tempco resistors

[Erik The ViKing]

>
> The collector current of a BJT (bipolar junction transistor) can be
> described as
>
> Ic = Is * exp(Vbe/Vt)
>
> where Is is the reverse collector saturation current,
> Vbe is the base-emitter voltage,
> Vt is the "thermal voltage".
>
> Both Is and Vt are heavily dependent upon temperature. The dependence
> of Is can be cancelled by the use of a second transistor with similar
> thermal behavior. This leaves the Vt term which affects the scaling
> factor of the expo converter. This secondary temperature effect can
> be reduced by multiplying the CV by a factor whose temperature coefficient
> is opposite to that of the transistor. This is where the so-called tempco
> resistor comes in useful.
>

To my knowledge this formula doesn't really seem to fit a transistor, it
works better for tubes!!!! This doesn't mean that is the WRONG formula, it's
just a very simple formula. There are all kind of effects (for instance a
lot of capicitance) which seem to occur!!!

Erik the ViKing