[sdiy] LM13700 VCA circuit

[James Patchell]

It does cancel out...or nearly so...it is so close, that one can say that 
it does.  If you look at equation (1) in that doc...which has the transfer 
function,  you will note that the denominator is "1 + gm * R44....as long 
as gm * R44 >> 1, then you can assume that the tempcos of the two Gms wil 
cancel out...which it turns out...they do in practice (I have measured it 
to make sure)....

At 02:56 AM 9/20/2004 -0500, Ryan Williams wrote:
>hi jim,
>
>thanks for the reply. The one you are using seems different without the 
>extra opamp. I saw that circuit reccently on a vca circuit but wasn't able 
>to solve the circuit equations so that everything cancels out, but maybe I 
>did something wrong? so I didn't want to build it without understanding it 
>first.
>
>-ryan
>
>James Patchell wrote:
>>Yes, it will, in fact...it has already been done...The only difference 
>>between what you did and what somebody else did was you have some extra 
>>gain in the feedback loop that the linearizing OTA is in.  I used the 
>>circuit I found in my temperature compensated VCO's (circuit in the 
>>following url)
>>http://www.oldcrows.net/~patchell/synthmodules/100-1007.pdf
>>If you look on page 5 of that doc you will see your circuit sans the 
>>extra opamp...
>>I measured about 0.1 % linearity...which is pretty darn good...
>
>

         -Jim
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