This circuit is a modification to the ripple carry adder. Since the carry ripples through the adder, propagation delay will be present, and to negate that, this circuit introduces look ahead logic, which will calculate the carry for each stage based on the inputs to each stage. The logic uses the propagate and genarate signals to calculate the carry. These signals are generated using the input signals. Thus as and when the input is introduced to the adder, the carry is generated for all units.