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# DIODE CIRCUITS

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CHAPTER 2 DIODE CIRCUITS 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 Vs Vd Vr Vd I R Unfortunately there are two unknowns in this equation I and Vd The relationship between voltage and current in the diode is given by I I 0 e V D VT 1 Where VT is 25 9 mV at 300 degrees K and IO is the reverse saturation current If we put the two equations together we get VS Vd RI 0 e V D VT 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 Diode Circuits 1 If not try another guess This process can be carried out several ways The simplest is to use a calculator while more complex methods would use a computer One interesting approach is to use a math solving program on a personal computer such as MathCAD Literally all circuits could be solved this way but that would be impractical especially for more complex circuits Secondly the diode equation mentioned above only represents the hole electron currents within the body of the diode There is a considerable leakage on

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