Supplementary Reader II EECS 40 Introduction to Microelectronic Circuits Prof C Chang Hasnain Fall 2006 EE 40 University of California Berkeley Professor Chang Hasnain Table of Contents Chapter 2 2 1 2 2 2 2 1 2 3 2 4 2 5 2 5 1 2 5 2 2 5 3 2 5 4 2 5 5 2 5 6 2 5 7 Diode Circuits 1 Physical Behavior of Diodes 1 Solving Diode Circuits 2 Proof by Contradiction Approach 2 Load Line Analysis 2 Zener Diodes 3 Applications for Diodes 4 Clipper Circuit a k a Limiter Circuit 4 Level Shift Circuit 6 Clamping Circuit a k a DC Restorer 7 Rectifier Circuit 7 Peak Detector 11 Voltage Doubler Circuit 12 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 i EE 40 University of California Berkeley Professor Chang Hasnain 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 iD iD iD Diode on Cathode Anode vD vD Diode off Fig 1a circuit symbol of a diode same as Hambley 10 1a Vth vD Fig 1b Ideal diode currentvoltage characteristics on and off states are labeled Fig 1c Simple piecewise model threshold voltage labeled iD iD VBD IS vD IS Fig 1d Schockley equation vD 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 the 1 EE 40 University of California Berkeley Professor Chang Hasnain 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 sense when dealing with diode problems I 0 V 0 2 2 Solving Diode Circuits 2 2 1 Proof by Contradiction Approach The proof by contradiction method uses guess and check also discussed in the text book For each diode in the circuit guess an on or off state and replace it with the corresponding model open circuit for off and 0 7 volt source for on If diode is assumed on the current should flow into the positive terminal of the diode If diode is assumed off the voltage should be negative across the diode i e reverse biased We apply the contra positive of the second bullet above to contradict our guess If A B then Not B Not A In other words if we solve the circuit and find a negative current through an on diode or a voltage greater than 0 7V across an off diode we guessed the wrong states 2 3 Load Line Analysis Load line analysis is simply about finding the intersection point based on the physical characteristics of a device the load the physical characteristics of a driving circuit based on fundamental laws such as KCL and KVL For our present discussion the diode is our load but this powerful technique can be applied to other loads as well From the physical behavior of a diode we know that a diode behaves in a certain way This presents an I V characteristic based on the model we choose When this diode is placed in a simple circuit the circuit will also want to dictate the way it behaves In particular by KVL the driving circuit and the load must see the same voltage at their interface In addition the current out of the driving circuit must match the current into the load For the circuit to be consistent both of these constraints must be satisfied Therefore we find the intersection of the curves representing the physical characteristic of a diode and the curve representing the manner in which the diode will behave when placed in a circuit the load s IV curve or load line We call this intersection point the operating point because it is the point at which the device will operate The physical characteristic of a diode or other device is sometimes represented …
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