600 but flows into the capacitors 610 and 612 through the switching transistor 603 as a result the capacitor 612 stores a voltage drop u03b4v equaling to a threshold voltage of the driving transistor 608 that is where v t u2014 t dv is the threshold voltage of the driving transistor 608 therefore during the compensation period the present invention pixel circuit 60 does not output current to the oled 600 next in the duration t 3 the data line signal stay at the high voltage level the first scan line signal change to the low voltage level and the second scan line signal change to the high voltage level so the switching transistors 601 and 603 are turned off the switching transistors 602 and 604 are turned on then data line signal v in charge the capacitor 612 through the switching transistor 602 and a gate voltage v g of the driving transistor 608 becomes if an output source voltage of the driving transistor 608 is v out then a current i oled flowing into the oled 600 is i oled k t dv u00b7 v gs u2212 v th u2014 t dv 2 k t dv u00b7 v in v th u2014 t dv u2212 v out u2212 v th u2014 t dv 2 k t dv u00b7 v in u2212 v out 2 therefore the current flowing into the oled 600 is not affected by the threshold voltage of the driving transistor 608 so as to improve a gray level increase a contrast ratio and decrease power.