A logic gate is an electronic device that makes decisions based on the different combinations

of digital signals present on its inputs.

It is a logical circuit device in computer, having one output channel and one or more input

channels, which gives signal only when specific input conditions are met. Logic gate is the

basic building block of a digital circuit.

There are three basic logic gates from which others can be constructed. These are;

‘ NOT’ gate

‘ AND’ gate

‘ OR’ gate

In logic diagrams, gates are represented by symbols. Inputs and output signals are represented

with arrow lines having labels.

The ‘ NOT’ Gate: The ‘ NOT’ gate is the most basic of all the logic gates and is sometimes

referred to as an inverting buffer or simply a digital inverter. This gate accepts a single input

and brings out the negative of its input as output. That is it inverts its input signal as output. I.e.

if its input is 0, output will be 1, and if its output is 0 it means input will be 1. Or true= false

and false =true.

The ‘ NOT’ gate accepts only one input signal.

Truth Table for ‘ NOT’ gate

INPUT

(A) OUTPUT

(A)

1 0

0 1 The Boolean expression is A=A

The overline ( ) over the expression signify the NOT

The ‘ AND’ Gate: Unlike the ‘ NOT’ gate, the ‘ AND’ gate accepts only one two inputs

signals and bring out one output signal. Hence there are two signal lines.

It is a logic circuit, used in computers that produce a high voltage signal as output if its two

inputs carry a high voltage; otherwise it produces a low voltage output if any of its input is low.

Truth Table for ‘ AND’ gate

INPUT (A) INPUT

(B)

OUTPUT

(Q)

1 1 1

1 0 0

0 1 0

0 0 0

Two inputs AND gate.

The Boolean expression is A.B=Q

The period (.) in the expression stands for multiplication

The ‘ OR’ Gate: This is a logic circuit used in computers, that gives a high voltage output if

all or one of its input carries a high voltage, and a low voltage output if all or one of its input

carries a low voltage. The ‘ OR’ gate also accepts two inputs.

Truth Table of ‘ OR’ Gate

INPUT

(A)

INPUT

(B)

OUTPUT

(Q)

1 1 1

1 0 1

0 1 1

0 0 0

Two inputs OR gate.

The Boolean expression is A+B=Q

The plus used in the expression stands for addition.

AND GATE

OR GATE

‘ NOT’ GATE

USES OF LOGIC Gate

1. They are used in integrated circuits

2. They are used in functioning of transistors, resistors and capacitors

3. They are used in making comparators

4. They are used in producing high speed CPU’ s

5. They are used as elemental building blocks for devices where signals must be selected or combined in a controlled form. This is obvious in telephone switching, satellite transmission and digital computer operations.

6. They are used in making micro-controllers.

7. They are used in making amplifiers, and oscillator circuits.

8. They are used in vacuum tubes

9. They are used in constructing alarms used in cars and homes.

Lesson Notes Library

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Logic circuit