Some references describe diode tempco as -2.1 mV per degree C and also as 3300 parts per million. If parts per million, then the tempco would vary with the forward voltage of the diode, for example, with a very low forward current, Vf could be half a volt; at 3300 parts per million that comes to about
-1.7 mV. So is it -1.7 or is it -2.1; in other words is tempco a constant or is it a proportion of Vf?
The voltage across an ideal diode is the natural log of a ratio of bunch of terms that involve doping densities and thicknesses and widths multiplied by the diode current. And that log is multiplied by k*T/Q.
The derivative of all that with respect to T is the same thing with the T replaced by 1. So as long as the current is constant (and all those other constants do not vary with temperature), the slope is constant, or a fixed increment per degree, not a fixed fraction of Vf per degree.
Some references describe diode tempco as -2.1 mV per degree C and also
vary with the forward
forward current, Vf
to about
constant or is it a proportion of Vf? Temperature coefficient of the p/n junction is approx -2mV/deg. C It does not depend on Vf.
And don't forget to factor in the IR drop due to bulk resistance.
When using diodes or Vbe like that to monitor temp I typically run them @ around 1mA or less.
Graham
I didn't notice this before, but in John Popelish's response he says that "as long as the current is constant," tempco is constant. Since Vf is a function of current, and tempco varies with current (if I interpret correctly), then tempco would vary with Vf. But I guess we're not talking about anything simple or remotely linear.
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