An image sensor includes a photoelectric conversion unit, a signal generation unit, and a feedback unit. The photoelectric conversion unit is formed above a substrate and detects incident light to generate photo-charges based on a drive voltage. The signal generation unit is formed on the substrate and generates an analog signal based on the photo-charges. The feedback unit generates the drive voltage based on an amount of the photo-charges generated from the photoelectric conversion unit. The image sensor may perform a wide dynamic range (WDR) function.