The present disclosure is directed to a CMOS active pixel sensor that compensates for variations in a threshold voltage of a source follower contained therein. A structure in accordance with an embodiment includes: a replica source follower transistor; a system for creating a current in said replica source follower transistor such that a gate-source voltage of said replica source follower is substantially equal to a threshold voltage of said replica source follower; and a current mirror for biasing the isolation source follower transistor at a same current density as the replica source follower transistor.