Two-stage amplifier for active pixel sensor cell array for reducing fixed pattern noise in the array output

Abstract

An active pixel sensor cell array in which a two-stage amplifier amplifies the output of each cell. The two-stage amplifier design reduces fixed pattern noise in the image data generated by reading the array, by providing increased gain for the output of each cell without impractically increasing the size and complexity of each cell. For each column of cells of the array, one part of the two-stage amplifier for each cell is shared by all cells of the column, and another part of the two-stage amplifier for each cell is included within the cell itself. Preferably, each cell includes only NMOS transistors (no cell includes a PMOS transistor). In preferred embodiments, a differential amplifier within each cell is the primary stage of the cell's output amplifier, PMOS load circuitry including a secondary output amplifier stage is shared by all cells of the column, and the two amplifier stages for each cell together comprise an op amp. In some such preferred embodiments, the op amp is provided with capacitor feedback for increased gain. Another aspect of the invention is an active pixel sensor cell including a differential amplifier, and PMOS transistor load circuitry coupled to the cell. Preferably, the differential amplifier is the first stage of an op amp, and the remainder of the op amp, including optional capacitor feedback circuitry, is coupled to the cell but is not included within the cell.