[sdiy] OTA as Voltage Controlled Resistor

[Donald Tillman]

On Apr 27, 2013, at 10:41 AM, Mattias Rickardsson <mr at analogue.org> wrote:

> > Yes, but... it wouldn't have the quirks that make the vactrol circuit charming, replacing them with these new OTA quirks.
> 
> Which quirks does the OTA solution have?


Let's see; off the top of my head:

   * The linearity of the OTA.

   * And the more extreme case, the situation where the OTA input gets overdriven.

   * Limitations on the output current of the OTA.

   * The characteristics of the OTA output driving into a voltage that's not ground.

   * The characteristics of the darlington transistors that are used to sample the voltages.

   * The current that the voltage sampling steals.

I'm sure there are more.

Every implementation has quirks.  Musicians have a tendency to find and apply those quirks just as much as they use the original intended function.

Note that a little later in the LM13700 data sheet, the schematic for the voltage controlled low pass filter looks very much like the single ended voltage controlled resistor.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com