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.
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