Q. Illustrate Edge-Triggered flip-flops ?

Edge-triggered flip-flop is used to synchronize state change at the time of a clock pulse transition in place of constant level. A number of edge-triggered flip-flops trigger on rising edge (0 to 1 transition) while others trigger on falling edge (1- to 0 transition).  Fig shows clock pulse signal in positive and negative edge-triggered flip-flops.

Effective positive clock transition involves a minimum time known as setup time for that the D input should be maintained at constant value before occurrence of clock transition. In the same way a minimum time known as hold time for which D input should not change after application of positive transition of pulse.