Carry look ahead adder in digital electronic

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Carry Look Ahead Adder-


  • In Ripple Carry Adder, each full adder has to wait for its carry-in from its previous stage full adder to start its operation which causes an unnecessary delay.
  • Carry Look Ahead Adder is an improved version of the ripple carry adder which generates the carry-in of each full adder simultaneously without causing any delay.

Logic Diagram for Carry Look Ahead Adder-



Working of Carry Look Ahead Adder-


  • The working of carry look ahead adder is based on the principle that the carry-in of any stage full adder is independent of the carry bits generated during intermediate stages and is only dependent of the following two parameters-

  1. Bits being added in the previous stages
  2. Carry provided in the beginning

  • Since, the above two parameters are always known, the carry-in of any stage full adder can be evaluated at any instant of time. Thus, a full adder need not wait until its carry-in is generated by its previous stage full adder.

Advantages and Disadvantages of Carry Look-Ahead Adder :
Advantages –
  • The propagation delay is reduced.
  • It provides the fastest addition logic.
Disadvantages –
  • The Carry Look-ahead adder circuit gets complicated as the number of variables increase.
  • The circuit is costlier as it involves more number of hardware.

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