AW: Lin/exp control

[Joachim Verghese]

Juergen wrote:

> Such low resistor values would have advantages, too.

Yes, Yamaha tended to use rather large integrating caps for
their VCOs, in the range of tens of nanofarads. The larger
control currents make the VCOs less susceptible to jitter,
I suppose, but place higher demands on V-to-I converter
linearity.

> I wonder if linearity is affected with relative high current thru the
> differential pair.

Yes, linearity is affected since the current gain of the transistors
starts to fall at those higher currents, i.e. an increasing portion
of the control current is drawn through the base instead of the
collector. Maybe some sort of darlington configuration would work
better. Adds to the complexity, though...

> Have you checked if the current distribution thru the pair is stable
> in the upper ranges ?

Yes, I didn't notice any problem there, at least with the particular
3046 array I used. What I did notice was that the 3086 array doesn't
work in this application, matching does matter.

-joachim