An alternator and a rectifier are provided. The rectifier includes a gate driving circuit, a logic circuit, and a comparison circuit. The gate driving circuit generates a gate voltage, and a control terminal of a transistor receives the gate voltage. The gate driving circuit receives a control signal, and adjusts the gate voltage according to the control signal, so as to control a conductivity degree of the transistor. The logic circuit generates the control signal and a switch signal according to a comparison result and selects a selected voltage according to the switch signal. The comparison result is generated by comparing a sensing voltage of a first terminal of the transistor with the selected voltage.