A comparator circuit includes a first comparator, a second comparator and an inverter. The first comparator includes two N-channel metal-oxide-semiconductor (NMOS) transistors, two first P-channel metal-oxide-semiconductor (PMOS) transistors and two second PMOS transistors. A gate of the NMOS transistors respectively receives first and second voltages, and sources of the first PMOS transistors are connected to first and second resistors, respectively. The first comparator outputs differential output signals from drains of the NMOS transistors according to the voltage difference between the first and second voltages. An output of the second comparator is connected to gates of the first PMOS transistors of the first comparator. An input of the inverter is connected to the output of the second comparator, and an output of the inverter is connected to gates of the PMOS transistors.