Lin/exp control

[Joachim Verghese]

Juergen said:

> So I guess it's finally time now for some Hz/V experiments !

Yeah, V/oct oscillators do get boring in the long run. :-)
Anyhow, I've also experimented with some linear stuff -- most
recently with a Yamaha-style oscillator. I've posted a (quick)
scan of the voltage-to-current converter at

http://www.netcontrol.fi/~jocke/circuits/v2i.gif

The circuit is based on the classic op-amp + transistor V-to-I
converter, but I have added an exponential modulation input.

CV is the control voltage input, usually 0 - 4V. The FT input
has R, 2R, 4R, and 8R resistors (R = 5kohm, typ.) switchable
to ground for octave selection. VIB is the exponential modulation
input, and CC is the control current output.

U2A is a low-offset op-amp which drives transistors U3A and U3B. 
With no modulation applied, the bases of U3A and U3B are at
equal potential, and the current through FT is divided equally.
(The current through CC is half the current through FT).
With a modulation voltage applied (to VIB) the balance is
disturbed, and the current through CC is altered in proportion
to the product of the voltages at CV and VIB.

Transistors U3A and U3B would preferrably be matched and have
high current gain. The 3046 array I've used isn't the most optimal
component in this respect (low hfe).

-joachim