Pulse and Digital Circuits

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1 Second Edition Pulse and Digital Circuits A. Anand Kumar

2 PULSE AND DIGITAL CIRCUITS SECOND EDITION A. ANAND KUMAR Dean K.L. University Vijayawada, Andhra Pradesh New Delhi

3 PULSE AND DIGITAL CIRCUITS, 2nd ed. A. Anand Kumar 2008 by PHI Learning Private Limited, New Delhi. All rights reserved. No part of this book may be reproduced in any form, by mimeograph or any other means, without permission in writing from the publisher. ISBN The export rights of this book are vested solely with the publisher. Fifteenth Printing (Second Edition) January, 20 Published by Asoke K. Ghosh, PHI Learning Private Limited, M-97, Connaught Circus, New Delhi-000 and Printed by Jay Print Pack Private Limited, New Delhi-005.

4 To the memory of my parents Shri A. Nagabhushanam and Smt. A. Ushamani (Freedom Fighters)

5 Contents Preface. LINEAR WAVE SHAPING 03. The Low-Pass RC Circuit.. Sinusoidal Input 2..2 Step-Voltage Input 3..3 Pulse Input 5..4 Square-Wave Input 7..5 Ramp Input 9..6 Exponential Input.2 The Low-Pass RC Circuit as an Integrator 2.3 The High-Pass RC Circuit 3.3. Sinusoidal Input Step Input Pulse Input Square-Wave Input Ramp Input Exponential Input 39.4 The High-Pass RC Circuit as a Differentiator 42.5 Double Differentiation 43.6 Attenuators Application of Attenuator as a CRO Probe 69.7 RL Circuits 76.8 RLC Circuits RLC Series Circuit RLC Parallel Circuit 79.9 Ringing Circuit 80 Short Questions and Answers 8 Review Questions 88 Fill in the Blanks 89 Objective Type Questions 9 Problems 00 v xi

6 vi Contents 2. NONLINEAR WAVE SHAPING Clipping Circuits Diode Clippers Shunt Clippers Series Clippers Clipping at Two Independent Levels Series and Shunt Noise Clippers Compensation for Variation of Temperature Transistor Clippers Emitter-Coupled Clipper Comparators Clamping Circuits The Clamping Operation Negative Clamper Positive Clamper Biased Clamping Clamping Circuit Taking Source and Diode Resistances into Account Clamping Circuit Theorem Practical Clamping Circuit Effect of Diode Characteristics on Clamping Voltage Synchronized Clamping Design of a Clamping Circuit 75 Short Questions and Answers 8 Review Questions 85 Fill in the Blanks 86 Objective Type Questions 88 Problems 9 3. SWITCHING CHARACTERISTICS OF DEVICES Junction Diode Switching Times Piece-Wise Linear Diode Characteristics Breakdown in p-n Junction Diodes Transistor as a Switch Transistor Switching Times Breakdown Voltages of a Transistor The Transistor Switch in Saturation Temperature Sensitivity of Saturation Parameters Design of Transistor Switch 209 Short Questions and Answers 26 Review Questions 29 Fill in the Blanks 29 Objective Type Questions 22

7 Contents vii 4. MULTIVIBRATORS Bistable Multivibrator A Fixed-Bias Bistable Multivibrator A Self-Biased Transistor Binary Commutating Capacitors A Non-Saturating Binary Triggering the Binary Triggering Unsymmetrically through a Unilateral Device (Diode) Triggering Symmetrically through a Unilateral Device A Direct-Connected Binary The Emitter-Coupled Binary (the Schmitt Trigger Circuit) Monostable Multivibrator The Collector Coupled Monostable Multivibrator The Emitter-Coupled Monostable Multivibrator Triggering the Monostable Multivibrator Astable Multivibrator The Collector-Coupled Astable Multivibrator The Emitter-Coupled Astable Multivibrator 32 Short Questions and Answers 36 Review Questions 322 Fill in the Blanks 323 Objective Type Questions 325 Problems TIME-BASE GENERATORS General Features of a Time-Base Signal Methods of Generating a Time-Base Waveform Exponential Sweep Circuit Unijunction Transistor Sweep Circuit Using UJT Sweep Circuit Using a Transistor Switch A Transistor Constant-Current Sweep Miller and Bootstrap Time-Base Generators Basic Principles The Transistor Miller Time-Base Generator The Transistor Bootstrap Time-Base Generator Current Time-Base Generators A Simple Current Sweep Linearity Correction Through Adjustment of Driving Waveform A Transistor Current Time-Base Generator 377 Short Questions and Answers 380 Review Questions 383 Fill in the Blanks 384 Objective Type Questions 385 Problems 387

8 viii Contents 6. SYNCHRONIZATION AND FREQUENCY DIVISION Pulse Synchronization of Relaxation Devices Frequency Division in the Sweep Circuit Other Astable Relaxation Circuits Monostable Relaxation Circuits as Dividers Phase Delay and Phase Jitters Synchronization of a Sweep Circuit with Symmetrical Signals Sine Wave Frequency Division with a Sweep Circuit 40 Short Questions and Answers 402 Review Questions 404 Fill in the Blanks 404 Objective Type Questions SAMPLING GATES Basic Operating Principles of Sampling Gates Unidirectional Diode Gate Unidirectional Diode Gates to Accommodate More than One Input Signal Bidirectional Sampling Gates Using Transistors Reduction of Pedestal in a Gate Circuit Bidirectional Diode Sampling Gate Four-Diode Sampling Gate Four-Diode Gate (Alternative Form) Six-Diode Sampling Gate Applications of Sampling Gates Chopper Amplifier Sampling Scope 426 Short Questions and Answers 427 Review Questions 429 Fill in the Blanks 429 Objective Type Questions LOGIC GATES The Basic Gates The OR Gate The AND Gate The NOT Gate (Inverter) The Universal Gates The NAND Gate The NOR Gate The Derived Gates The Exclusive-OR (X-OR) Gate The Exclusive-NOR (X-NOR) Gate 447

9 Contents ix 8.4 Inhibit Circuits Pulsed Operation of Logic Gates 453 Short Questions and Answers 456 Review Questions 459 Fill in the Blanks 460 Objective Type Questions 46 Problems LOGIC FAMILIES Digital IC Specification Terminology Logic Families Transistor Transistor Logic (TTL) Two-Input TTL NAND Gate (Standard TTL) Totem-Pole Output Open-Collector Gates Tri-State (3-State) TTL Schottky TTL Integrated Injection Logic (IIL or I 2 L) I 2 L Inverter I 2 L NAND Gate I 2 L NOR Gate Emitter-Coupled Logic (ECL) ECL OR/NOR Gate Metal Oxide Semiconductor (MOS) Logic Symbols and Switching Action of NMOS and PMOS Resistor NMOS Inverter NMOS NAND Gate NMOS NOR Gate Complementary Metal Oxide Semiconductor (CMOS) Logic CMOS Inverter CMOS NAND Gate CMOS NOR Gate Transmission Gate Dynamic MOS Logic Dynamic MOS Inverter Dynamic MOS NAND Gate Dynamic MOS NOR Gate 486 Short Questions and Answers 487 Review Questions 49 Fill in the Blanks 492 Objective Type Questions 493

10 x Contents 0. BLOCKING OSCILLATORS Monostable Blocking Oscillator (Base Timing) Monostable Blocking Oscillator (Emitter Timing) Astable Blocking Oscillator (Diode Controlled) Astable Blocking Oscillator (RC Controlled) Applications of Blocking Oscillators 53 Short Questions and Answers 53 Review Questions 55 Fill in the Blanks 56 Objective Type Questions 57 Problems 57 GLOSSARY ANSWERS TO FILL IN THE BLANKS ANSWERS TO OBJECTIVE TYPE QUESTIONS ANSWERS TO PROBLEMS INDEX

11 Preface After nearly thirty years of my experience in the classroom, I have strived to develop this comprehensive text on pulse circuitry in order to provide students with a solid grounding in the foundations of analysis and design of pulse and digital circuits. The second edition of this textbook with various new features is suitable for use as one-semester course material by undergraduate students of Electronics and Communication Engineering, Electrical and Electronics Engineering, Electronics and Instrumentation Engineering, and Telecommunication Engineering. Appropriate for self-study, the book will also be useful to AMIE and IETE students. The text is organized into 0 chapters. The outline of the book is as follows. When non-sinusoidal signals are transmitted through a linear network, the shape of the waveform undergoes a change. This process called linear wave shaping is discussed in Chapter. Particularly in communication systems, quite often, it is required to remove a part of the waveform above or below some reference level. This process is called clipping. In many pulse systems, quite often a dc level is required to be added to a waveform to fix the top or bottom of the waveform at some reference level. This process is called clamping. Clipping and clamping together is called nonlinear wave shaping. Chapter 2 deals with the various clipping and clamping circuits. The switching characteristics of junction diodes and transistors as required for a clear understanding of the pulse circuits are covered in Chapter 3. Memory is the basic requirement of all computers. The basic memory element is a flipflop, i.e. the bistable multivibrator. The monostable multivibrator is the basic gating circuit. The astable multivibrator is used as a master oscillator, and the Schmitt trigger circuit as a basic voltage comparator. The various types of multivibrators are discussed in Chapter 4. Time-base generators are essential for display of signals on the screen. Voltage and current time-base generators are presented in Chapter 5. A large pulse and digital system consists of a number of waveform generators which need to be synchronized with or without frequency division. Synchronization and frequency division of various generators with pulse type as well as symmetrical signals are the topics treated in Chapter 6. xi

12 xii Preface When signals are to be transmitted only for specified intervals of time and are to be blocked during other intervals of time, we require sampling gates. Various types of sampling gates are explained in Chapter 7. Logic gates are the fundamental building blocks of any digital system. Realization of logic gates using diodes and transistors is discussed in Chapter 8. Most of the logic gates, flip-flops, counters, shift registers, arithmetic circuits, encoders, decoders, etc. are available in several digital families. The TTL, ECL, IIL, MOS and CMOS class of logic families are discussed in Chapter 9. When pulses of very large peak power are to be generated, we require blocking oscillators. Several types of monostable and astable blocking oscillators are discussed in Chapter 0. A large number of design examples have been worked out to help students understand each new concept or analysis method as it is introduced. Extensive short questions and answers and also review questions are included at the end of each chapter to enable the students to prepare for examinations confidently. Fill-in-the-blank type questions, objective type multiple choice questions and numerical problems are provided at the chapter-ends to enable students to build a clear understanding of the subject matter discussed in the text and also to assess their learning. Answers to fill-in-the-blanks, objective type questions and numerical problems are given at the end of the book. Most of the solved and unsolved problems presented in this book have been classroom tested. I express my profound gratitude to all those individuals without whose assistance and cooperation this book would not have been completed. First of all, I thank Sri. V. Srinivasa Rao, Technician of Adam s Engineering College, Palvancha who typed the entire original manuscript and drew all the figures in this book. I also thank Mr. P. Venkateswara Rao of our college for helping me in the revision of this book. I am grateful to Mr. Burugupalli Venugopala Krishna, Chairman Sasi Educational Society, Velivennu for encouraging and providing me with all the facilities for the revision of this book. I also thank Mr. B. Ravi Kumar, our Executive Director, for his cooperation. I thank Mr. Koneru Satyanarayana, Chancellor, K.L. University, Vijayawada, AP for his constant encouragement. I express my sincere appreciation to my brother Mr. A. Vijaya Kumar and to my friends, Dr. K. Koteswara Rao, Chairman, Gowtham Educational Society, Gudivada and Mr. Y. Ramesh Babu and Smt. Y. Krishna Kumari of Detroit for their encouragement. I thank Dr. K. Raja Rajeswari, Professor, ECE Department and Dr. K.S. Lingamurthy, Professor and Head of EEE Department of Andhra University College of Engineering, Visakhapatnam for their constant words of encouragement. I thank my publishers PHI Learning and their staff, in particular Mr. Darshan Kumar, senior editor, who edited the manuscript for the first edition and Mr. Sudarshan Das, editor, who made this second edition possible. Finally, I am indebted to my wife, A. Jhansi, for putting up with my spending countless hours working on the manuscript. Our sons Dr. A. Anil Kumar and Mr. A. Sunil Kumar and daughters-in-law Dr. A. Anureet Kaur and A. Apurupa and granddaughter Khushi supported me with their constant words of encouragement. The author will gratefully acknowledge constructive criticism from both students and teachers for further improvement of this book. A. Anand Kumar

13 Linear Wave Shaping Chapter Linear Wave Shaping A linear network is a network made up of linear elements only. A linear network can be described by linear differential equations. The principle of superposition and the principle of homogeneity hold good for linear networks. In pulse circuitry, there are a number of waveforms, which appear very frequently. The most important of these are sinusoidal, step, pulse, square wave, ramp, and exponential waveforms. The response of RC, RL, and RLC circuits to these signals is described in this chapter. Out of these signals, the sinusoidal signal has a unique characteristic that it preserves its shape when it is transmitted through a linear network, i.e. under steady state, the output will be a precise reproduction of the input sinusoidal signal. There will only be a change in the amplitude of the signal and there may be a phase shift between the input and the output waveforms. The influence of the circuit on the signal may then be completely specified by the ratio of the output to the input amplitude and by the phase angle between the output and the input. No other periodic waveform preserves its shape precisely when transmitted through a linear network, and in many cases the output signal may bear very little resemblance to the input signal. The process whereby the form of a non-sinusoidal signal is altered by transmission through a linear network is called linear wave shaping.. THE LOW-PASS RC CIRCUIT Figure. shows a low-pass RC circuit. A low-pass circuit is a circuit, which transmits only low-frequency signals and attenuates or stops high-frequency signals. At zero frequency, the + R + v t i () it () C vo () t Figure. The low-pass RC circuit.

14 2 Pulse and Digital Circuits reactance of the capacitor is infinity (i.e. the capacitor acts as an open circuit) so the entire input appears at the output, i.e. the input is transmitted to the output with zero attenuation. So the output is the same as the input, i.e. the gain is unity. As the frequency increases the capacitive reactance (X C = /2pfC) decreases and so the output decreases. At very high frequencies the capacitor virtually acts as a short-circuit and the output falls to zero... Sinusoidal Input The Laplace transformed low-pass RC circuit is shown in Figure.2(a). The gain versus frequency curve of a low-pass circuit excited by a sinusoidal input is shown in Figure.2(b). This curve is obtained by keeping the amplitude of the input sinusoidal signal constant and varying its frequency and noting the output at each frequency. At low frequencies the output is equal to the input and hence the gain is unity. As the frequency increases, the output decreases and hence the gain decreases. The frequency at which the gain is / 2 (= 0.707) of its maximum value is called the cut-off frequency. For a low-pass circuit, there is no lower cut-off frequency. It is zero itself. The upper cut-off frequency is the frequency (in the high-frequency range) at which the gain is / 2, i.e. 70.7%, of its maximum value. The bandwidth of the low-pass circuit is equal to the upper cut-off frequency f 2 itself. R + + V Is () i () s /Cs V o () s Figure.2 (a) A Bandwidth 0 f 2 f (a) Laplace transformed low-pass RC circuit and (b) its frequency response. For the network shown in Figure.2(a), the magnitude of the steady-state gain A is given by Vo() s Cs A = = = Vi () s R + + RCs = = + jw RC + j2p frc Cs (b) \ A = +(2 p frc) 2 At the upper cut-off frequency f 2, A = 2 \ 2 = 2 +(2 p f RC) 2

15 Linear Wave Shaping 3 Squaring both sides and equating the denominators, \ The upper cut-off frequency, f 2 = 2 = + (2pf 2 RC) 2. 2p RC So A = and A = f + j f f 2 + f2 The angle q by which the output leads the input is given by f q = tan f..2 Step-Voltage Input A step signal is one which maintains the value zero for all times t < 0, and maintains the value V for all times t > 0. The transition between the two voltage levels takes place at t = 0 and is accomplished in an arbitrarily small time interval. Thus, in Figure.3(a), v i = 0 immediately before t = 0 (to be referred to as time t = 0 ) and v i = V, immediately after t = 0 (to be referred to as time t = 0 + ). In the low-pass RC circuit shown in Figure., if the capacitor is initially uncharged, when a step input is applied, since the voltage across the capacitor cannot change instantaneously, the output will be zero at t = 0, and then, as the capacitor charges, the output voltage rises exponentially towards the steadystate value V with a time constant RC as shown in Figure.3(b). v i V 0 (a) 2 t v o t=5rc v i V v 0.9V o 90% t = RC 0.632V 63.2% 0.V 0% 0 t t tr = 2.2 RC t 2 (b) 2 5t 99.3% t Figure.3 (a) Step input and (b) step response of the low-pass RC circuit. Let V be the initial voltage across the capacitor. Writing KVL around the loop in Figure., vi () t = Ri() t + i() t dt C Differentiating this equation, dvi () t di() t = R + i( t) dt dt C

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R a) Explain the operation of RC high-pass circuit when exponential input is applied.

R a) Explain the operation of RC high-pass circuit when exponential input is applied. SET - 1 1. a) Explain the operation of RC high-pass circuit when exponential input is applied. 2x V ( e 1) V b) Verify V2 = = tanhx for a symmetrical square wave applied to a RC low 2x 2 ( e + 2 pass circuit.