Supplementary Reader II EECS 40 Introduction to Microelectronic Circuits Prof. C. Chang-Hasnain Fall 2006EE 40, University of California Berkeley Professor Chang-Hasnain i Table of Contents Chapter 2. Diode Circuits......................................................................................................................... 1 2.1 Physical Behavior of Diodes ....................................................................................................... 1 2.2 Solving Diode Circuits ................................................................................................................ 2 2.2.1 Proof by Contradiction Approach........................................................................................... 2 2.3 Load Line Analysis ..................................................................................................................... 2 2.4 Zener Diodes ............................................................................................................................... 3 2.5 Applications for Diodes............................................................................................................... 4 2.5.1 Clipper Circuit (a.k.a. Limiter Circuit) ................................................................................... 4 2.5.2 Level Shift Circuit .................................................................................................................. 6 2.5.3 Clamping Circuit (a.k.a. DC Restorer) ................................................................................... 7 2.5.4 Rectifier Circuit ...................................................................................................................... 7 2.5.5 Peak Detector........................................................................................................................ 11 2.5.6 Voltage Doubler Circuit ....................................................................................................... 12 2.5.7 Diode Logic Gates ................................................................................................................ 13 Acknowledgements We thank the writing assistance of Abhinav Gupta, Kevin Wang, Henry Wang, and Wendy Xiao-Xue Zhao, and detailed editing by Isaac Seetho, Yu Ben, Timothy C Loo, Jia Zou, and Michael Krishnan.EE 40, University of California Berkeley Professor Chang-Hasnain 1 Chapter 2. Diode Circuits 2.1 Physical Behavior of Diodes A diode is a simple two terminal device. The two terminals are labeled anode (positive side) and cathode (negative side). The diode symbol used in a circuit is shown in Fig. 1a, with the definition of the plus and minus directions of voltage and current. A positive voltage applied to the diode is referred as a forward bias and negative, a reverse bias. In this part of the course, we will introduce three models to describe the current-voltage (I-V) characteristics of a diode: the ideal diode model, a simple piecewise model and the Shockley equation. An ideal diode has only two modes of operation: off and on, as shown in Fig. 1b. When the diode is “off”, it passes through no current but the voltage can be any value less than zero. It behaves like an open circuit. When the diode is “on”, the voltage is clamped at zero while its current can be any positive value. Hence, it acts like a short circuit. AnodeCathode+—DvDiAnodeCathode+—DvDi DvDiDiode offDiode onDvDiDiode offDiode on DvDithVDvDithV Fig. 1a circuit symbol of a diode (same as Hambley 10.1a) Fig. 1b Ideal diode current-voltage characteristics (on and off states are labeled) Fig. 1c Simple piecewise model (threshold voltage labeled) DiDvSIDiDvSI BDVDiDvSIBDVDiDvSI Fig. 1d Schockley equation Fig. 1e real diode with breakdown voltage The simple piecewise model is similar to the ideal diode model with “off” and “on” states being open and short circuit, respectively. The only difference is the inclusion of a threshold voltage. As shown in Fig. 1c, when the diode is biased below a certain threshold voltage, it is “off”, i.e. the current passing through the diode is zero. When the diode is “on”, voltage is clamped at this threshold voltage while current can be any positive value depending on the rest of the circuit. In EE 40, this threshold voltage is set to be 0.7 V. Note, this number is just a matter of convention and not based on fundamental physical laws. In this set of notes, this simple piecewise model is slightly different from the model used in section 10.5 of Hambley. This model is a simpler version of the piecewise-linear model described in 10.5, hence the name “simple piecewise model.” Shockley Equation model, shown by Fig. 1d, is more accurate than the first two. With forward bias, the current increases exponentially with voltage. For reverse bias, the current is negative and saturates at a saturation current. Since it is difficult to use this model to reach analytical solutions, for the circuits in this chapter, we use the ideal and simple piecewise model. None of the models describe what happens when the voltage bias becomes a large but negative value, which is known as reserve bias breakdown voltage, for example, as in Zener diodes (Hambley 10.3). In the circuit analysis part of this course, we will simply add a reverse voltage as in 10.3. The physics behind theEE 40, University of California Berkeley Professor Chang-Hasnain 2 reverse breakdown phenomenon and the Shockley equation will be briefly discussed in the next Chapter. However, you will not see more detailed discussion until EE 105 and EE130, which I hope you will take next year. Note, no matter which model you use, a diode has its I-V curve passes through the origin, i.e. with zero voltage there should be no current flow. The only exception is when unless there is an external source to generate electrons, e.g. in the case of sun light or laser beam shinning on a photodiode. In general, if we did not specifically mention photo-generation of electrons, zero voltage bias across a diode leads to zero current. Do not lose this “common
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