expo mix up

[Magnus Danielson]

From: "jhaible" <jhaible at debitel.net>
Subject: Re: expo mix up
Date: Wed, 6 Oct 1999 12:08:05 +0200

Hi!

Another favorite topic just poped up:

> > >I sink the VCO/VCFs current in the transistor that has the control
> > voltage applied. This is the same as the Formant and the ASM-1 I think.
> > But the minimoog VCO uses the second one, with the op-amp's feedback
> > going to the first.
> >
> > Sorry, this is the wrong way round. But the question remains the same.
> > What advantages are there in doing it either way?
> 
> The first difference is the CV polarity, of course. If you have a certain
> CV polarity (after a certain number of inverting adder stages, for instance)
> and a given polarity of the expo converter (npn or pnp), you can choose
> the right input of the converter to avoid an extra inverter stage.
> 
> The two inputs have different requirements for the driving impedance,
> however. The "reference" transistor (in the feedback loop) can work
> with a medium driving resistance, like the 1k (tempco) of a passive
> resistor divider. The same resistor divider applied to the other, the "expo"
> transistor will cause a remarkable HFT error. Why ? The external
> impedance adds to the internal rB, which is transformed to an increased
> rE. This will linearize the expo function at higher currents.

It is noteworthy to point out that the rBE is being closely matched in many
cases and that the rBE is typical 300 mOhm in a MAT-02. The dual transistor
configuration will compensate out the rBE of the expo-transistor by using
its match in the ref-transitor. One cannot stress enougth that the resistance
as seen on the base of the expo transistor to some voltage source should be
really low.

Juergen where about to use the expo base once as an extra input, until someone
point out that his migth not be the best of ideas...

> You can use the base of the "expo" transistor as an input, but then make
> sure you drive it with a really low Z. (tempco resistor in the feedback
> loop of an opamp ... which will - damn ! - change the polarity as well,
> so you might just as well use the other input ..)
> There are some configurations where it makes sense, though. I just
> don't remember which they are.

Maybe it is time to collect material on expo converters and make a propper
presentation on the subject.

Cheers,
Magnus