[sdiy] OTA to voltage-control resonance?

[Antti Huovilainen]

On Sun, 30 Aug 2009, David G. Dixon wrote:

> Antti, can you point the dummies on the list (like me) to a decent source of
> information about OTAs which would help us to understand the statements you
> have made in the preceding paragraph?  The LM13700 datasheet simply isn't
> doing it for me.

The following paper by Achim Gratz derives the transfer function:
http://synth.stromeko.net/diy/OTA.pdf

Iout = Ictrl*tanh(Vin/2Vt)

1. Place both halves of the OTA after a common opamp.
2. Wire second half to use constant Ictrl (say 1mA). Connect VCA control
    current to Ictrl of first half.
3. Connect both outputs to identical resistors / buffers so that gain of
    second half is at most one.
4. Now connect buffered output of the second half to - input of the opamp
    and connect input signal to + input of the opamp.
5. First half output now acts as distortion free VCA output (assuming
    perfect matching between OTA halves).
6. You will probably need to apply compensation capacitor from opamp
    output to - input to keep the circuit stable.

The opamp makes sure that output of second OTA half is always equal input 
signal. Since the outputs of both OTA halves are linearly related, the 
circuit output must be linearly scaled version of the input signal.

Antti

"No boom today. Boom tomorrow. There's always a boom tomorrow"
   -- Lt. Cmdr. Ivanova