A bias circuit includes a first transistor, a second transistor, a first resistor and a second resistor. The first end of the first transistor is coupled to a first voltage source. One end of the first resistor is coupled to the second end of the first transistor, and the other end of the first resistor is coupled to the control terminal of the first transistor. The first end of the second transistor is coupled to a second voltage source, and the second end of the second transistor is coupled to the control terminal of the first transistor. One end of the second resistor is coupled to the other end of the first resistor, and the other end of the second resistor is coupled to the control terminal of the second transistor.