Lecture - 4: Applications of Diode

Diode Approximation: (Large signal operations):

1. Ideal Diode:

  • When diode is forward biased, resistance offered is zero,
  • When it is reverse biased resistance offered is infinity. It acts as a perfect switch.

The characteristic and the equivalent circuit of the diode is shown in fig. 1.

Fig. 1

2. Second Approximation:

  • When forward voltage is more than 0.7 V, for Si diode then it conducts and offers zero resistance. The drop across the diode is 0.7V.
  • When reverse biased it offers infinite resistance.

The characteristic and the equivalent circuit is shown in fig. 2.

Fig. 2

3. 3rd Approximation:

•  When forward voltage is more than 0.7 V, then the diode conducts and the voltage drop across the diode becomes 0.7 V and it offers resistance Rf (slope of the current)

VD= 0.7 + ID Rf.

The output characteristic and the equivalent circuit is shown in fig. 3.

Fig. 3

  • When reverse biased resistance offered is very high & not infinity, then the diode equivalent circuit is as shown in fig. 4.

Fig. 4

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