[sdiy] Thought experiment...

[Harry Bissell Jr]

I tried the Mike Sims circuit without much success...
ie. I did not see any improvement.

Does someone have a different result ???

H^) harry




--- James Patchell <patchell at cox.net> wrote:

> The Mike Sims VCA has the main advantage of having a
> very high 
> performance/price ratio.  It may be posible that
> using a CA3280 in the same 
> circuit you might be able to squeeze out some more
> linearity, but the 
> LM13700 has proved its worth in that I can't seem to
> see any problems that 
> it causes...plus, the LM13700 is only what, 50
> cents, and the CA3280 is $5+ 
> (unless you buy them from me :-)...I originally used
> an AD633 for the 
> multiplier, but it did not perform as good as the
> LM13700, in this application.
> 
> At 10:06 PM 3/22/2005 -0600, Ryan Williams wrote:
> >hi Dave,
> >
> >I also cannot see how you mean for this to cancel
> the Vt variable. If you 
> >are calculating otherwise, can I see the math? 
> This type of circuit, with 
> >error compensation or pre-distortion fascinates me,
> my favorite type.  I 
> >haven't spent a great deal of time on this yet, so
> excuse me if I have any 
> >mistakes.
> >
> >About a month ago, I decided to solve this problem.
> The only solution I 
> >could think of involved multiplying the input CV by
> the Vt temperature 
> >variable.  This should be the same as in the
> circuit Jim Patchell has 
> >posted although I have only just glanced at it. The
> way I've done it, 
> >(again which is probably similar to Jim's) is to
> use the second pair to 
> >generate the Vt. I have an opamp holding the first
> transistor at a 
> >constant current, and another opamp holding the
> other transistor's current 
> >constant. the second opamp's output after a voltage
> divider connects to 
> >the base of transistor 2. the base of the
> transistor 2 sits at 
> >ln(Ic2/Ic1)*Const*Vt. I had it setup as
> ln(50)*10*Vt. the 10 came from the 
> >voltage divider. I just made up these numbers.
> Haven't put any thought 
> >into what would be best. I really need to spend
> some time learning how 
> >Jim's circuit is better (just asuming it is). I
> planned on trying the 
> >THAT300 quad npn, don't have them yet though.
> >
> >The difficult part I think is the multiplier. I
> expect when I actually 
> >build this I'll use the two OTA circuit (also used
> in Jim's circuit, Mike 
> >Sims VCA in EDN magazine, and other places)
> although I've not yet measured 
> >it's performance. Incase Jim or anyone else who is
> reading is familiar 
> >with this circuit; is there any advantage to using
> the CA3280 in place of 
> >the LM13700. I was under the impression that it's
> transistors were matched 
> >well, better perhaps? What about a multiplier IC?
> If i remember correctly, 
> >the two LM13700 VCA doesn't completely remove it's
> temperature error, but 
> >only makes it much smaller. I don't really care
> about a couple of dollars 
> >more if performance is better, atleast not for my
> purposes.
> >
> >-Ryan Williams
> >http://www.sdiy.org/destrukto
> >
> >
> >D A F 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
> >
> >         -Jim
>
>***************************************************************
> >http://www.oldcrows.net/~patchell
> >
>
>***************************************************************
> 
> 
>