16 Chapter 1 ■ Introduction
PRACTICE 1. Assume that only the digits shown in 8.75 ⫻ 2.446 ⫻ 9.15 are significant. Deter-
PROBLEMS 4 mine their product and show it with the correct number of significant digits.
2. For the numbers of Problem 1, determine
8.75 ⫻ 2.446
ᎏᎏ
9.15
3. If the numbers in Problems 1 and 2 are exact, what are the answers to eight
digits?
4. Three currents are measured as 2.36 A, 11.5 A, and 452 mA. Only the digits
shown are significant. What is their sum shown to the correct number of sig-
nificant digits?
Answers: 1. 196 2. 2.34 3. 195.83288; 2.3390710 4. 14.3 A
1.7 Circuit Diagrams
Electric circuits are constructed using components such as batteries, switches,
resistors, capacitors, transistors, interconnecting wires, etc. To represent these
circuits on paper, diagrams are used. In this book, we use three types: block
diagrams, schematic diagrams, and pictorials.
Block Diagrams
Block diagrams describe a circuit or system in simplified form. The overall
problem is broken into blocks, each representing a portion of the system or
circuit. Blocks are labelled to indicate what they do or what they contain,
then interconnected to show their relationship to each other. General signal
flow is usually from left to right and top to bottom. Figure 1–5, for example,
represents an audio amplifier. Although you have not covered any of its cir-
cuits yet, you should be able to follow the general idea quite easily—sound
is picked up by the microphone, converted to an electrical signal, amplified
by a pair of amplifiers, then output to the speaker, where it is converted back
to sound. A power supply energizes the system. The advantage of a block
diagram is that it gives you the overall picture and helps you understand the
general nature of a problem. However, it does not provide detail.
Sound Power Sound
Amplifier Amplifier
Waves Waves
Microphone Power Speaker
Supply
Amplification System
FIGURE 1–5 An example block diagram. Pictured is a simplified representation of an
audio amplification system.
Section 1.7 ■ Circuit Diagrams 17
Current
Switch
Lamp
(load)
Interconnecting wire
Jolt
Battery
(source)
FIGURE 1–6 A pictorial diagram. The battery is referred to as a source while the lamp
is referred to as a load. (The ⫹ and ⫺ on the battery are discussed in Chapter 2.)
Switch
Pictorial Diagrams
Pictorial diagrams are one of the types of diagrams that provide detail. ⫹
They help you visualize circuits and their operation by showing components Battery ⫺ Lamp
as they actually appear. For example, the circuit of Figure 1–6 consists of a
battery, a switch, and an electric lamp, all interconnected by wire. Operation
is easy to visualize—when the switch is closed, the battery causes current in
the circuit, which lights the lamp. The battery is referred to as the source and (a) Schematic using lamp symbol
the lamp as the load.
Switch
Schematic Diagrams
While pictorial diagrams help you visualize circuits, they are cumbersome to
draw. Schematic diagrams get around this by using simplified, standard
symbols to represent components; see Table 1–7. (The meaning of these ⫹
symbols will be made clear as you progress through the book.) In Figure Battery ⫺ Resistance
1–7(a), for example, we have used some of these symbols to create a
schematic for the circuit of Figure 1–6. Each component has been replaced
by its corresponding circuit symbol.
When choosing symbols, choose those that are appropriate to the occa- (b) Schematic using resistance symbol
sion. Consider the lamp of Figure 1–7(a). As we will show later, the lamp
FIGURE 1–7 Schematic representa-
possesses a property called resistance that causes it to resist the passage of tion of Figure 1–6. The lamp has a cir-
charge. When you wish to emphasize this property, use the resistance symbol cuit property called resistance (dis-
rather than the lamp symbol, as in Figure 1–7(b). cussed in Chapter 3).
18 Chapter 1 ■ Introduction
TABLE 1–7 Schematic Circuit Symbols
⫹ ⫹ ⫹
⫺ ⫺
⫺
Single Multicell AC Current Fixed Variable Fixed Variable Air Iron Ferrite
cell Voltage Source Core Core Core
Source
Batteries Resistors Capacitors Inductors
SPST Earth
SPDT Chassis
Wires Wires
Lamp Switches Microphone Speaker Joining Crossing Grounds Fuses
V
Voltmeter
kV
I
Ammeter Air Core Iron Core Ferrite Core
A
Circuit Dependent
Breakers Ammeter Transformers Source
When you draw schematic diagrams, draw them with horizontal and ver-
tical lines joined at right angles as in Figure 1–7. This is standard practice.
(At this point you should glance through some later chapters, e.g., Chapter 7,
and study additional examples.)
1.8 Circuit Analysis Using Computers
Personal computers are used extensively for analysis and design. Software
tools available for such tasks fall into two broad categories: prepackaged
application programs (application packages) and programming languages.
Application packages solve problems without requiring programming on
the part of the user, while programming languages require the user to write
code for each type of problem to be solved.
Circuit Simulation Software
Simulation software is application software; it solves problems by simulating
the behavior of electrical and electronic circuits rather than by solving sets of
equations. To analyze a circuit, you “build” it on your screen by selecting
components (resistors, capacitors, transistors, etc.) from a library of parts,
which you then position and interconnect to form the desired circuit. You can
Section 1.8 ■ Circuit Analysis Using Computers 19
FIGURE 1–8 Computer screen showing circuit analysis using Electronics Workbench.
change component values, connections, and analysis options instantly with
the click of a mouse. Figures 1–8 and 1–9 show two examples.
Most simulation packages use a software engine called SPICE, an acro-
nym for Simulation Program with Integrated Circuit Emphasis. Popular
products are PSpice, Electronics Workbench® (EWB) and Circuit Maker. In
this text, we use Electronics Workbench and OrCAD PSpice, both of which
have either evaluation or student versions (see the Preface for more details).
Both products have their strong points. Electronics Workbench, for instance,
more closely models an actual workbench (complete with realistic meters)
than does PSpice and is a bit easier to learn. On the other hand, PSpice has a
FIGURE 1–9 Computer screen showing circuit analysis using OrCAD PSpice.
20 Chapter 1 ■ Introduction
more complete analysis capability; for example, it determines and displays
important information (such as phase angles in ac analyses and current
waveforms in transient analysis) that Electronics Workbench, as of this writ-
ing, does not.
Prepackaged Math Software
Math packages also require no programming. A popular product is Mathcad
from Mathsoft Inc. With Mathcad, you enter equations in standard mathe-
matical notation. For example, to find the first root of a quadratic equation,
you would use
⫺b ⫹ 兹苶b2苶苶
⫺苶4苶⭈苶a苶⭈苶c
x: ⫽ ᎏᎏᎏ
2⭈a
Mathcad is a great aid for solving simultaneous equations such as those
encountered during mesh or nodal analysis (Chapters 8 and 19) and for plot-
ting waveforms. (You simply enter the formula.) In addition, Mathcad incor-
porates a built-in Electronic Handbook that contains hundreds of useful for-
mulas and circuit diagrams that can save you a great deal of time.
Programming Languages
Many problems can also be solved using programming languages such as
BASIC, C, or FORTRAN. To solve a problem using a programming lan-
guage, you code its solution, step by step. We do not consider programming
languages in this book.
A Word of Caution
With the widespread availability of inexpensive software tools, you may
wonder why you are asked to solve problems manually throughout this book.
The reason is that, as a student, your job is to learn principles and concepts.
Getting correct answers using prepackaged software does not necessarily
mean that you understand the theory—it may mean only that you know how
to enter data. Software tools should always be used wisely. Before you use
PSpice, Electronics Workbench, or any other application package, be sure
that you understand the basics of the subject that you are studying. This is
why you should solve problems manually with your calculator first. Follow-
ing this, try some of the application packages to explore ideas. Most chapters
(starting with Chapter 4) include a selection of worked-out examples and
problems to get you started.
