Diodes 1

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith2 Figure 3.1 The ideal diode: (a) diode circuit symbol; (b) i– v characteristic; (c) equivalent circuit in the reverse direction; (d) equivalent circuit in the forward direction.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith3 Figure 3.2 The two modes of operation of ideal diodes and the use of an external circuit to limit the forward current (a) and the reverse voltage (b).

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith4 Figure 3.3 (a) Rectifier circuit. (b) Input waveform. (c) Equivalent circuit when v I  0. (d) Equivalent circuit when v I  0. (e) Output waveform.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith5 Figure E3.1

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith7 Figure 3.4 Circuit and waveforms for Example 3.1. EXAMPLE 3.1

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith8 Figure 3.5 Diode logic gates: (a) OR gate; (b) AND gate (in a positive-logic system).

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith9 Figure 3.6 Circuits for Example 3.2. EXAMPLE 3.2

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith11 Figure E3.4

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith13 Figure 3.7 The i– v characteristic of a silicon junction diode.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith14 Figure 3.8 The diode i– v relationship with some scales expanded and others compressed in order to reveal details.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith15

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith16 EXAMPLE mA at 0.7 V Find I s for n=1 and 2

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith17 Figure 3.9 Illustrating the temperature dependence of the diode forward characteristic. At a constant current, the voltage drop decreases by approximately 2 mV for every 1  C increase in temperature. Forward bias Temperature Coefficient

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith18 Reverse bias I s doubles for every 10ºC temperature rise

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith19 Figure E3.9 At 20ºC, V=1 V V? at 40ºC and 0ºC

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith20 Figure 3.10 A simple circuit used to illustrate the analysis of circuits in which the diode is forward conducting.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith21 Figure 3.11 Graphical analysis of the circuit in Fig using the exponential diode model.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith22 EXAMPLE 3.4 V DD = 5 R=1 k  Diode 1 mA at 0.7 V V 2 -V 1 = 0.1 V for I 2 /I 1 =10

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith23 Figure 3.12 Approximating the diode forward characteristic with two straight lines: the piecewise-linear model.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith24 Figure 3.13 Piecewise-linear model of the diode forward characteristic and its equivalent circuit representation.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith25 Figure 3.14 The circuit of Fig with the diode replaced with its piecewise-linear model of Fig EXAMPLE 3.5 V D0 = 0.65 V r D = 20 

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith26 Figure 3.15 Development of the constant-voltage-drop model of the diode forward characteristics. A vertical straight line (B) is used to approximate the fast-rising exponential. Observe that this simple model predicts V D to within  0.1 V over the current range of 0.1 mA to 10 mA.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith27 Figure 3.16 The constant-voltage-drop model of the diode forward characteristics and its equivalent-circuit representation.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith28 Ideal Diode Model

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith29 Figure E3.12 Find R for V out =2.4 V Diode  V=0.1 V for 1 decade current change 0.7 V at 1 mA

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith30 Figure 3.17 Development of the diode small-signal model. Note that the numerical values shown are for a diode with n = 2. Small Signal Model

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith33 Figure 3.18 (a) Circuit for Example 3.6. (b) Circuit for calculating the dc operating point. (c) Small-signal equivalent circuit. EXAMPLE 3.6 V + = 10 V + sin (120  t) V R = 10 k  Diode: 1 mA at 0.7 V and n=2

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith34

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith35 Figure 3.19 Circuit for Example 3.7. EXAMPLE 3.7

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith36 Figure E3.16

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith39 Figure 3.20 Circuit symbol for a zener diode.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith40 Figure 3.21 The diode i– v characteristic with the breakdown region shown in some detail.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith41 Figure 3.22 Model for the zener diode.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith42 Figure 3.23 (a) Circuit for Example 3.8. (b) The circuit with the zener diode replaced with its equivalent circuit model. EXAMPLE 3.8 V z =6.8 V at I z = 5 mA, r z =20  I ZK =0.2 mA V o ? V o with V + changing  1V V o with I L =1 mA (a)V o with R L

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith43

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith44

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith45 Figure 3.24 Block diagram of a dc power supply.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith46

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith47 Figure 3.25 (a) Half-wave rectifier. (b) Equivalent circuit of the half-wave rectifier with the diode replaced with its battery-plus-resistance model. (c) Transfer characteristic of the rectifier circuit. (d) Input and output waveforms, assuming that r D ! R.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith48

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith49

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith50 Figure 3.26 Full-wave rectifier utilizing a transformer with a center-tapped secondary winding: (a) circuit; (b) transfer characteristic assuming a constant-voltage-drop model for the diodes; (c) input and output waveforms.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith51 Figure 3.27 The bridge rectifier: (a) circuit; (b) input and output waveforms.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith52

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith53 Figure 3.28 (a) A simple circuit used to illustrate the effect of a filter capacitor. (b) Input and output waveforms assuming an ideal diode. Note that the circuit provides a dc voltage equal to the peak of the input sine wave. The circuit is therefore known as a peak rectifier or a peak detector.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith54 Figure 3.29 Voltage and current waveforms in the peak rectifier circuit with T. The diode is assumed ideal.

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith56

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith57 EXAMPLE 3.9

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith58 Figure 3.30 Waveforms in the full-wave peak rectifier.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith59 Figure 3.31 The “superdiode” precision half-wave rectifier and its almost-ideal transfer characteristic. Note that when v I > 0 and the diode conducts, the op amp supplies the load current, and the source is conveniently buffered, an added advantage. Not shown are the op-amp power supplies.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith60 Figure 3.32 General transfer characteristic for a limiter circuit. Limiter = Clipper Double Limiter Cf. Single Limiter Hard limiter Cf. Soft Limiter

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith61 Figure 3.33 Applying a sine wave to a limiter can result in clipping off its two peaks.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith62 Figure 3.34 Soft limiting.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith63 Figure 3.35 A variety of basic limiting circuits.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith64 Figure E3.27

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith65 Figure 3.36 The clamped capacitor or dc restorer with a square-wave input and no load.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith66 Figure 3.37 The clamped capacitor with a load resistance R.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith67 Figure 3.38 Voltage doubler: (a) circuit; (b) waveform of the voltage across D 1.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith68 Figure 3.39 Simplified physical structure of the junction diode. (Actual geometries are given in Appendix A.)

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith69 Figure 3.40 Two-dimensional representation of the silicon crystal. The circles represent the inner core of silicon atoms, with +4 indicating its positive charge of +4q, which is neutralized by the charge of the four valence electrons. Observe how the covalent bonds are formed by sharing of the valence electrons. At 0 K, all bonds are intact and no free electrons are available for current conduction.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith70 Figure 3.41 At room temperature, some of the covalent bonds are broken by thermal ionization. Each broken bond gives rise to a free electron and a hole, both of which become available for current conduction.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith71

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith72 Figure 3.42 A bar of intrinsic silicon (a) in which the hole concentration profile shown in (b) has been created along the x-axis by some unspecified mechanism.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith73

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith74 Figure 3.43 A silicon crystal doped by a pentavalent element. Each dopant atom donates a free electron and is thus called a donor. The doped semiconductor becomes n type.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith75

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith76 Figure 3.44 A silicon crystal doped with a trivalent impurity. Each dopant atom gives rise to a hole, and the semiconductor becomes p type.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith77

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith78 Figure 3.45 (a) The pn junction with no applied voltage (open-circuited terminals). (b) The potential distribution along an axis perpendicular to the junction.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith79

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith80 Figure 3.46 The pn junction excited by a constant-current source I in the reverse direction. To avoid breakdown, I is kept smaller than I S. Note that the depletion layer widens and the barrier voltage increases by V R volts, which appears between the terminals as a reverse voltage.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith81 Figure 3.47 The charge stored on either side of the depletion layer as a function of the reverse voltage V R.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith82

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith84 Figure 3.48 The pn junction excited by a reverse-current source I, where I > I S. The junction breaks down, and a voltage V Z, with the polarity indicated, develops across the junction.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith85 Figure 3.49 The pn junction excited by a constant-current source supplying a current I in the forward direction. The depletion layer narrows and the barrier voltage decreases by V volts, which appears as an external voltage in the forward direction.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith86 Figure 3.50 Minority-carrier distribution in a forward-biased pn junction. It is assumed that the p region is more heavily doped than the n region; N N D.

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith93 Figure 3.51 The SPICE diode model.

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith95 Figure 3.52 Equivalent-circuit model used to simulate the zener diode in SPICE. Diode D 1 is ideal and can be approximated in SPICE by using a very small value for n (say n = 0.01).

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith96 Figure 3.53 Capture schematic of the 5-V dc power supply in Example 3.10.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith97

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith98 Figure 3.54 The voltage v C across the smoothing capacitor C and the voltage v O across the load resistor R load = 200  in the 5-V power supply of Example 3.10.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith99 Figure 3.55 The output-voltage waveform from the 5-V power supply (in Example 3.10) for various load resistances: R load = 500 , 250 , 200 , and 150 . The voltage regulation is lost at a load resistance of 150 .

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith100 Figure E3.35 (a) Capture schematic of the voltage-doubler circuit (in Exercise 3.35).

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith101 Figure E3.35 (Continued) (b) Various voltage waveforms in the voltage-doubler circuit. The top graph displays the input sine-wave voltage signal, the middle graph displays the voltage across diode D 1, and the bottom graph displays the voltage that appears at the output.

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith102 Figure P3.2

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith103 Figure P3.3

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith104 Figure P3.4 (Continued)

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith105 Figure P3.4 (Continued)

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith106 Figure P3.5

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith107 Figure P3.6

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith127 Figure P3.102

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Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith129 Figure P3.105

Copyright  2004 by Oxford University Press, Inc. Microelectronic Circuits - Fifth Edition Sedra/Smith130 Figure P3.108