Diodes ELZ 206 - Elektronik I Microelectronic Circuits Fourth Edition Adel S. Sedra, Kenneth C. Smith, 1998 Oxford University Press Department of Electrical and Electronics Engineering Dicle University The ideal diode diode circuit symbol i v characteristic equivalent circuit in the reverse direction equivalent circuit in the forward direction 1
The ideal diode - Example The two modes of operation of ideal diodes and the use of an external circuit to limit the forward current and the reverse voltage. Microelectronic Circuits - Fifth Edition Sedra/Smith Rectifier circuit Input waveform Equivalent circuit when v I 0 Equivalent circuit when v I 0 Output waveform 2
Rectifier circuit Transfer characteristic Output waveform Input Input waveform Rectifier circuit v D waveform? Input waveform v D waveform Microelectronic Circuits - Fifth Edition Sedra/Smith 3
Example v D waveform v D waveform Microelectronic Circuits - Fifth Edition Sedra/Smith Example Diode logic gates OR gate AND gate Microelectronic Circuits - Fifth Edition Sedra/Smith 4
Example # D 1, D 2 : ideal V =? I =? Microelectronic Circuits - Fifth Edition Sedra/Smith Example # # D 1, D 2 : ideal V =? I =? Microelectronic Circuits - Fifth Edition Sedra/Smith 5
The i v characteristic of a silicon junction diode. Microelectronic Circuits - Fifth Edition Sedra/Smith The diode i v relationship with some scales expanded and others compressed in order to reveal details. 6
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. Microelectronic Circuits - Fifth Edition Sedra/Smith Physical Operation of Diodes Simplified physical structure of the junction diode. Microelectronic Circuits - Fifth Edition Sedra/Smith 7
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. 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. 8
Diffusion and Drift 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. n type semiconductor A silicon crystal doped by a pentavalent element. Each dopant atom donates a free electron and is thus called a donor. The doped semiconductor 2004 by Oxford becomes University Press, n Inc. type. 9
p type semiconductor A silicon crystal doped with a trivalent impurity. Each dopant atom gives rise to a hole, and the semiconductor becomes p type. Analysis of Diode Circuits A simple circuit used to illustrate the analysis of circuits in which the diode is forward conducting. For solution Graphical analysis Iterative analysis 10
Graphical Analysis Example I D, V D =? V DD =5V R=1k ohms Assuming that I D =1mA at V D =0.7V and its voltage drop changes 0.1V for every decade change in current. 11
Simplified diode models Approximating the diode forward characteristic with two straight lines: the piecewise-linear model. Piecewise-linear model of the diode forward characteristic and its equivalent circuit representation. 12
Example I D, V D =? V DD =5V R=1k ohms Use piecewise linear model (V D0 =0.65V, r D =20 ohms) Example - solution 13
Simpler model of the diode for forward region 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. The constant-voltage-drop model of the diode forward characteristics and its equivalent-circuit representation. 14
Example I D =? V DD = 5V R = 1k ohms Use simpler piecewise linear model (V D0 =0.65V) Modeling the Diode Forward Characteristic 15
Modeling the Diode Forward Characteristic The small-signal model and its application Development of the diode small-signal model. Note that the numerical values shown are for a diode with n = 2. 16
Application The small-signal approximation allows one to seperate the dc analysis from the signal analysis Example V + = 10V (DC) + 1V peak amplitude sinus (AC) I D =? r d =? v d (peak-to-peak) =? Assume that V D =0.7V I D =1mA, n=2 17
Example - solution Circuit for Example Circuit for calculating the dc operating point. Small-signal equivalent circuit Example A string of diodes is used to provide a constant voltage of about 2.1V. n=2, V T = 25mV a) Diode current (no load) b) r d =? c) Input (10±1 V) Output (2.1±? V) d) Calculate the change of output voltage v o =? (with load) Microelectronic Circuits - Fifth Edition Sedra/Smith 18
Example A string of diodes is used to provide a constant voltage of about 2.8V. n=2, V T = 25mV a) Diode current b) r d =? c) Input (15±1 V) Output (2.8±? V) Operation in the reverse breakdown region zener diode Circuit symbol for a zener diode. The diode i v characteristic with the breakdown region shown in some detail. 19
Example Example 6.8V zener diode is specified to have V z =6.8V at I z =5mA. r z =20 ohms and I ZK =0.2mA. a) Vo =? (no load) b) Vo =? for a ±1V change in V + (no load) c) RL=2k ohms, Vo =? d) RL=0.5k ohms, Vo =? e) R Lmin =? For which the diode still operates in breakdown region. 20
Example - solution Circuit for Example The circuit with the zener diode replaced with its equivalent circuit model. Design of the zener shunt regulator 21
Example It is required to design a zener shunt regulator to provide an output voltage of approximately 7.5V The raw supply varies between 15 and 25V The load current varies over the range 0 to 15mA. (V z =7.5V at a current I z =20mA and r z =10 ohms) a) R=? b) Line and load regulation =? c) Vo =? for full change in V s. d) Vo =? for full change in I L. Example 22
Example Rectifier Circuits Block diagram of a dc power supply 23
The Half-Wave Rectifier Full-Wave Rectifier Transfer characteristic assuming a constant-voltage-drop model for the diodes Full-wave rectifier utilizing a transformer with a center-tapped secondary winding 24
The Bridge Rectifier The rectifier with a filter capacitor A simple circuit used to illustrate the effect of a filter capacitor. Diodes: ideal The circuit provides a dc voltage equal to the peak of the input sine wave. 25
The rectifier with a filter capacitor - more practical situation - Voltage and current waveforms in the peak rectifier circuit with CR @ T. The diode is assumed ideal. Limiter Circuits General transfer characteristic for a limiter circuit. Applying a sine wave to a limiter can result in clipping off its two peaks. 26
Limiter Circuits Soft limiting. Limiter Circuits A variety of basic limiting circuits. 27