Lecture -3: Diode Operating Point

Example - 1:

When a silicon diode is conducting at a temperature of 25°C, a 0.7 V drop exists across its terminals. What is the voltage, VON, across the diode at 100°C?

Solution:

The temperature relationship is described by

 VON (TNew) VON(Troom) = KT (TNew Troom)

or,           VON (TNew ) = VON (Troom) + KT (Tnew Troom)

Given     VON (Troom) = 0,7 V, Troom= 25° C, TNew= 100° C

Therefore, VON (TNew ) = 0.7 + (-2 x 10-3 ) (100-75)  = 0.55 V

Example - 2:

Find the output current for the circuit shown in fig.1(a).

Fig.1- Circuit for Example 2

Solutions:

Since the problem contains only a dc source, we use the diode equivalent circuit, as shown in fig. 1(b). Once we determine the state of the ideal diode in this model (i.e., either open circuit or short circuit), the problem becomes one of simple dc circuit analysis.

It is reasonable to assume that the diode is forward biased. This is true since the only external source is 10 V, which clearly exceeds the turn-on voltage of the diode, even taking the voltage division into account. The equivalent circuit then becomes that of fig. 1(b). with the diode replaced by a short circuit.

The Thevenin's equivalent of the circuit between A and B is given by fig. 1(c).

The output voltage is given by

or,         

If VON= 0.7V, and Rf= 0.2 W , then

Vo = 3.66V

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