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FacebookThe simplest diode circuit, shown in Figure 1 (a), consists of a supply voltage with a diode and resistor in series. From previous lectures we have seen that the diode in forward bias has the current voltage elationship as shown in Figure 1 (b).
We can analysis the circuit shown in Figure 1 (a) using Kirchoff’s voltage and current laws. We can easily see that the current flowing through the diode is the same as that flowing through the resistor. Also applying Kirchoff’s voltage law in the clockwise direction yields the following voltage relationship,
Finding the solution for Equations (1) and (2) requires solving non-linear equations, so it is easier to solve these equations graphically.
A change in the load will change the intercept on the current axis and as a result the slope of the load line causing the Q-pioint to change
Figure 1 (a) Simple Diode circuit (b) V-I relationship for forward bias.
We can analysis the circuit shown in Figure 1 (a) using Kirchoff’s voltage and current laws. We can easily see that the current flowing through the diode is the same as that flowing through the resistor. Also applying Kirchoff’s voltage law in the clockwise direction yields the following voltage relationship,
where VD is the voltage across the diode, and DI is the current through the diode. In analysising circuits in EE2201 we considered circuit elements that had linear voltage and current relationships. For the case of the diode, however, the voltage and current relationship is not linear, but has a non-linear relationship as shown in Figure 1 (b). To determine the voltage across the diode, VD, and its current, DI the solution must satisfy
both Equation (1) and the relationship V-I diode characteristics in Figure 1 (b). To satisfy both conditions requires the solution of simultaneous equations. The first is given by Equation 1, and the second is given by Shockley’s equation defined as,
Figure 2 Solving Simultaneous Equations Graphically
Figure 2 shows two curves the first is the V-I characteristics of the diode and the second curve is a straight line that interests the first. The second curve (Line) is referred to as the load line and has a slope of1/R (R is the load). The intercepts on the current and voltage axis are obtained by setting the current through the diode to zero, hence from Equation (1) the voltage across the diode isE. Similarly, by setting the voltage across the diode to zero the current through the diode is given by ID= E/R. The point where the load line intercepts the V-I characteristics of the diode is the solution of the two simultaneous equations, and is referred to as the operating point or (quiescent) Q-pointA change in the load will change the intercept on the current axis and as a result the slope of the load line causing the Q-pioint to change
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