A regulator circuit includes an OP-amp, buffer, power transistor, voltage divider, load, and feedback current generator. The OP-amp generates a first voltage signal by amplifying a difference between input and feedback voltage signals, and drives a first node as the first voltage signal. The buffer drives a second node as a second voltage signal based on the first voltage signal. The power transistor includes drain, gate and source terminals respectively connected to a supply voltage, the second node, and a third node. The voltage divider generates the feedback voltage signal by dividing an output voltage signal of the third node. The load includes a terminal connected to the third node and another terminal receiving a ground voltage. The feedback current generator provides a first feedback current corresponding to a ripple of the output voltage signal to the first node for enhancing a speed at which the ripple reduced.