[sdiy] Re: Old unique VCO - T?==?windows-125 6?Q?une drifting problem - adapting a servo c ?= =?iso-8859-1?Q?onversion‏

[Ian Fritz]

At 02:33 PM 5/11/2012, David G Dixon wrote:
> > The tempco resistor only compansates VT parameter of
> > transistor but not IEBS of transistors( Emetor -Bas
> > saturation current of transistors) witch drifts too much with
> > ambient temperature. But heating the transistors as used in
> > uA726 compansates both...
>
>Actually, if the transistors are relatively well matched and in thermal
>contact (as they would be in a dual package), then wiring them in
>common-emitter mode automatically compensates for I_S.

Correct. The I_S temperature dependence can be removed either by using a 
matched pair or by thermostating.  There are designs that do both, for some 
reason, and this can be confusing.  But as pointed out above, if you use a 
matched pair you additionally need to add the tempco resistor to compensate 
the 1/T term in the exponent.  This is not needed with thermostating.

Ian