Abstract: A circuit for generating a reference voltage of an image sensor is provided. The circuit comprises a signal differential amplifier, a gain amplifier, a source follower and a clamp circuit. The signal differential amplifier is adapted for receiving and comparing a bias voltage and the reference voltage, and outputting a first voltage according to a comparison result. The gain amplifier is coupled to the signal differential amplifier, and is adapted for receiving the first voltage and outputting a second voltage. The source follower, coupled to the gain amplifier, and is adapted for receiving the second voltage and outputting the reference voltage. The clamp circuit is coupled to the source follower, and is adapted for receiving the reference voltage and limiting the reference voltage to below a clamp voltage.