[sdiy] OTA to voltage-control resonance?

[cheater cheater]

Thanks a lot Antti, that was very helpful of you. I can now understand
the papers mentioned here better.

D.

On Mon, Aug 31, 2009 at 12:14 PM, Antti Huovilainen<ajhuovil at cc.hut.fi> wrote:
> 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
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