[sdiy] Thought experiment...

[Ryan Williams]

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