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# DE: Basic Logic Gates

Digital Electronics (Module 2)
by

## Ayman Odawa

on 26 September 2013

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#### Transcript of DE: Basic Logic Gates

Basic Logic Gates
Introduction
Logic gates are the building blocks of any digital circuit which is the building block of modern digital systems.
A Logic gate has one or more inputs and only one output.
A logic gate is defined as an
electronic circuit
or device which makes logical decisions.
The logic gates
AND
,
OR
, and
NOT
are referred to as
Basic Logic Gates
.
The basic gates combine to form more complex logic circuits called
Combinational Logic Gates.
The available combinational logic gates are:
NAND
,
NOR
,
XOR
, and
XNOR
.
Logic States
In digital systems there are only two possible states represented by two voltage levels.
The two states or voltage-levels are:
HIGH and it represents Closed switches,Lights ON, or Logic-1.
LOW and it represents Open switches, Lights OFF, or Logic-0.
Basic Logic Gates
The three basic logic gates are:
1) AND
2) OR
3) NOT
Series Switching Circuit
Such a series switching circuit is also known as AND Switching Circuit because the lamp is
ON
only if both,
S1

AND

S2
are CLOSED.
AND Logic Gate
The digital circuit which can act exactly like an AND switching circuit is
known as AND logic gate.
Logic Symbol
The AND logic gate can have a maximum number of 8-inputs but
only
one output.
Logic Operation and Truth Table
Boolean Expression
X
=
A

AND

B
X
=
A

B
X
=
A

B
Parallel Switching Circuit
What is the number of all possible combinations for 2-input logic gate ?
Parallel Switching Circuit
Parallel Switching Circuit Operation
Such a parallel switching circuit is also known as
OR Switching Circuit
because the lamp is ON if
S1

OR

S2

OR
both are CLOSED.
OR Logic Gate
The digital circuit which can act exactly like an OR switching circuit is known as OR logic gate.
Logic Symbol
The OR logic gate can
have a maximum number of 8-inputs but only one output.
Logic Operation and Truth Table
OR Boolean Expression
X
=
A

OR

B
X
=
A

+

B
Inverter Switching Circuit
Inverter Circuit Operation
Such an Inverting Switching circuit is also known as
NOT Circuit
because the lamp is
ON
if
S
is
NOT
closed.
NOT Logic Gate
The digital circuit which can act exactly like a
NOT
switching circuit is known as
NOT
logic gate.
Logic Symbol
The NOT gate, unlike other gates, has only one input.
Boolean Expression
X
=
NOT

A
Timing Diagram
Universal Logic Gates
The NAND Gate
It is called a universal gate because it can be used to construct all basic gates or any combination of these gates.
Logic Symbol
The term
NAND
is a short form of
N
OT-
AND
. It means an AND function with inverted output.
Logic Operation and Truth Table
Boolean Expression
The NOR Gate
Logic Symbol
The term
NOR
is a short form of
N
OT-
OR
and is equivalent to an OR function with inverted output.
Logical Operation and Truth Table
Boolean Expression
XOR gate is made of the combination of AND, OR, and NOT gates

Logic Circuit and Symbol

The Exclusive-OR (XOR)

Logical Operation and Truth Table

Note that the XOR gate has only two inputs.

Boolean Expression

Logic Circuit
The Exclusive-NOR (XNOR)

Symbol
Logical Operation and Truth Table

The XNOR gate has only two inputs.

Boolean Expression

The bubble on the output indicates that its output is opposite that of the XOR gate.
Full transcript