A converting circuit for receiving an input voltage and generating an output current, including: a transistor, coupled to a supply voltage at a drain of the transistor, and a source of the transistor is coupled to a first voltage, and a gate of the transistor is coupled to the input voltage and a fixed voltage; and a resistor, coupled to the input voltage and the gate of the transistor, and the output current flows through the resistor, wherein the output current is related to the fixed voltage, the input voltage and the resistor.