Diode Circuits

Abstract

As discussed in the previous section, diodes are essentially one-way valves. They carry current in one direction, but block current in the other. In addition, in the forward conduction mode, the voltage must go above a threshold before significant conduction occurs. Diodes are used in a multitude of ways that utilize these characteristics. The objective of this course is not to look at hole and electron flows, but to see how the diodes can be used in circuits and how to analyze those circuits. For example, consider the circuit below, Figure 1. We are generally not interested in the holes and electrons in the diode, but rather, the current in the resistor or the voltage across the resistor. Figure 1. A simple diode circuit NUMERICAL METHOD The diode being a non-linear element means that the basic methods of circuit analysis learned in your circuits course cannot be used. Then, how does one go about analyzing the circuit? For example, we can write a loop equation Unfortunately, there are two unknowns in this equation, I and V d. The relationship between voltage and current in the diode is given by I I e V V D T = − 0 1 () () Where V T is 25.9 mV at 300 degrees K, and I O is the reverse saturation current. If we put the two equations together we get V V RI e S d V V D T = + − 0 1 () () (1) This equation cannot be solved analytically. But it can be solved numerically. You can make a guess of the diode voltage and plug into the equation and see if you get a balance.