# 17 figure 241 shows two diodes with reverse

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17. Figure 2.41 shows two diodes with reverse saturation currents of and placed in series. 1 D 1 D V D2 I B V D1 V B Figure 2.41 Calculate , , and in terms of , , and . 18. In the circuit of Problem 17, we wish to increase by a factor of 10. What is the required change in ? 19. Consider the circuit shown in Fig. 2.42, where A. Calculate and for V X 1 D R 1 2 k I X Figure 2.42 V, 0.8 V, 1 V, and 1.2 V. Note that changes little for V. 20. In the circuit of Fig. 2.42, the cross section area of is increased by a factor of 10. Determine and for V and 1.2 V. Compare the results with those obtained in Problem 19.

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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 58 (1) 58 Chap. 2 Basic Physics of Semiconductors 21. Suppose in Fig. 2.42 must sustain a voltage of 850 mV for V. Calculate the required . 22. For what value of in Fig. 2.42, does sustain a voltage equal to ? Assume A. 23. We have received the circuit shown in Fig. 2.43 and wish to determine and . We note V X 1 D I X R 1 Figure 2.43 that mA and mA. Calculate and . 24. Figure 2.44 depicts a parallel resistor-diode combination. If A, calculate for mA, 2 mA, and 4 mA. 1 D R 1 1 k I X Figure 2.44 25. In the circuit of Fig. 2.44, we wish to carry a current of 0.5 mA for mA. Determine the required . 26. For what value of in Fig. 2.44, does carry a current equal to ? Assume A. 27. We have received the circuit shown in Fig. 2.45 and wish to determine and . Measure- 1 D R 1 I X V X Figure 2.45 ments indicate that V and V. Calculate and . 28. The circuit illustrated in Fig. 2.46 employs two identical diodes with A. 1 D R 1 I X D 2 2 k Figure 2.46 Calculate the voltage across for mA. 29. In the circuit of Fig. 2.47, determine the value of such that this resistor carries 0.5 mA.
BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 59 (1) Sec. 2.4 Chapter Summary 59 1 D R 1 I X D 2 1 mA Figure 2.47 Assume A for each diode. 30. Sketch as a function of for the circuit shown in Fig. 2.48. Assume (a) a constant- voltage model, (b) an exponential model. 1 D R 1 I X V X Figure 2.48 SPICE Problems In the following problems, assume A. 31. For the circuit shown in Fig. 2.49, plot as a function of . Assume varies from 0 to 2mA. 1 D I V out in Figure 2.49 32. Repeat Problem 31 for the circuit depicted in Fig. 2.50, where k . At what value of are the currents flowing through and equal? 1 D I in R 1 V out Figure 2.50 33. Using SPICE, determine the value of in Fig. 2.50 such that carries 1 mA if mA. 34. In the circuit of Fig. 2.51, . Plot as a function of if varies from V to V. At what value of are the voltage drops across and equal? V in 1 D R 1 out V Figure 2.51

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BR Wiley/Razavi/ Fundamentals of Microelectronics [Razavi.cls v. 2006] June 30, 2007 at 13:42 60 (1) 60 Chap. 2 Basic Physics of Semiconductors 35. In the circuit of Fig. 2.51, use SPICE to select the value of such that V for V. We say the circuit “limits” the output.
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