Basic logic gates with truth table & diagram (2 & 3 inputs)

In digital electronics, logic gates have significant roles to construct logic circuits in computer. Logical gates are the basic building block of digital electronics. Most of the digital devices contain integrated circuits (ICs) in it which are made of the logical gates. In this article, we are going to discuss all the basic logic gates with truth table, circuit diagrams, Boolean expressions, operations and uses. This will be helpful for the students of Physics, Electronics and Computer Science.

There are seven logic gates in Computer out of which three are basic logic gates. These basic logic gates are OR gate, AND gate and NOT gate. Here, we are going to discuss these three gates only. NAND gate, NOR gate, XOR gate and XNOR gate are discussed in other articles. Links are at the below of this post.

Contents in this article:

  • What are Logic gates?
  • How the Logic gates work?
  • Basic Logic Gates with truth table
  • OR gate with truth table
  • AND gate with truth table
  • NOT gate with truth table
  • Uses of Logic gates

What are Logic Gates?

Computer Logic gates are the physical electronic devices which can perform logical operations with one or more binary inputs and give a single binary digit at the output. The operation is basically switching operations. All the logic gates are made of resistor and diodes or transistors. Here we are dealing with classical logic gates. These logic gates obey Boolean algebra. There are some quantum logic gates which are very much different from these. Here these are not the parts of our discussion.

How the logic gates work?

Logic gates performs classical switching operations. If a logic gate have more than one inputs, it allows to pass the desired binary bit at the output of the logic gate. Even a logic gate can give the complement of input at the output. Sometimes it gives the complement of the sum or multiplication of the inputs at its output. The construction elements (Diode or Transistor) of logic gates is designed to be operated as switches. Check the Diode as a switch and Transistor as a switch.

Basic Logic gates with Truth Table and diagram

OR gate, AND gate, NOT gate are the three basic classical logic gates. Here, I am going to discuss the Boolean equation, logical diagram and operation of each of these logic gates. There are some other logic gates like – NAND gate, NOR gate, XOR gate, XNOR gate. But these gates are not the basic logic gates. These gates are discussed in other articles separately.

OR gate with Truth Table

OR gate is a classical logical gate which give the output as the Boolean sum of the inputs of the gate. An OR gate can have infinite number of inputs and gives only one output. If A, B, C, D,….. be the inputs of the OR gate then the output of the OR gate will be

Y = A+B+C+D+…….. This is the Boolean expression defining the relation between the inputs and the output of the OR gate.

Circuit symbol for Two input OR gate

Here is the circuit symbol of an OR gate with two inputs A and B.

Circuit symbol of OR gate with two inputs
Circuit symbol of OR gate with two inputs

Truth Table for two Input OR gate

Here is the OR gate truth table with two inputs A and B.

Input (A)Input (B)Output, Y= A+B
000
011
101
111
Truth Table for Two input OR gate

Circuit diagram for three input OR gate

Here is the circuit symbol of OR gate with three inputs A, B and C.

Circuit symbol of OR gate with three inputs
Circuit symbol of OR gate with three inputs

Truth Table for three Input OR gate

Here is the truth table of OR gate with 3 inputs A, B and C.

Input (A)Input (B)Input (C)Output, Y= A+B+C
0000
0011
0101
0111
1001
1101
1011
1111
Truth Table of Three input OR gate

AND gate with Truth Table

AND logic gate is a classical logic gate which gives the output equal to the Boolean multiplication of the inputs. It also can have infinite number of binary inputs. If A, B, C, D,… be the inputs of AND gate then the Boolean expression for the output will be as following –

Y = ABCD….

The logical diagram and Truth table for two-input and three-input AND gate are as below –

Circuit diagram for two Input AND logic gate

Here is the circuit symbol of AND gate with two inputs A and B.

Circuit symbol of AND gate with two inputs
Circuit symbol of AND gate with two inputs

Truth Table for two Input AND gate

Here is the truth table of AND gate with 2 inputs A and B.

Input (A)Input (B)Output, Y=AB
000
010
100
111
Truth Table of two input AND gate

Circuit Diagram for Three Input AND gate

This is the circuit symbol of AND gate with three inputs A, B and C.

Circuit symbol of AND gate with three inputs
Circuit symbol of AND gate with three inputs

Truth Table for three input AND gate

Here is the truth table of AND gate with 3 inputs A, B and C.

Input (A)Input (B)Input (C)Output, Y=ABC
0000
0010
0100
0110
1000
1010
1100
1111
Truth Table for three input AND gate

NOT logic gate with Truth Table

NOT gate is the classical logic gate which gives the output as the complement of its input. A NOT gate can have only one binary input and it gives the output as the inverter of the input. If we consider that the input of NOT gate is A, then the Boolean expression for the output will be \small \textbf{Y=} \overline{\textbf{A}}.

One can see the logic diagram and Truth Table of NOT gate at below.

Circuit Symbol for NOT gate

NOT gate has very simple circuit symbol as shown below.

NOT gate circuit symbol
NOT gate symbol

Truth Table for NOT gate

Input (A) Output \small \textbf{Y=} \overline{\textbf{A}}
01
10
Truth Table of NOT gate

Use of Logic gates

  1. Logic gates have wide applications in logical binary operations.
  2. Logic gates can store data. Therefore logic circuits like Flipflops, Resisters, counters are used as memory in computers and PCs.
  3. Switching operation of Logic gates is very much popular. Logic gates used as switches in digital circuits.
  4. Logic gates are the main constitutions of integrated circuits (ICs).

These are the basic logic gates with truth table and diagram. If you have any doubt on this topic feel free to ask me in the comment section.

Thank you!

Related Posts:

  1. Basic logic gates using NAND gate only
  2. XOR gate using only NAND gate and only NOR gate
  3. NAND gate
  4. NOR gate
  5. XNOR gate