A circuit includes an input impedance, an operational amplifier, a voltage-adjusting circuit, a pulse-generating circuit, and a drive circuit. The input impedance is coupled to an input terminal of the operational amplifier, receives an input voltage, and outputs an input current. The operational amplifier is coupled to a first power voltage and outputs an amplified signal according to an input operating voltage and a feedback signal. The voltage-adjusting circuit adjusts the input operating voltage of the operational amplifier. The pulse-generating circuit generates a pulse width modulation signal according to the amplified signal. The drive circuit is coupled to a second power voltage and generates a driving signal according to the pulse width modulation signal. The feedback signal is correlated with the driving signal.