An image sensor comprising: - a pixel sensor comprising: - a photo detector circuit arranged to provide a detection quantity having a level that depends on an amount of light to which the photo detector circuit has been exposed during a photo detection time interval; and - a transistor-based circuit arranged to provide an output signal indicative of the amount of light in response to the detection quantity according to an input-output relationship, the transistor-based circuit comprising a transistor having a threshold voltage that affects the input-output relationship of the transistor-based circuit, and a biasing node adapted to apply a biasing voltage to a semiconductor region that affects the threshold voltage of the transistor; and - characterized in that said image sensor further comprises a biasing circuit comprising a calibrating control loop (1401, 1408) that includes a replica of the transistor-based circuit, the calibrating control loop being adapted to calibrate the input-output relationship of the replica of the transistor-based circuit by means of the biasing voltage that is applied to the biasing node in the replica, the biasing circuit being arranged to apply the same biasing voltage also to the biasing node in the transistor-based circuit. - a pixel sensor comprising: - a photo detector circuit arranged to provide a detection quantity having a level that depends on an amount of light to which the photo detector circuit has been exposed during a photo detection time interval; and - a transistor-based circuit arranged to provide an output signal indicative of the amount of light in response to the detection quantity according to an input-output relationship, the transistor-based circuit comprising a transistor having a threshold voltage that affects the input-output relationship of the transistor-based circuit, and a biasing node adapted to apply a biasing voltage to a semiconductor region that affects the threshold voltage of the transistor; and - a photo detector circuit arranged to provide a detection quantity having a level that depends on an amount of light to which the photo detector circuit has been exposed during a photo detection time interval; and - a transistor-based circuit arranged to provide an output signal indicative of the amount of light in response to the detection quantity according to an input-output relationship, the transistor-based circuit comprising a transistor having a threshold voltage that affects the input-output relationship of the transistor-based circuit, and a biasing node adapted to apply a biasing voltage to a semiconductor region that affects the threshold voltage of the transistor; and - characterized in that said image sensor further comprises a biasing circuit comprising a calibrating control loop (1401, 1408) that includes a replica of the transistor-based circuit, the calibrating control loop being adapted to calibrate the input-output relationship of the replica of the transistor-based circuit by means of the biasing voltage that is applied to the biasing node in the replica, the biasing circuit being arranged to apply the same biasing voltage also to the biasing node in the transistor-based circuit.