[sdiy] DETERMINING FLOATING RESISTOR RANGE OF OTA??

[David G. Dixon]

> Looking at the FLOATING RESISTOR in the lm13700/lm13600 datasheet, I am
> trying to figure out how one gets the right values when building this to
> specifiy a range of resistance
> 
> i am building a circuit which requires a variable resistance over the
> range of 1k-3k

Dan, I have several ideas for this, but I need to have a better idea of the
application.  Does the resistor really need to be floating?  Also, do you
require a strictly linear response?

An SSM2164 (or Coolaudio V2164) VCA may be a better choice for making VC
resistors, even though it has an exponential response.  If you put the VCA
in series with a 1.3k resistor, and in parallel with a fixed resistor at
3.9k, then a voltage from 0V to 0.667V at the VCA's control voltage pin will
give a parallel resistance from 1k to 3k.  The exponential response of the
VCA cancels out the nearly logarithmic response of the mismatched parallel
resistors, giving an almost perfect linear response in overall resistance
vs. VCA control voltage.

Of course, I may also just be smokin' crack!