Applied Electronic II
Adama University
EENG 2202
1
Electrical Engineering
CHAPTER 6
DIGITAL ELECTRONICS
INTRODUCTION
Our world is an analog world. Measurements that we make of the physical objects around us
are never in discrete units, but rather in a continuous range. We talk about physical constants
such as 2.718281828… or 3.141592…. To build analog devices that can process these values
accurately is next to impossible. Even building a simple analog radio requires very accurate
adjustments of frequencies, voltages, and currents at each part of the circuit.
To make things simpler, we work with a digital abstraction of our analog world. Instead of
working with an infinite continuous range of values, we use just two values! Yes, just two
values: 1 and 0, on and off, high and low, true and false, black and white, or whatever you want
to call it. It is certainly much easier to control and work with two values rather than an infinite
range. We call these two values a binary value for the reason that there are only two of them. A
single 0 or a single 1 is then a
binary digit
or
bit
. This sounds great, but we have to remember
that the underlining building block for our digital circuits is still based on an analog world.
This chapter discusses how digital circuits are implemented at the physical level. As you know,
transistors are the fundamental building blocks for all digital circuits. They are the actual
physical devices that implement the binary switch and, therefore, also for the logic gates. There
are many different transistor technologies for creating a digital circuit. Some of these
technologies are the diode-transistor logic (DTL), transistor-transistor logic (TTL), bipolar
logic, and complementary metal-oxide semiconductor (CMOS) logic. Among them, the most
widely used is the CMOS technology.
6.1 BASIC AND DERIVED LOGIC GATES
The three basic building blocks of digital circuits are the NOT, the OR, and the AND gates.
NOR, NAND, XOR, and XNOR gates are examples of derived logic gates. The operation of
these gates is explained in the upcoming sections.
a.
NOT Gate (inverter)
The
NOT
gate simply inverts its input, so a
0
input will produce a
1
output, and a
1
becomes
0
.
Logic symbol for NOT gate is shown in fig. 6.1.
x
y
Figure 6.1 Logic symbol for NOT gate

Applied Electronic II
Adama University
EENG 2202
2
Electrical Engineering
b.
OR & NOR Gates
OR
gate takes two or more inputs and produce an output of
1
if at least one of this
inputs is
1
, and
0
if all are
0’
s.
NOR
gate is derived by combining
OR
gate and Inverter
which does the opposite of
OR
gate.
NOR
gate give an output of
1
whenever all the
inputs are
0
’s.
The logic symbol for two-input OR & NOR gates is depicted in fig. 6.2.
x
y
F
x
y
F
(a) OR Gate Symbol
(b) NOR Gate Symbo
Figure 6.2 Logic symbols for OR & NOR gates
Truth table for OR & NOR Gate is given in the table of figure 6.3.

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