The invention provides detection on adaptive voltage amplifiers and battery chargers. Adaptive voltages are applied to an output terminal by an amplifier. Bias currents which are provided by a bias circuit under a first external connection condition when a pull-up resistor connected with the output terminal is unavailable are higher than bias currents which are provided when the pull-up resistor is available. Voltages, which are different from voltages provided when the pull-up resistor is available, are applied to the output terminal by the amplifier when the pull-up resistor is unavailable. The bias circuit can be applied to a portable device which is used for receiving charge currents from the battery chargers via connectors, the connectors are provided with pins D+ and are connected with the output terminal of the amplifier so as to be used for detecting the battery chargers, and the pins D+ are used for being connected with the battery chargers. Standard revision 1.2 of the USB (universal serial bus) battery chargers can be met by the portable device.