A read-out circuit of an image sensor includes a ramp signal generator, a bias voltage generator and a conversion circuit. The ramp signal generates a ramp signal that linearly varies at a constant slope. The bias voltage generator generates a bias voltage based on a power supply voltage having a first noise component. The conversion circuit generates a reference voltage based on the bias voltage and the ramp signal, and performs an analog-to-digital conversion on an analog signal from a pixel to generate a digital signal corresponding to the analog signal. The analog signal has second noise component. The bias voltage generator adjusts an alternating current component included in the bias voltage such that a magnitude of a third noise component added to the reference voltage is substantially the same as a magnitude of the second noise component.