An image forming apparatus comprising: a developing unit for applying developer on a photosensitive body, the developing unit comprising a toner/carrier compartment including toner and carrier therein, the toner/carrier compartment having an initial toner/carrier ratio of toner and carrier when the developing unit is initially mounted in the image forming apparatus; a toner/carrier ratio sensor configured to receive an input control signal and to generate an output voltage corresponding to a toner/carrier ratio in the toner/carrier compartment of the developing unit; a controller configured to generate the input control signal and to adjust the input control signal based on the output voltage, such that the output voltage reaches a target value corresponding to the initial toner/carrier ratio; and a memory unit configured to store information relating to a first value of the input control signal that causes the toner/carrier ratio sensor to generate an output voltage which reaches the target value, wherein the input control signal corresponds to a pulse width modulation signal, wherein the information stored in the memory unit corresponds to a duty ratio of the PWM signal, and wherein the controller is further configured to: detect the output voltage generated by the toner sensor according to the PWM signal, and if the output voltage is equal to or less than a minimum reference value, increase the duty ratio of the PWM signal, or if the output voltage is equal to or greater than a maximum reference value, decrease the duty ratio of the PWM signal, and if the level of the output voltage is not increased according to the increase in the duty ratio of the PWM signal, or if the level of the output voltage is not decreased according to the decrease in the duty ratio of the PWM signal, indicate that the toner sensor is in an abnormal state. a developing unit for applying developer on a photosensitive body, the developing unit comprising a toner/carrier compartment including toner and carrier therein, the toner/carrier compartment having an initial toner/carrier ratio of toner and carrier when the developing unit is initially mounted in the image forming apparatus; a toner/carrier ratio sensor configured to receive an input control signal and to generate an output voltage corresponding to a toner/carrier ratio in the toner/carrier compartment of the developing unit; a controller configured to generate the input control signal and to adjust the input control signal based on the output voltage, such that the output voltage reaches a target value corresponding to the initial toner/carrier ratio; and a memory unit configured to store information relating to a first value of the input control signal that causes the toner/carrier ratio sensor to generate an output voltage which reaches the target value, wherein the input control signal corresponds to a pulse width modulation signal, wherein the information stored in the memory unit corresponds to a duty ratio of the PWM signal, and wherein the controller is further configured to: detect the output voltage generated by the toner sensor according to the PWM signal, and if the output voltage is equal to or less than a minimum reference value, increase the duty ratio of the PWM signal, or if the output voltage is equal to or greater than a maximum reference value, decrease the duty ratio of the PWM signal, and if the level of the output voltage is not increased according to the increase in the duty ratio of the PWM signal, or if the level of the output voltage is not decreased according to the decrease in the duty ratio of the PWM signal, indicate that the toner sensor is in an abnormal state. detect the output voltage generated by the toner sensor according to the PWM signal, and if the output voltage is equal to or less than a minimum reference value, increase the duty ratio of the PWM signal, or if the output voltage is equal to or greater than a maximum reference value, decrease the duty ratio of the PWM signal, and if the level of the output voltage is not increased according to the increase in the duty ratio of the PWM signal, or if the level of the output voltage is not decreased according to the decrease in the duty ratio of the PWM signal, indicate that the toner sensor is in an abnormal state.