A device includes a current source and sampling units. Each of the sampling units includes a transistor and a capacitor electrically coupled to a gate of the transistor. The sampling units are sequentially activated such that the capacitor samples a voltage of a column line of a pixel array and are activated together such that the transistor is turned on according to the sampled voltage of the capacitor, to drain a current from the current source through an output node to generate an output voltage thereat.