Models of Diode (Ideal and Constant Voltage)

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Models of Diodes


  1.    Why use different models for diode?

  • we have seen how the I-V characteristic of a PN Junction diode reveals exponential dependence of junction current on biasing voltage, given by
  • This implies the necessity to solve exponential equations for obtaining exact values of current and voltages in circuits having diodes, which actually can't be expressed in closed form.
  • So to simplify our life, we adopt standard approximations for behaviour of diode, called models for analysing various circuits.



  2.    The IDEAL Diode Model

  • This model assumes that as soon as the diode is forward biased, even if the biasing voltage is just tiny bit greater than 0 volts, the diode acts as a short circuit and can be modelled by a short circuiting wire.
  • On the other hand, as soon as its reverse biased, it starts behaving as an open circuit, even though the biasing voltage is just infinitesimally negative.
  • In essence, all this approximation says is that as soon as you see the p side of diode facing positive(relative to n side) voltage, replace it with a short circuit, and if negative voltage( relative to n side) faces the p side, replace it with an open circuit.


Ideal model for a diode.

  3.    Constant Voltage Model

  • As seen earlier in the exponential behaviour, the current through the junction attains a significant value only after the biasing voltage reaches a certain value called built in potential, which is around 700mV for a typical silicon diode.
  • So this model tries to incorporate this built in potential for attaining greater accuracy.
  • Here, as the diode is considered to be switching between its "ON & OFF"  states for the biasing voltage V = builtin potential. 
  • Also, when diode is considered to be in its "ON" state, a drop of builtin potential is accounted for by adding a battery of emf equal to that potential. The following figure makes it very clear:


Constant voltage model of diode. Note: a battery is also included in this model, unlike the ideal model.

These models reduce our work tremendously, which will be clear when we'll see some applications.


Next post will be on Rectifiers!

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