tempco again

[Haible_Juergen#Tel2743]

Oops, how could I forget about the Is temperature effect!
Joachim, you're absolutely right with your description, of course.

Thinking of it again, one could even use a single transistor plus
tempco resistor, if it wasn't for the Is effect. But the Is effect being
the most important factor, we use a transistor pair. The pair helps
to cancel the Is effect, as well as the expo temperature effect
on what I called "bias BE voltage". What remains, is the scale
temperature effect on the "delta BE voltage", which still needs the
tempco resistor.

BTW, the Is effect might be the reason that you have to use similar
transistors (best: monolithic). If it would  be the expo effect only, any
two transistors would do, as long as they are thermally coupled.

JH.