[sdiy] VC voltage divider?

[David G. Dixon]

> I need a "voltage controlled voltage divider". Is that a 
> synonym for something else?

If you only want to control one resistor in the divider, then a synonym for
this is a "linear VCA".  I always use Mike Irwin's trusty linearized 2164
circuit for these applications.  OTAs such as LM13700 are another approach,
but they are actually current-controlled and require severely limited input
voltages to avoid distortion.

If a "voltage-controlled potentiometer" is what you are after, then one
synonym for this is a "crossfader", which is handily done with two
linearized 2164 VCAs.  Another synonym for this is "balanced modulator" or
"four-quadrant multiplier".  Basically, the two linearized VCAs are driven
by the normal and inverted versions of the same control voltage, such that
one is on when the other is off, and they cross over each other in the
middle.  If the inputs to each VCA represent the taps of the pot, then the
output of the multiplier is the wiper.  This can be achieved with one 2164,
one quad opamp (e.g., TL074), and a few resistors.