[sdiy] LM13700 VCA circuit

[James Patchell]

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...

At 12:04 AM 9/20/2004 -0500, Ryan Williams wrote:
>hi all,
>
>A while back I thought of a VCA circuit that uses the LM13700. both 
>sections are used and the first is in the feedback loop of an opamp. The 
>idea is to compensate for the non-linearity. the output of the opamp 
>inputs into the second OTA also. I noticed whole lot of high frequency 
>oscillation, most of this was fixed with a 100pF cap also in the feedback loop.
>
>so my question is: Is there any reason besides the high frequency 
>oscillation that this circuit won't work as well as my calculations say it 
>will? (if I'm doing the algebra correctly, it should cancel out the tanh 
>and Vt from the OTA equations). It seems to sound ok but my ears aren't 
>great anyway.
>
>the circuit that I tested can be seen here:
>http://www.sdiy.org/destrukto/schematics/linearized-lm13700-vca.pdf
>the only difference is that I left off the second opamp (TL072B) and used 
>the LM13700 buffer instead. this was because I don't care about DC offset 
>in my application. I left off that offset trimmer also. those diodes that 
>are just to protect the OTA from the opamp in case the opamp tries to 
>swing to a rail (too much voltage at the input), that could also be done 
>by increasing the input and output resistors.
>
>credits: that current source is from Thomas Henry's CA3080 book.
>
>-Ryan

         -Jim
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