an output voltage vout at an output terminal out according to an input voltage vin of an input terminal in assume the output stage circuit 60 is utilized in a half supply voltage structure and the supply voltage is within a range from the voltage u00bd vdd to ground i e the second voltage vdd_h is the voltage u00bd vdd as shown in fig6 the output stage circuit 60 comprises transistors 600 and 602 and a current source 604 the current source 604 is a constant current source coupled to the output terminal out for providing a constant current i_bias in contrast with the output stage circuit 40 shown in fig4 the sources of the transistors 600 and 602 are coupled to the second power terminal pw 2 and to ground respectively for receiving the second voltage vdd_h and ground voltage the base of the transistors 400 and 403 are coupled to the first power terminal pw 1 and to ground gnd respectively the output stage circuit 60 needs to provide a constant current when the input voltage vin is at a normal biasing point in such a condition the transistor 600 is cut off and the required constant current is provided by the constant current i_bias generated by the current source 604 when the input voltage vin decreases the output voltage vout increases in such a condition the transistor 600 is turned on for providing additional current.