Section IV - ELEN-325. Introduction to Electronic Circuits:...

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ELEN-325. Introduction to Electronic Circuits: Design Approach Jose Silva-Martinez - - 1 ELEN-325. Part IV. Diode’s Applications 1.- The PN junction (diode). The diode is a unidirectional device with two modes of operation: Forward bias when current can flow through the device and its intrinsic resistance is small and the second mode of operation is reverse biased. In the second mode of operation, the current flowing through the device is extremely small (less than 0.1 nA for most of the diodes used in micro-electronics applications), and the diode can be considered as an open circuit. In forward bias region, the current flowing through the device is determined by the voltage applied to the diode; both parameters are approximately related as follows: ± ² ³ ³ ³ ³ ´ µ - = 1 e I i th d V v S d (4.1) where I S is the saturation current and V th is the thermal voltage. The saturation current is function of the diode dimensions and other physical parameters, and its value is strongly affected by temperature variations; typical values are in the order of 10 -10 -10 -15 A. Usually its value increases by factor of two when the temperature increases by 10 degrees. The thermal voltage V th is temperature dependent as well, and it can be computed by using the following expression q kT V th = (4.2) where k is the Boltzmann constant (=); T is the temperature in kelvin degrees (300° K = 27° centigrade), and q is the fundamental charge of the electron (=1.6x10 -19 C). At room temperature (300° K), the thermal voltage is roughly 26 mV. The symbol and typical i-v diode’s curve are shown in figure 4.1 + v d - i d i d v d -i S 0.5 0.8 1 μ A 10 mA Fig. 4.1. Diode symbol and typical id-vd curve. Since the I-v curve is obtained by sweeping the dc voltage across the diode at very low frequencies, none of the high frequency diode’s limitations are then considered in this plot; the i-v plot is known as the DC diode’s characteristics. For negative voltages, the diode operates as a very large resistor; the output current is around -I S (less than -0.1 pA) and in most of the practical cases it is negligible. For instance if -1 volt is applied to the diode’s terminals the current flowing through it is around –I S ; if I S =0.1 pA, the equivalent diode’s resistance is in the order of r diode =1V/0.1 pA=10 13 . The diode can be used for the processing of AC signals. The circuit shown in figure 4.2 for instance operates as an AC rectifier if the amplitude of the input signal is larger than 0.7V. The circuit can be described by the following equations:
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ELEN-325. Introduction to Electronic Circuits: Design Approach Jose Silva-Martinez - - 2 ± ² ³ < - = > ´ ´ ´ µ · · · ¸ ¹ - = = - 0 v if 0 R I v 0 v if 1 e R I R i v i L S 0 i TH V 0 v i v L S L d 0 (4.3) R L i d Fig. 4.2. A signal rectifier using a diode Notice in equation 4.3 that the resulting circuit equation is non-linear and there is not a closed form solution. Usually computer based methods are used for plotting the output voltage as function of the input voltage. Even if the
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This note was uploaded on 01/01/2012 for the course ECEN 325 taught by Professor Silva during the Spring '08 term at Texas A&M.

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Section IV - ELEN-325. Introduction to Electronic Circuits:...

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