An input to a rail-to-rail, FET, operational amplifier having a transconductance that is constant throughout the operating range of the operational amplifier is presented. The first output shaft and the second output shaft are adapted to be selectively coupled to the load. The positioning member includes an arm portion defining a receiving cavity for receiving the guiding protrusion therein, a positioning portion having an engaging bar received in the slot of the casing, and a middle portion connecting the arm portion and the positioning portion. The wireless transmission is preferably carried out as inductive transmission.