in state the first scan line 130 is enabled to switch on the transistor m 1 and the second scan line 135 and the third scan line 140 are disabled such that the transistors m 2 and m 3 are switched off a driving voltage vdrv is transmitted via the data line 150 since the voltage at the node u201c a u201d equals to the driving voltage vdrv the voltage applied to the capacitor c 2 will equal to vdrv thus the gate voltage v g of the transistor m 4 is computed as v g vdrv u2212 vth in the emission state the third scan line 140 is enabled to switch on the transistor m 3 and the first scan line 130 and the second scan line 135 are disabled such that the transistors m 1 and m 2 are switched off the source voltage vdd is also transmitted via the data line 150 at this time the voltage v gs crossing the gate and the source electrodes of the transistor m 4 is computed as v gs v g u2212 v s vdrv u2212 vth u2212 vdd where vs is the source voltage thus the driving current id passing through the organic light emitting diode oled can be obtained according to the following formula id u2062 1 2 u00d7 k u00d7 v gs uf603 vth uf604 2 u2062 1 2 u00d7 k u00d7 vdrv uf603 vth uf604 vdd uf603 vth uf604 2 u2062 1 2 u00d7 k u00d7 vdrv vdd 2 as will be understood.