4 Octave Expo Converter Question

[Inventor]

Hi, I'm working on a Karplus Strong string synthesis analog board and I need an expo converter for the VCO. The situation is that the board only operates over a four octave range due to the range of the bucket brigade device that I am using for the delay line. For this reason I believe that I can do the job with a really simplified expo converter circuit.

Now I realize that I will need tempco for proper tracking, however if at all possible I'd like to make do without the matched pair of transistors. This will save cost, complexity, board space, and prevent me from having to add another supplier. I don't mind having a trimpot since that may help deal with the fact that I'd like to use a single transistor for the expo conversion.

The output of the circuit is a current referenced to the negative supply rail since it will be driving the current into an LM13700 OTA, so what is needed is a special circuit topology that takes a voltage input from 0V to 4V (or 4.8V depending on the system in which it resides), and an output current flowing down into the negative rail (or close to it).

So my question is: "What are your recommendations for such a circuit?"

Les

[frijitz]

[Inventor]

[Dan Lavin]

Les,

I think Thomas Henry has some nice simple designs from his Making Music With the 3080 OTA book. Pls check your PM for some notes I sent.

[Inventor]