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Digital ElectronicsArithmetic Circuits

A 4-bit binary adder uses identical ripple-carry full-adder stages. If each stage requires 12 ns for its carry output to become valid after carry-in changes, approximately what is the worst-case carry propagation time through all four stages?

  • A12 ns
  • B24 ns
  • C36 ns
  • D48 ns
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Correct Answer: D. 48 ns
Explanation: In a ripple-carry adder, a carry may propagate serially through all four stages. The approximate worst-case carry delay is 4 × 12 ns = 48 ns.
Reference: Standard digital electronics: ripple-carry adder delay
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