1. A driver circuit, the driver circuit comprising:
a current mirror having first and second PMOS transistors;
a bipolar transistor coupled to the second PMOS transistor of the current mirror to receive a first current; and
a plurality of serially connected diode devices coupled between a collector and a base of the bipolar transistor;
wherein the collector of the bipolar transistor is configured to provide a driving voltage according to the number of the
plurality of serially-connected diode devices and the size ratio between the first and the second PMOS transistors.