[sdiy] OTA to voltage-control resonance?

[Antti Huovilainen]

On Sun, 30 Aug 2009, Jerry Gray-Eskue wrote:

> If you put it in the feed back loop I believe you will need the "Floating
> Voltage Controlled Resistor" shown in figure 10 of the National LM13700 data
> sheet.

That's just a fairly contrived way of doing the same as using an OTA 
directly as a VCA. The problems stay the same - namely, distortion caused 
by the tanh-shape of the transfer function.

If you truly want to eliminate the distortion (while keeping the SNR 
same), you can either use the linearizing diodes or use the other half 
inside opamp feedback loop to pre-distort the signal. The tanh shape is 
applied before multiplying by Ictrl. Hence the second half can be run with 
constant gain and as long as the same signal is applied to inputs of both 
halves, the distortion should be cancelled.

Antti

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