Combinational Logic Circuits

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 What is the Combinational Circuit

A combinational logic circuit is one in which the present state of the combination of the logic inputs decides the output . The term combination logic means combining two or more logic gates to form a required function where the output at a given time depends only on the input.

In Combinational circuit as output does not depend on the time instant, no feedback is required for its next output generation.

Combinational circuits do not required to have a memory.


Examples of Combinational Circuits

These are some examples of Combinational Circuits.

half adders 

full adders 

encoders and decoders

 subtractors

Multiplexer

De-multiplexer

                             Procedure of Creating Combinational logic

 

  • First read the problem and understand it 
  • specify number of inputs and outputs and label them
  • Derive the truth table for each of the outputs based on their relationships to the input.
  • Simplify the Boolean Expression from using Karnaugh Map and Boolean algebra
  • Draw a logic diagram that represents the simplified Boolean expression. Verify the design by analyzing or simulating the circuit

 Example - 01

 Design a combinational circuit with three inputs and one output? The output is equal to logic 1 when the binary value of the input is less than 3, and the output is logic 0 otherwise.

Truth Table                                                                                             

A

B

C

F (Out Put)

0

0

0

1

0

0

1

1

0

1

0

1

0

1

1

0

1

0

0

0

1

0

1

0

1

1

0

0

1

1

1

0


Karnaugh Map


Circuit











Here when change the input logic state that's how output changes.

Simulate Circuit




                      

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