Page 54 - Digital Electronics by harish
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2.3 Full adder
When we add two bits, we may get a carry bit. Hence, an adder circuit needs to add three bits
(ie. two input bits A and B and one carry bit C). The logic circuit which adds three bits is
called Full adder. The truth table of full adder is given in table.
Inputs Outputs
A B C CARRY 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
A full adder can be implemented using two half adders and an OR gate as shown in the
figure.
Figure : Block diagram
Figure : Block diagram of full adder using two half adders
The logic diagram and logic equations for SUM and CARRY are given in figure.
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