2 VCO's - 1 Log convertor (polysynth design)

[Tony Clark]

>>    However, how could you then separate the tuning of your VCO's?  You'd 
>> lose the ability to detune (except by changing the reset level of one 
>> VCO which would also affect that VCO's output level) or have 
>> fifths/octave separations.  Seems like a bigger negative than dealing 
>> with the added parts count, IMO.
> 
> Say you have the expo converter design where the base of the second
> transistor (expo transistor) is grounded, the emitter is driven by the
> first transistor and an opamp to generate a Vbe offset, and the collector
> goes to a current controlled oscillator.  You can add a second transistor
> by connecting the emitter of another, matched transistor to the emitter of
> the other expo transistor (the three transistor current mirror mentioned
> above) driving the second oscillator.  If the base of the second expo
> transistor is connected to a small variable voltage source, such as the
> wiper of a pot fed through a resistive voltage divider to the base, you can
> offset the frequency of one oscillator relative to the other.  The offset
> will alter the standing current of the second expo transistor without
> changing the scale factor, the oscillators should still track each other. 
> Plus or minus 18 millivolts will give a plus or minus  one octave tuning
> range.

   Ah, silly me.  Hadn't thought about that.  :)  That would probably 
negate the need to heavily match everything too.

   Tony

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