Preface – Pulse and Digital Circuits


Writing a textbook on a topic that has matured over the years is, for obvious reasons, truly daunting. First, a subject that continues to be important enough to be taught in classrooms even after eight decades demands that those studying it for the first time not only understand the critical concepts, but also learn to appreciate the finer nuances so that the subject and the practitioners continue to grow together. Second, almost every such mature area has some established books that have stood the test of time and continue to hold sway over a large majority of the readers. To match up to the standards set by such books is far from easy.

At the same time, we maintain that there is a need for books that meet the requirements of present-day undergraduate students of electronics and electrical engineering. Sometimes, books are written on the premise that not every minute detail needs to be presented to the student, but some things should be left to their imagination to encourage repeated reading and to stimulate their analytical ability. However, we have observed that not all students benefit from such a textbook. Neither do we believe that re-reading to reinterpret the written word improves the analytical ability of all students. A student who has tried and failed at this loses the motivation to pursue the subject further. Dealing with such a student is not easy either.

Through this book, we attempt to address these problems. This textbook, written to cater to the needs of undergraduate students of electronics and electrical engineering in various Indian universities, uses numerous simplified circuits and illustrations to explain concepts and facilitate a clear understanding of the subject matter in students, motivating them to read further. Drawing upon our experience in the classroom, we have attempted to provide lucid answers and explanations to common queries and problematic concepts for students, making this book a great resource for self-study. We have made a concerted effort to make the presentation and illustrations more student-friendly. Many examples and solved problems have been included.

The Organization of the Book

The book is organized into eighteen chapters.

Chapter 1, “An Introduction to Pulse Waveforms”, introduces pulse waveforms, touching upon the prerequisites for the study of pulse and digital circuits and recapitulating the method of analysis of various amplifier configurations and their relevance. The chapter briefly describes the relevant network theorems and their applications in pulse and digital circuits.

Chapter 2, “Linear Waveshaping: High-pass Circuits”, analyses the response of linear high-pass RC and RL circuits to various types of input signals. Except for the sinusoidal signal, all other signals undergo distortion in amplitude when passed through a high-pass circuit, the amount of distortion being dependent on the time constant employed in the circuit. The presentation is so arranged that the student is able to arrive at the response from their earlier knowledge of mathematics by writing and solving the necessary differential equations using Laplace transforms. The specific application of a high-pass circuit as a differentiator is also discussed.

Chapter 3, “Linear Waveshaping: Low-pass Circuits, Attenuators and RLC Circuits”, examines the response of linear low-pass RC and RL circuits to various types of input signals. Specific applications of low-pass RC circuits as integrators and compensated attenuators are discussed. Finally, the response of the RLC circuit to step input and the under-damped RLC circuit as a ringing circuit are presented.

Chapter 4, “Non-linear Waveshaping: Clipping Circuits and Comparators”, describes the use of non-linear circuit elements such as diodes and transistors in waveshaping applications such as clippers. The chapter concludes with a discussion of the application of a diode clipper as an amplitude comparator.

Chapter 5, “Non-Linear Waveshaping: Clamping Circuits”, talks about non-linear waveshaping in applications such as clamping in which either the positive or the negative peak of the input is clamped to either a zero level or to an arbitrarily chosen dc reference level. This, in turn, means that the clamping circuit introduces a dc level that is lost while transmitting a signal through a capacitor-coupled network.

Chapter 6, “Switching Characteristics of Devices”, considers the switching characteristics of devices such as diodes and transistors in view of the influence of inter-electrode capacitances on the switching speeds of these devices. The dependence of the parameters of a transistor on temperature is also presented.

Chapter 7, “Astable Multivibrators”, examines regenerative circuits that use transistors. These circuits, essentially used to generate a square-wave output (clock pulses), can also be used to trigger some other circuits. The applications of an astable multivibrator as a voltage-to-frequency converter and as a frequency modulator are also considered.

Chapter 8, “Monostable Multivibrators”, discusses monostable multivibrators, which generate a gating signal or a pulse. Two possible circuit configurations—collector-coupled and emitter-coupled monostable multivibrators—are presented. The application of a monostable multivibrator as a voltage-to-time converter is also described.

Chapter 9, “Bistable Multivibrators”, focuses on bistable multivibrators, which are used as 1-bit memory elements in digital circuits. Two circuit variations—fixed-bias and self-bias arrangements—are presented. Both symmetric and unsymmetric methods of triggering bistable multivibrators are presented. The chapter concludes with a detailed discussion on emitter-coupled bistable multivibrators called Schmitt triggers used in waveshaping applications.

Chapter 10, “Logic Gates”, presents logic gates belonging to various families such as RTL, DTL, TTL and CMOS, while discussing the relative merits and demerits of each. The interfacing of TTL and CMOS logic gates is also described.

Chapter 11, “Sampling Gates”, analyses sampling gates that transmit the input signal to the output terminals only during the time interval decided by the control signal. Unidirectional and bidirectional sampling gates that transmit signals of either or both polarities are considered along with the methods to eliminate pedestal in the output.

Chapter 12, “Voltage Sweep Generators”, introduces voltage sweep generators that find application in CROs to move the electron beam linearly along the time axis so as to be able to display a time-varying signal applied to vertical deflecting plates. Miller and bootstrap sweep generators, which linearize an exponential sweep, are also discussed.

Chapter 13, “Current Sweep Generators”, considers current sweep generators that are used to produce large deflections of electron beams for specific applications such as in television and radar receivers. A method to improve the linearity of the current sweep by adjusting the driving waveform is also included.

Chapter 14, “Blocking Oscillators”, considers single-transistor regenerative circuits, called blocking oscillators, which use a pulse transformer to derive positive feedback by a proper choice of the winding polarities. Variations of astable and monostable blocking oscillators are also covered.

Chapter 15, “Synchronization and Frequency Division”, talks about the synchronization of the output of various waveform generators. Synchronization of the output of relaxation circuits like sweep generators and multivibrators with pulses and symmetric signals is discussed.

Chapter 16, “Op-amps, 555 Timers and Negative Resistance Devices in Switching Applications”, describes the applications of linear ICs, including op-amps and 555 timers for wave generation and waveshaping. The applications of certain negative resistance devices such as UJTs, tunnel diodes and p–n–p–n diodes in wave generation and waveshaping are discussed here.

Chapter 17, “Combinational Circuits: Implementation and Design”, focuses on the design and implementation of combinational circuits. NAND and NOR—called universal gates—are used to derive various other gates and their combinations. Boolean algebra and Karnaugh maps are used as tools to simplify complex gate circuits.

Chapter 18, “Sequential Circuits: Flip-flops and Counters”, gives the principle of working of various types of flip-flops, both un-clocked and clocked, with the help of truth tables and timing diagrams. Synchronous and asynchronous counters and their design procedures are also presented.

For quick reference, some important information on h-parameter conversion, Laplace transforms, most commonly used derivatives and integrals are appended at the end of the book.