Linear current source

[Christopher_List at Sonymusic.Com]

  Fellow Scholars-

  (It's nice to see all this new traffic on the list - it's been so quiet
  for so long)

  I have need (for several applications) to linearly control an OTA. Now,
  the easiest way to control it and get a very large range of sweep seems
  to be exponential - the PNP pair.

  The A of E has a "Good Circuit Idea" that is called "Howland Current
  Source for driving current controled voltage-current devices(or something
  like that)" - that's OTAs - right? So I built this, seeing a linear
  relationship bewteen Vin and control current - but it didn't seem to do
  anything when I hooked it up to an OTA. Do I need a resistor bewteen the
  PNP and the OTA pin (I thought this might be the problem this morning -
  so I didn't have time to try it)? Will this work for 3080/13600 type
  OTA's where the control pin is at -V+.6?

  They say the "Howland current source" is quite imperfect in the body of
  the chapter - but I guess it's OK to use for this application. The rest
  of the current sources they talk about are ground referenced - so I guess
  they wouldn't work for our usual OTAs.

  Does anybody have any good linear voltage->current source for OTA
  circuits that can drive the OTA over it's full range?

  - Thanks,
  Chris