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Unformatted text preview: Attia, John Okyere. “Diodes.” Electronics and Circuit Analysis using MATLAB. Ed. John Okyere Attia Boca Raton: CRC Press LLC, 1999 © 1999 by CRC PRESS LLC CHAPTER NINE DIODES In this chapter, the characteristics of diodes are presented. Diode circuit analysis techniques will be discussed. Problems involving diode circuits are solved using MATLAB. 9.1 DIODE CHARACTERISTICS Diode is a twoterminal device. The electronic symbol of a diode is shown in Figure 9.1(a) . Ideally, the diode conducts current in one direction. The cur rent versus voltage characteristics of an ideal diode are shown in Figure 9.1(b) . i anode cathode ( a ) i v ( b ) Figure 9.1 Ideal Diode (a) Electronic Symbol (b) IV Characteristics The IV characteristic of a semiconductor junction diode is shown in Figure 9.2 . The characteristic is divided into three regions: forwardbiased, reversed biased, and the breakdown. © 1999 CRC Press LLC © 1999 CRC Press LLC i v reversed biased forward biased breakdown Figure 9.2 IV Characteristics of a Semiconductor Junction Diode In the forwardbiased and reversedbiased regions, the current, i , and the voltage, v , of a semiconductor diode are related by the diode equation i I e S v nV T = − [ ] ( / ) 1 ( 9 . 1 ) where I S is reverse saturation current or leakage current, n is an empirical constant between 1 and 2, V T is thermal voltage, given by V kT q T = ( 9 . 2 ) and k is Boltzmann’s constant = 138 10 23 . x − J / o K, q is the electronic charge = 16 10 19 . x − Coulombs, T is the absolute temperature in o K At room temperature (25 o C), the thermal voltage is about 25.7 mV. © 1999 CRC Press LLC © 1999 CRC Press LLC 9.1.1 Forwardbiased region In the forwardbiased region, the voltage across the diode is positive. If we assume that the voltage across the diode is greater than 0.1 V at room temperature, then Equation (9.1) simplifies to i I e S v nV T = ( / ) ( 9 . 3 ) For a particular operating point of the diode ( i I D = and v V D = ), we have i I e D S v nV D T = ( / ) ( 9 . 4 ) To obtain the dynamic resistance of the diode at a specified operating point, we differentiate Equation (9.3) with respect to v , and we have di dv I e nV s v nV T T = ( / ) di dv I e nV I nV v V s v nV T D T D D T = = = ( / ) and the dynamic resistance of the diode, r d , is r dv di nV I d v V T D D = = = ( 9 . 5 ) From Equation (9.3), we have i I e S v nV T = ( / ) thus ln( ) ln( ) i v nV I T S = + ( 9 . 6 ) Equation (9.6) can be used to obtain the diode constants n and I S , given the data that consists of the corresponding values of voltage and current. From © 1999 CRC Press LLC © 1999 CRC Press LLC Equation (9.6), a curve of v versus ln( ) i will have a slope given by 1 nV T and yintercept of ln( ) I S . The following example illustrates how to find n and I S from an experimental data. Since the example requires curve fitting, the MATLAB function polyfit will be covered before doing the example....
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