It is a usual structure of focalplanearray (FPA) readout integrated circuit (ROIC) which uses a capacitance transimpedance amplifier (CTIA) as the input stage, a correlated double sampling (CDS) circuit to get signal and reduce noises, and a source follower (SF) circuit to output the signal. The output swing and linearity range of the ROIC were compared, using different combinations of the CDS switch and SF structure. The inputoutput characteristics of different kinds of switches and SF structures were discussed and the conclusion is that, in order to get the maximal output swing and linearity range, appropriate CDS switch and SF structure should be chosen based on the integral direction of photocurrent.