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Electronics Forum / Basics / December 2005diode temperature coefficient
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? > Some references describe diode tempco as -2.1 mV per degree C and also > as 3300 parts per million. [quoted text clipped - 3 lines] > -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 > > as 3300 parts per million. [quoted text clipped - 14 lines] > constant, or a fixed increment per degree, not a fixed fraction of Vf > per degree. 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 > kellwrote Some references describe diode tempco as -2.1 mV per degree C an als > as 3300 parts per million > If parts per million, then the tempco would vary with the forwar > voltage of the diode, for example, with a very low forward current V > could be half a volt; at 3300 parts per million that comes to abou > -1.7 mV. So is it -1.7 or is it -2.1; in other words is tempco > constant or is it a proportion of Vf Temperature coefficient of the p/n junction is approx -2mV/deg. It does not depend on Vf >Temperature coefficient of the p/n junction is approx -2mV/deg. C >It does not depend on Vf. --- That's only true at a single temperature and at a single Vf, so yes, it does depend on Vf.  SignatureJohn Fields Professional Circuit Designer > >Temperature coefficient of the p/n junction is approx -2mV/deg. C > >It does not depend on Vf. [quoted text clipped - 6 lines] > John Fields > Professional Circuit Designer 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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