Logic Circuit I (Standard Single Logic Gate)

Topic: Logic Circuit I (Standard Single Logic Gate)

Class: SSS Two

Definitions of Logic Gate
1. A logic gate is an elementary building block of a digital circuit.
2. It is also defined as an electrical circuit, that performs one or more logical operations on one or more input signals.
3. Logic gates are devices that implement Boolean functions, i.e. it does a logic operation on one or more bits of input and give a bit as an output.
Most logic gates have two inputs and one output. At any given moment, every terminal is in one of the two binary conditions low (0) or high (1), represented by different voltage levels. In most logic gates, the low state is approximately zero volts (0 V), while the high state is approximately five volts positive (+5 V).

Types of Standard Single Logic gate
There are three types of standard single logic gates. They are:
1. AND gate
2. OR gate
3. NOT gate

AND gate`
An AND gate will give a high output only if all of the inputs are high. For example, in a simple lighting circuit with two switches in series, the lamp will light only if both switches are pressed.

Logic Symbol for AND gate

Truth Table of the AND gate

INPUT A	INPUT B	OUTPUT C
o	o	o
o	1	o
1	o	o
1	1	1

OR Gate
An OR gate is a digital logic gate that gives an output of 1 when any of its inputs are 1, otherwise 0.

Logic Symbol for OR Gate

Truth Table of the OR Gate

INPUT A	INPUT B	OUTPUT C
o	o	o
o	1	1
1	o	1
1	1	1

A NOT gate is often called an inverter. A NOT gate is slightly different because it has just one input. It will give a high output if the input is low and vice versa.

Logic Symbol for NOT Gate

Truth Table of the NOT gate

INPUT A	OUTPUT C
0	1
1	0

Logic Equations
Aside from representing the function of a logic gate with a truth table and grammatical definition, the use of logic equations can be used to represent logic gates and circuits.
The AND gate Equation
The AND gate operation can be expressed by a Boolean algebra equation. For a 2-input AND gate, the equation is:
X=A.B
The symbol for the AND operation is a centre dot (.). It does not mean multiplication. The expression reads ‘X’ equal to A and B.
The OR Gate Equation
The OR gate operation can also be expressed by a Boolean algebra equation. For a 2-input OR gate, the equation is:
X= A+B
The symbol for the OR operation is a +. It does not mean addition. The expression reads X equal to A or B.
NOT Gate Equation
The NOT gate operation can also be expressed by a Boolean algebra equation
X= Ā
A complement bar (also called an over bar) is placed over the assigned input letter. The expression reads ‘X’ is equal to not ‘A’.