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kell
Guest
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 Posted:
Sat Nov 26, 2005 1:35 am Post subject:
diode temperature coefficient |
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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?
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John Popelish
Guest
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| Posted:
Sat Nov 26, 2005 1:35 am Post subject:
Re: diode temperature coefficient |
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kell wrote:
| Quote: | 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. |
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Pooh Bear
Guest
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| Posted:
Sat Nov 26, 2005 9:35 am Post subject:
Re: diode temperature coefficient |
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John Popelish wrote:
| Quote: | kell wrote:
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.
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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
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pebe
Guest
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| Posted:
Sat Nov 26, 2005 9:35 am Post subject:
Re: diode temperature coefficient |
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| Quote: | kellwrote
Some references describe diode tempco as -2.1 mV per degree C an |
als
| Quote: | 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 |
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neon
Joined: 25 Feb 2006
Posts: 570
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Tue Mar 14, 2006 9:51 am Post subject:
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| the temp coeficiency of a diose is totaly dependent on current but take any diode and run some current thru and you will find a point where there is no drift with temp, the problem is why run extra current to achieve no drift? |
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