For semiconductor diode circuit

Semiconductor diodes are non-linear devices. That is why the electrical circuits with such devices should be analysed and calculated by non-linear methods. One of them is a graph-analytical method.

The electrical circuit with a non-linear device (diode), linear resistor and a source voltage are shown in Fig. 3.5.

Fig. 3.5. Calculation of circuit with a semiconductor diode: a – general non-linear circuit, b – separation of non-linear circuit from linear one

It is necessary to find a voltage drop across the diode and current flowing through the diode.

Let’s separate the linear circuit from the non-linear one and define the current-voltage characteristic of the linear circuit first. In other words, we define the connection between and

where: and are voltage and resistance of the linear circuit; and are load voltage and current.

Fig. 3.6. Current-voltage characteristics of the linear and non-linear circuits

Let’s draw the plot of this equation. It will be a straight line, because the equation expresses linear dependence. It is enough to find two points to draw this plot. These points can be found at and . They correspond to a short circuit and a break of a linear circuit output.

At , we have got the point (from equation).

At , = E, it is possible to find the point .

Having connected these points, we got linear circuit characteristic. The tangent of slope angle of this line equals . It means that, when is low, the line tends to the vertical. When is high, the line will tend to the horizontal. Further, using the data sheet or the result of measurements, we can find a diode characteristic.

The intersection of these plots will correspond to: ; . The point is called a working point. The linear current-voltage characteristic is called a load line.