Lecture Electrical Engineering: Lecture 9 - Dr. Nasim Zafar

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COMSATS Institute of Information Technology Virtual campus Islamabad Dr. Nasim Zafar Electronics 1 EEE 231 – BS Electrical Engineering Fall Semester – 2012 The Diode Circuits: Lecture No: 9 Contents:  Introduction.  The Ideal Diode.  Terminal Characteristics of Junction Diodes.  Modeling the Diode Forward Characteristics.  Load Line Analysis Dr. Nasim Zafar 2 References:  Microelectronic Circuits: Adel S. Sedra and Kenneth C. Smith.  Electronic Devices and Circuit Theory: Robert Boylestad & Louis Nashelsky ( Prentice Hall )  Electronic Devices : Thomas L. Floyd ( Prentice Hall ) Dr. Nasim Zafar 3 Introduction:  The simplest and most fundamental nonlinear circuit element is the diode.  Just like a resistor, the diode has two terminals; but unlike the resistor , which has a linear (straight-line) relationship between the current flowing through it and the voltage appearing across it, the diode has a nonlinear i-v characteristic.  Let us discuss an ideal diode in order to understand the essence of the diode function.  We can then study the real silicon p-n junction diode and explain its current-voltage characteristics. Dr. Nasim Zafar 4 Introduction: Applications of the Diode:  One of the important application of a diode is their use in the design of the rectifiers, which converts an ac signal into a dc signal.  We will also briefly discuss some other specialized diodes such as the light emitting diodes LED’s and photodiodes. Dr. Nasim Zafar 5 Diode Equation and Models:  In this lecture we will discuss some models for the operation and design of the diode to explain diode characteristics.  We can use these models instead of the diode equation in circuit analysis.  Later on, we will be developing similar models (or equivalent circuits) to represent the behaviour of transistors when they are used as linear amplifiers. Dr. Nasim Zafar 6 Modeling The Diode:  The Ideal Diode Model  The Exponential Model  Load Line Analysis  Piecewise-Linear Model The Diode Models 1. The Ideal Diode Model The Diode: P-N Junction Diode Schematic Symbol: Anode Cathode p Dr. Nasim Zafar n 9 Diode Circuits: anode Reversed bias + - + - Forward bias cathode The left hand diagram shows the reverse biased junction. No current flows flows. The other diagram shows forward biased junction. A current flows. Dr. Nasim Zafar 10 Forward-biased diode Circuit: R R IF > 0A IF > 0A IF V IF V +V -V R R IF IF Dr. Nasim Zafar 11 Reverse-biased diode Circuit: R R V 0A 0A IT IT V +V -V R R Dr. Nasim Zafar 12 The Ideal Diode Model: Current-Voltage Characteristic: The ideal diode the most fundamental nonlinear circuit element. Useful for circuits with more than one diode IF Forward operating region II I VR VF III IV Reverse operating region IR Zafar Dr. Nasim 13 I-V Characteristics of an Ideal Diode V V    I  0 R  0  I   R R R  If the voltage across anode and cathode is greater than zero, the resistance of an ideal diode is zero and current becomes infinite.  However, if the voltage is less than zero, the resistance becomes infinite and current is zero. Dr. Nasim Zafar 14 Effect of VF. I VS 5V 4.3 V VD1 0.7V R1 1 k VR1 VS  VD1 5V  0.7V 4.3V VR1 4.3V I  4.3mA R1 1kΩ D1 Value VF Ideal 0V Practical 0.7 V VR1 5V 4.3 V I 5 mA 4.3 mA Dr. Nasim Zafar 15 Two Modes of Operation: On or Off Forward Biased Diode:  If a positive voltage is applied to an ideal diode, zero voltage drop appears across the diode and it behaves as a short circuit. Diodes operated in this mode are called forward biased.  Current must flow in the forward biased diode. A forwardbiased diode is said to be turned-on or simply “on”. Dr. Nasim Zafar 16 Two Modes of Operation: On or Off Reverse Biased Diode:  When a negative voltage is applied to the p-side of a diode, no current flows and the diode behaves as an open circuit. Diodes operated in this mode are called reverse biased diodes.  An ideal diode has zero current in the reverse biased mode and is said to be cut-off or off. Dr. Nasim Zafar 17 The Ideal Diode Model:  Consider the two modes of operation for an ideal diode, either “on” or “off”. It acts as a switch since: I  In the ON state it is short circuit.  In the OFF state it is open circuit. Dr. Nasim Zafar OFF ON V 18 Ideal diode characteristics: Forward bias Biasing polarities (+) Reverse Bias (-) (-) (+) IF Equivalent switch state ON OFF Device resistance Zero Infinite Device current A-to-K voltage A-to-K current determined by external resistance and voltage Zero Dr. Nasim Zafar Zero Equal to the applied voltage 19 Terminal Characteristics of Junction Diodes The Forward-Bias Region, V ≥ 0 The Reversed-Bias Region, V ≤ 0 Current-Voltage Characteristic: Ideal Diode Real Diode  Positive voltage yields finite current  Negative voltage yields zero current Dr. Nasim Zafar 21 Diode Voltages: To forward bias a diode, the anode must be more positive than the cathode or LESS NEGATIVE. To reverse bias a diode, the anode must be less positive than the cathode or MORE NEGATIVE. A conducting diode has about 0.6 volts across if silicon, 0.3 volts if germanium. Dr. Nasim Zafar 22 Example 1- Forward Biased: I R1 1 k VS 5V D1 Dr. Nasim Zafar 23 Example 2-Reverse Biased: I R1 1 k VS 5V D1 Dr. Nasim Zafar 24 Diode i-v Characteristics: (Ref No. 3) less than 1mA at 300K Vknee Dr. Nasim Zafar 25 The Diode Models 2. The Exponential Model The Exponential Model Current-Voltage Characteristic: The general equation linking the diode current I to the applied voltage V is:   eV   I Io  exp    nkT   VT ~ 26 mV Dr. Nasim Zafar 27 The Exponential Model: I-V Characteristic of a PN Junction:  Current increases exponentially with applied forward bias, and “saturates” at a relatively small negative current level for reverse bias in a p-n junction. Dr. Nasim Zafar 28 The Diode Models 3. The Load Line Analysis The Load Line Analysis of the Diode Circuit: Graphical Analysis:  Another important concept, that we will need for the transistor analysis, is that of the “Load Line” for a non-linear device.  Graphical analysis is performed by plotting the diode currents (exponential model) and the voltages in a diode circuit on the i-v plane.   eV   I Io  exp  nkT    V  Ri  v ss D D Dr. Nasim Zafar 30 The Load Line Analysis:  A sketch of the graphical construction is shown in the next slide.  The curve represents the exponential diode equation and the straight line represent the diode equation obtained from the Kirchoff loop equation. Such a straight line is know as the Load Line.  The load line intersects the diode I-V curve about some operating point of the circuit. This point is also known as the ‘Q’ or quiescent point.  Co-ordinates of Q-point give the values ID, VD. Dr. Nasim Zafar 31 Load Line Analysis: V  Ri  v ss D D VSS/R Slope=-1/R VSS Dr. Nasim Zafar 32 Analysis of Diode Circuit: + Thevenin equivalent KCL + Vo - KVL io vD - iD V o v D io  iD Dr. Nasim Zafar Their characteristics intersect 33 Analysis of Diode Circuit: V  Ri  v ss D D VSS/R Slope=-1/R VD Dr. Nasim Zafar 34 Load-Line Analysis: (Solve a Problem) If the circuit shown below has: Vss=2V and R=1kW. Find the diode voltage and current at the operating point. V  Ri  v ss D D Repeat for: Vss=10V and R=10k W VDQ=0.68V and iDQ=0.93mA Dr. Nasim Zafar 35 Summary Dr. Nasim Zafar 36
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