[sdiy] VCO Linear Voltage

[David G. Dixon]

Oh...my...Gawd!!!!

Yes, the current conveyor is just the ticket for sending a current into any
arbitrary voltage unaltered, and will indeed allow the SSM2164 to have "safe
sex" with the LM13700.

And, what is the simplest, most straightforward implementation of the
current conveyor...?

a Supertransistor!

...which is just a transistor on the end of an opamp!  The current is fed
into the opamp's - input, the + input is grounded (thus providing virtual
ground at the - input), the opamp's output feeds the base, the emitter is
fed back to - input, and the collector sources or sinks (depending on the
transistor type) virtually the same current to whatever voltage.  Hook a
2164 VCA to the - input, make the trannie a pnp (a la 2N3906), and this will
feed the Iabc pin of LM13700 quite happily.  Simulations suggest virtually
perfect equivalence between the input and output currents.

(I read about this in Horowitz and Hill many moons ago, but managed to
forget all about it.)

The 2164 Expo Triangle-Core VCO is back in business!!



> It only seems exotic until you know what it is called - try "current 
> conveyor" in your favourite search engine.  It is also sometimes 
> called an "ideal transistor", but that is not the most useful search 
> term because different folks have different ideals.