scan line driver 107 and the falling edge of the goe signal is synchronous with the rising edge of output of the scan line driver 107 so as to control the output of the scan line driver therefore the charging voltage of the capacitor c 2 rises in the predetermined inclination angle synchronously with the rising edge of the output of the scan line driver and falls synchronously with the falling edge of the output of the scan line driver the charging voltage of the capacitor c 2 is respectively outputted to a non inverter node of a comparator ic 1 a and an inverter node u2212 of a comparator ic 1 b through a resistor r 4 the inverter node u2212 of the comparator ic 1 a and the non inverter node of the comparator ic 1 b are coupled to a second basis voltage point vref 2 r 6 u00d7 vref r 5 r 6 wherein the second basis voltage is determined by a resistance ratio of two resistors r 5 r 6 which are serially connected between the basis voltage vref and the ground the comparator ic 1 a compares the voltage of the capacitor c 2 and the second basis voltage vref 2 the turn on path referring to fig5 c of the transistor q 3 is turned off when the voltage of the capacitor c 2 exceeds the second basis voltage verf 2 moreover the comparator ic 1.