Tag: Full Adder Circuit Theory

Full Adder | Definition | Circuit Diagram | Truth Table

Half Adder-


Before you go through this article, make sure that you have gone through the previous article on Half Adder.


We have discussed-

  • Half Adder is used for the purpose of adding two single bit numbers.
  • Half adders have no scope of adding the carry bit resulting from the addition of previous bits.
  • To overcome this drawback, full adder comes into play.



In this article, we will discuss about Full Adder.


Full Adder-


  • Full Adder is a combinational logic circuit.
  • It is used for the purpose of adding two single bit numbers with a carry.
  • Thus, full adder has the ability to perform the addition of three bits.
  • Full adder contains 3 inputs and 2 outputs (sum and carry) as shown-



Full Adder Designing-


Full adder is designed in the following steps-




Identify the input and output variables-

  • Input variables = A, B, Cin (either 0 or 1)
  • Output variables = S, Cout where S = Sum and Cout = Carry




Draw the truth table-


A B Cin Cout (Carry) S (Sum)
0 0 0 0 0
0 0 1 0 1
0 1 0 0 1
0 1 1 1 0
1 0 0 0 1
1 0 1 1 0
1 1 0 1 0
1 1 1 1 1

Truth Table




Draw K-maps using the above truth table and determine the simplified Boolean expressions-



Also Read- Full Subtractor




Draw the logic diagram.

The implementation of full adder using 1 XOR gate, 3 AND gates and 1 OR gate is as shown below-



To gain better understanding about Full Adder,

Watch this Video Lecture


Next Article- Half Subtractor


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