > *-----------------------------------------------
> > *I'm sure glad Merry Christmas comes just once a year
> > * -Yogi Yorgensen
> > ------------------------------------------------
>
> --
> Scott Bernardi
> sbernardi at attbi.com

--
 -Jim
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* Visit:http://www.silcom.com/~patchell/
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*I'm sure glad Merry Christmas comes just once a year
* -Yogi Yorgensen
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