A fan circuit controls a speed of a fan. A control chip outputs a pulse width modulation (PWM) signal that changes with the temperature of an electronic device that the fan is mounted in. An input circuit converts the PWM signal into a continual voltage signal. An amplifier circuit amplifies the voltage signal and outputs a control signal. When the control signal is less than a threshold voltage, the control circuit is turned on and the fan receives current which is less than preset current. When the control signal is greater than the threshold voltage, the control circuit is turned off and the fan receives current which is greater than the preset current.