Lab09_Diodes - DIODE CIRCUITS LABORATORY A solid state diode consists of a junction of either dissimilar semiconductors pn junction diode or a

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DIODE CIRCUITS LABORATORY A solid state diode consists of a junction of either dissimilar semiconductors ( pn junction diode) or a metal and a semiconductor (Schottky barrier diode). Regardless of the type, the circuit symbol for a diode is as shown in Fig. 8.1a, and the corresponding device in Fig 8.1b Fig. 8.1a Fig 8.1b If V is positive, the diode is forward-biased . Then, the diode can conduct a significant positive current I even though V is a small voltage of typically 0.7 V for the most common diode (silicon diode). If V is negative, the diode is reverse-biased . This negative current is so small that it is often considered to be zero. Thus, the usual function of a diode is to allow current to flow in the direction of the arrow (the forward direction) for positive V’ s, but not allow any current to flow in the reverse direction for negative V’ s. Only a small forward bias (positive V ) is required to cause a diode to conduct a significant current I , and the less this voltage, the better. Ideally, this voltage would be zero volts. Also, ideally, a diode can conduct any value of current I in the forward direction, with this value being determined not by the diode, but by other components in the circuit in which the diode is connected. Also, ideally, a diode conducts zero amperes for a negative V , regardless of the voltage magnitude. Put another way, an ideal diode is a short circuit for a voltage V that tends to be positive (but it cannot be more than 0 V). Also, an ideal diode is an open circuit for a negative V. Thus, an ideal diode acts like a switch that is closed for current flow in the direction of the arrow in the diode circuit symbol, and open otherwise. Essentially, it is an electronically operated switch. This ideal approximation is satisfactory for analyzing many circuits that contain diodes, provided that the voltage levels are much greater than 0.7 V. Fig. 8.2 shows the I-V characteristic for an ideal diode. Fig. 8.2
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Fig. 8.3 shows the I-V characteristic of an actual, physical diode. The part of the curve in the first quadrant is the forward characteristic , and the part in the third quadrant is the reverse characteristic . The current I s is called the reverse saturation current . For a reverse voltage V B , the diode “breaks down” and draws a large reverse current. Fig. 8.3 The diode forward characteristic is shown on an expanded scale in Fig. 8.4. Observe the “turn-on” voltage V T . For forward voltages less than V T , a diode conducts very little current. Also, in the normal forward operating range, the diode voltage is approximately V T , almost irrespective of the current value. For the common silicon diode, V T is approximately 0.7 V. Fig. 8.4
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This note was uploaded on 07/10/2011 for the course EEL 3113c taught by Professor Staff during the Spring '11 term at University of Florida.

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Lab09_Diodes - DIODE CIRCUITS LABORATORY A solid state diode consists of a junction of either dissimilar semiconductors pn junction diode or a

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