to a positive input end 511 and a negative input end 512 the function of the error amplifier 50 is to amplify the voltage difference v i b between the power source terminal 621 and the load terminal 622 of the nmos field effect transistor 62 for controlling on state resistance of the nmos field effect transistor 62 and further controlling the charging power source 11 to charge the battery 13 the error amplifier 50 shown in fig5 comprises a gate control amplifier 51 and two current sources 52 53 the gate control amplifier 51 provides a control end 513 in addition to the positive input end 511 and the negative input end 512 the positive input end 511 couples with the power source terminal 621 of the nmos field effect transistor 62 and the negative input end 512 couples with the load terminal 622 the current source 52 is a constant current source and couples with the power source terminal 621 and the gate terminal 623 of the nmos field effect transistor 62 the current source 53 couples with the control end 513 of the gate control amplifier 51 the gate terminal 623 of the nmos field effect transistor 62 and the ground end.