Small signal model for ideal diodes

Discussion in 'General Electronics Chat' started by anhnha, Mar 3, 2016.

  1. anhnha

    Thread Starter Well-Known Member

    Apr 19, 2012
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    With normal diode, it is easy to define small signal model by derivative id as a function of Vd and this is equivalent to a resistor. However, what is the small signal model of an ideal diode (for example, one with Vd always exactly 0V or 0.7V)?
     
  2. WBahn

    Moderator

    Mar 31, 2012
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    What diode model says that Vd is always exactly one of two possible voltages?
     
  3. anhnha

    Thread Starter Well-Known Member

    Apr 19, 2012
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    From this page there are two ideal models for diode:
    A mathematically idealized diode:
    [​IMG]
    Ideal diode in series with voltage source:
    [​IMG]
     
  4. WBahn

    Moderator

    Mar 31, 2012
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    But look at those figures. Both of them allow for voltages other than 0 V for the top one or Vt for the bottom one. For instance, both allow a voltage of -1.5 V.

    I think you mean to say that an ideal diode model has a Vd = Vt when Id ≥ 0 and Id = 0 when Vd ≤ Vt.

    In that case, you use the same definition. Because the model is piecewise linear, you need two resistances, one for each region.

    In the conducting region, what is the change in voltage due to a change in current?

    In the non-conducting region, what is the change in voltage due to a change in current? The answer is easier to see if you think of it as what is the reciprocal of the change in current due to a change in voltage?
     
  5. dannyf

    Well-Known Member

    Sep 13, 2015
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    It is a piece wise function, or a minimum function,

    Vfwd=min(Vfwd, 0.7v).
     
  6. dannyf

    Well-Known Member

    Sep 13, 2015
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    This function has a dynamic resistance of infinity when Vfwd is less than 0.7v, and 0 otherwise.
     
  7. anhnha

    Thread Starter Well-Known Member

    Apr 19, 2012
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    In that case, you use the same definition. Because the model is piecewise linear, you need two resistances, one for each region.

    I think I've understood it now.
    Voltage remains constant regardless of current variation. So it is like a voltage source here and it acts as a resistor with zero resistance in small signal model.
    In this region, voltage changes but current remains the same so it is kind of current source that acts as a resistor with infinite resistance.

    Yes, I see now.
     
  8. Bordodynov

    Active Member

    May 20, 2015
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    See

    Diode.png
     
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