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diode temperature coefficient

Discussion in 'Electronic Basics' started by kell, Nov 25, 2005.

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  1. kell

    kell Guest

    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?
     
  2. 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.
     
  3. Pooh Bear

    Pooh Bear Guest

    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
     
  4. pebe

    pebe Guest

    kellwrote
    Some references describe diode tempco as -2.1 mV per degree C an
    als
    Temperature coefficient of the p/n junction is approx -2mV/deg.
    It does not depend on Vf
     
  5. John Fields

    John Fields Guest

    On Sat, 26 Nov 2005 00:01:08 -0600,
     
  6. kell

    kell Guest

    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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