This document provides an introduction to basic logic gates. It includes: 1. An introduction that defines logic gates and their use of diodes to allow or block signals based on logic conditions. 2. Descriptions of three basic logic gates - OR, AND, and NOT - including their symbols, Boolean expressions, and truth tables. 3. Sections on each of the three basic gates that explain how to simulate them using components like diodes, transistors, and resistors, and provide examples of their truth tables. The document serves as an overview of logic gates, their components, representations, and functions as basic building blocks of digital circuits.

1. TABLE OF CONTENT
S.NO CONTENT PAGE
1. Introduction 1
2. Basic Terminology 2
3. Basic Gate 4
4. OR Gate 6
5. AND Gate 8
6. NOT Gate 10
7. Bibliography 12

2. INTRODUCTION
A logic gate can be defined as digital circuit which either allows
a signal to pass through or stops it. These gates are related to
Boolean Algebra . These gates allow signals to pass through
them only when some logic is satisfied. A semiconductor diode
(P‐N junction) acts as a closed switch when it is forward biased,
i.e. it allows current to pass through it. It acts as an open circuit
when it is reversed biased, i.e. it allows very little or no current
to pass through it. This unique property of diode is employed in
the design of logic gates and the circuits.
The logic gates are building blocks of electronic devices. Logic
gate may have one or more inputs but can only have one
output.
Possible values of input and output of logic gates are expressed
in form of tables called truth tables.
Two values of logic statement are denoted by binary numbers 1
and 0 where 1 represent higher value.
1.

3. BASIC TERMINOLOGY
SIGNAL :-
Information converted in electrical form and suitable for
transmissionis called a signal. There are two types of signals:‐
i) Analog signal
ii)Digital signal
ANALOG SIGNAL :-
Analog signals are continuous variation of voltage or current.
They are essentially single‐valued function of time. Sine wave is
fundamental analog signal
DIGITAL SIGNAL :-
Digital signal are those which can take only discrete step wise
values. Binary system that is extensively used in digital
electronics employs just two levels of a signal. ‘O’ corresponds
to low level and ‘1’ corresponds to high level of voltage or
current.
2.

4. LOGICGATE :-
A digital circuit which allows a signal to pass through it or stops
is called a gate. When such gate allows the signal to pass
through only when some logical condition is satisfied, they are
called logic gates. Each logic gate follows certain logical
relationship between input and output voltage. It is used in
calculators, etc.
DIGITAL CIRCUIT :-
The electrical circuit which uses only digital signals is called
digital circuit.
3.

5. BASIC GATES
There are three basic logic gates. They are combined together
to form several other logic gate. These logic gates are the
building block of digital circuit. The basic logic gates are:‐
1. OR GATE
2. AND GATE
3.NOT GATE
The logic gates are represented with specific symbols in digital
circuit.
1. OR GATE
Boolian expression = A+B =Y
Where A and B are input while Y is output and (+) indicates OR.
4.

6. 2. AND GATE
Boolian expression = A.B =Y
Where A and B are input while Y is output and (.) indicates AND.
3. NOT GATE
Boolian expression = Ã =Y
Where A and B are input while Y is output and (~) indicates
NOT.
5.

7. OR GATE
AIM
To simulate and design the OR Gate
COMPONENTS
Two ideal p-n junction diodes (D1 and D2)
THEORY AND CONSTRUCTION
An OR Gate can be idealized by using two identical diode as
shown in figure.Here negative terminal of battery is grounded
and corresponded to 0 level and positive terminal correspond to
1 level.
TRUTH TABLE
INPUT A INPUT B OUTPUT Y
1 0 1
1 1 1
0 1 1
0 0 0

8. 6.
.
INTERFERENCE
a) IF A and B are open LED do not glow {A=0 B=0}
so A+B=Y , hence Y=0.
b) IF A and B are closed LED will glow {A=1 B=1}
so A+B=Y , hence Y=1.
c) IF A is open and B is closed LED will glow {A=0 B=1}
so A+B=Y , hence Y=1.
d) IF A is closed and B is open LED will glow {A=1 B=0}
so A+B=Y , hence Y=1.
7.

9. AND GATE
AIM
To simulate and design the AND Gate
COMPONENTS
Two n-p-n transistors and Resistance R and R2
THEORY AND CONSTRUCTION
An OR Gate can be idealized by using two identical diode as
shown in figure. Resistance R2 is earthed while R is connected to
both ends of Battery.
Here negative terminal of battery is grounded and
corresponded to 0 level and positive terminal correspond to 1
level.
TRUTH TABLE
INPUT A INPUT B OUTPUT Y
1 0 0
1 1 1
0 1 0

10. 0 0 0
8.
.
INTERFERENCE
a) IF A and B are open LED do not glow {A=0 B=0}
so A.B=Y , hence Y=0.
b) IF A and B are closed LED will glow {A=1 B=1}
so A.B=Y , hence Y=1.
c) IF A is open and B is closed LED will glow {A=0 B=1}
so A.B=Y , hence Y=0.
d) IF A is closed and B is open LED will glow {A=1 B=0}

11. so A.B=Y , hence Y=0.
9.
NOT GATE
AIM
To simulate and design the NOT Gate
COMPONENTS
n-p-n transistor
THEORY AND CONSTRUCTION
NOT Gate cannot be realized by using diodes but can be done
by using n-p-n transistor.
Base b is connected to input A while Emitter is earthed and
collector is connected to battery.
TRUTH TABLE
INPUT A OUTPUT Y

12. 1 0
0 1
10.
.
INTERFERENCE
a) IF A is closed LED do not glow {A=1}
so à =Y , hence Y=0.
b) IF A is open LED will glow {A=0}
so à =Y , hence Y=1.

13. 11.
BIBLIOGRAPHY
1. www.wikipedia.com
2. www.google.com
3. www.answers.com
4. NCERT Vol. 1 and 2 Physics
Class 12
5. Pradeep Class 12 Physics
6. HC Verma Physics Part 2