Since an ideal diode behaves as a short or an open we

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Since an ideal diode behaves as a short or an open, we first construct the I/V characteristics for two special cases of Ohm’s law: (3.1) (3.2) The results are illustrated in Fig. 3.5(a). For an ideal diode, we combine the positive-voltage region of the first with the negative-voltage region of the second, arriving at the charac- teristic in Fig. 3.5(b). Here, , and is defined as the current flowing into the anode and out of the cathode. V D I D V I (a) V I R = 0 R = Reverse Bias Bias Forward (b) Figure 3.5 I/V characteristics of (a) zero and infinite resistors, (b) ideal diode. Example 3.2 We said that an ideal diode turns on for positive anode-cathode voltages. But the characteristic in Fig. 3.5(b) does not appear to show any values for . How do we interpret this plot? Solution This characteristic indicates that as exceeds zero by a very small amount, then the diode turns on and conducts infinite current if the circuit surrounding the diode can provide such a current. Thus, in circuits containing only finite currents, a forward-biased ideal diode sustains a zero voltage—similar to a short circuit. Exercise How is the characteristic modified if we place a 1- resistor in series with the diode? Example 3.3 Plot the I/V characteristic for the “antiparallel” diodes shown in Fig. 3.6(a). Solution If , is on and is off, yielding . If , is off, but is on, again leading to . The result is illustrated in Fig. 3.6(b). The antiparallel combination therefore acts as a short for all voltages. Seemingly a useless circuit, this topology becomes much more interesting with actual diodes (Section 3.5.3).
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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 65 (1) Sec. 3.1 Ideal Diode 65 1 D D 2 V A I A V A I A (a) (b) Figure 3.6 (a) Antiparallel diodes, (b) resulting I/V characteristic. Exercise Repeat the above example if a 1-V battery is placed is series with the parallel combination of the diodes. Example 3.4 Plot the I/V characteristic for the diode-resistor combination of Fig. 3.7(a). V I A A 1 D R 1 1 D R 1 I A R 1 1 D R 1 R 1 I A V A I A R 1 1 V I A A R 1 (c) (a) (b) (d) (e) 1 D Figure 3.7 (a) Diode-resistor series combination, (b) equivalent circuit under forward bias, (c) equivalent circuit under reverse bias, (d) I/V characteristic, (e) equivalent circuit if is on. Solution We surmise that, if , the diode is on [Fig. 3.7(b)] and because for an ideal diode. On the other hand, if , is probably off [Fig. 3.7(c)] and . Figure 3.7(d) plots the resulting I/V characteristic. The above observations are based on guesswork. Let us study the circuit more rigorously. We begin with , postulating that the diode is off. To confirm the validity of this guess, let us assume is on and see if we reach a conflicting result. If is on, the circuit is reduced to that in Fig. 3.7(e), and if is negative, so is ; i.e., the actual current flows from right to left. But this implies that carries a current from its cathode to its anode, violating the definition of the diode. Thus, for , remains off and .
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