Question

What is one disadvantage of the ripple-carry adder?

(a) The interconnections are more complex

(b) More stages are required to a full adder

(c) It is slow due to propagation time

(d) All of the Mentioned

I had been asked this question by my college director while I was bunking the class.

My question is taken from 4-Bit Parallel Adder/Subtractor topic in chapter Arithmetic Circuits of Digital Circuits

Answer 1

Correct choice is (c) It is slow due to propagation time

Explanation: The main disadvantage in using this type of adders is that the time delay increases as for each adder to add the carry should be generated in the previous adder, and for that to add the carry from the one before is required. However, this disadvantage is taken care of in Carry Look Ahead adder in which the ripple carry is converted in such a way that the carry over a group of bits of the adder becomes 2-level logic.