An active driving circuit for a display panel includes first to fourth transistors, a capacitor, a constant current source, and a capacitor. The first transistor is connected with a positive power source. The second transistor has a common gate terminal together with the first transistor and a mirror circuit against the first transistor. Also, the second transistor is turned on by a common gate signal applied to the common gate terminal to supply the positive power source to a display device. The third transistor sets a saturated threshold voltage for the common gate terminal by allowing the first transistor and the second transistor to constitute a mirror circuit against each other in accordance with a scan line signal. The constant current source supplies a current with a ground one side and controlled by a gray signal of a data line. The fourth transistor sets the common gate voltage corresponding to the controlled current of the constant current source by the scan line signal. The capacitor accumulates charges corresponding to the difference between the positive power source and the common gate voltage.