A ramp generator includes a row decoder, a column decoder, a current cell matrix and a current-voltage converter. The row decoder generates row selection signals, and the column decoder generates column selection signals. Each of current cells included in the current cell matrix is turned on to provide a unit current if a corresponding row selection signal and a corresponding column selection signal are activated. Each of the current cells is maintained to be turned on even if the corresponding row selection signal or the corresponding column selection signal is deactivated. The current cell matrix outputs an output current by summing unit currents provided from the current cells that are turned on. The current-voltage converter converts the output current of the current cell matrix into a ramp voltage. Therefore, the ramp generator may have a small size and prevent a glitch.