Description of combinational circuit and Half Adder & Full Adder and their truth table

1. Combinational Circuit
Prof. Neeraj Bhargava
Kapil Chauhan
Department of Computer Science
School of Engineering & Systems Sciences
MDS University, Ajmer

2. Combinational circuit
 Combinational circuit is a circuit in which we combine the different
gates in the circuit Means this is the combination of different logic
circuits.
 Characteristics of combinational circuits are following −
 The output of combinational circuit at any instant of time,
depends only on the levels present at input terminals.
 The combinational circuit do not use any memory. The
previous state of input does not have any effect on the
present state of the circuit.
 A combinational circuit can have an n number of inputs and
m number of outputs.

3. Types of Combinational circuit
Combinational Circuit are categorized into three
categories:
 Arithmetic and Logical Function
 Data Transmission
 Code Convertor

4. Types of Combinational circuit
 Arithmetic & Logical Function
 Adders
 Subtractors
 Comparators
 PLDs
 Data Transmission
 Multiplexer - DeMultiplexer
 Encoder – Decoder
 Code Convertor
 Binary
 BCD

5. Block diagram of Combinational
circuit
Combinational Circuit m outputs
n outputs

6. Adder
Adder is a combinational circuit.
It perform Arithmetic addition of inputs.
Adders are of Two Types :
1. Half Adder
2. Full Adder

7. Half Adder
This circuit is perform Addition of 2 bits and output produced by this
circuit is Sum(s) and Carry(c).
Block Diagram:
A Sum(s)
B Carry(c)
Half Adder

8. Half Adder
 Truth table
Inputs Outputs
A B SUM(xor) CARRY(And)
0 0 0 0
0 1 1 0
1 0 1 0
1 1 0 1

9.  This circuit is perform Addition of 3 binary bits and output produced
by this circuit is Sum(s) and Carry(c).
 Sum is the additions of bits and Carry represent carry from previous
lower significant bits.
Block Diagram:
A Sum(s)
B Carry(c)
C
FULL ADDER
Full Adder

10. Full adder
 Truth Table
Inputs Output
A B C SUM CARRY
0 0 0 0 0
0 0 1 1 0
0 1 0 1 0
0 1 1 0 1
1 0 0 1 0
1 0 1 0 1
1 1 0 0 1
1 1 1 1 1