Circuit Model of a Diode

Circuit Model of a Diode

• We have seen that a diode has a very important property that it permits only unidirectional conduction. It conducts very well in forward direction when forward biased and conducts very poorly in reverse direction when reverse biased. An ideal diode should conduct instantaneously when forward biased and should not conduct at all when reverse biased. The V-I characteristics of such an ideal diode would be as shown in the Fig. 1.13.1. An ideal diode acts like a switch.

• But in a practical diode, there is a voltage drop across a diode and it starts conducting heavily after its cut-in voltage V., is exceeded by the forward biased voltage applied. Similarly it is not a perfect conductor with zero resistance in forward biased condition. It has some finite resistance in forward biased condition.

• So voltage drop across forward biased diode is made up of,

1. The drop equal to cut in voltage which is to be maintained across the diode to keep it forward biased.

2. The drop due to its forward resistance due to forward current (If rf ).

• Hence in analyzing various devices involving diodes it is necessary to replace diode by its circuit model which considers its cut-in voltage and forward resistance. So a diode is replaced by a model with a battery equal to cut-in voltage of diode, the forward resistance of a diode in series with an ideal diode as shown in Fig. 1.13.2.

Key Point : Hence the total voltage drop across the diode is sum of the cut-in voltage and the drop across its forward resistance due to the current flowing through it.

• If the forward resistance is not given, a diode can be replaced by a battery having voltage equal to cut-in voltage of diode.

Key Point : A reverse biased diode can be replaced by an open circuit from the circuit analysis point of view.

• Such a circuit model of diode helps to understand the working of various electronic circuits and applications.

Review Question

1. What is circuit model of a diode ?