US Patent 7683953 Intra-pixel fixed-pattern-noise cancellation circuit and method

A pixel sensor comprises a first reset transistor having a drain coupled to a first potential, a gate coupled to a first reset line, and a source. A second reset transistor has a drain coupled to the first reset transistor source, a gate coupled to a second reset line, and a source coupled to a photodiode cathode. A source-follower transistor has a drain coupled to the first reset transistor source, a gate coupled to the photodiode cathode, and a source. A row-select transistor has a drain coupled to the source-follower transistor source, a gate coupled to a row-select signal line, and a source coupled to a column output line. An array of these pixel sensors further comprises timing and readout circuits that control the transistors in the pixel sensor to effect a reset operation that cancels the fixed pattern of threshold variations of the source-follower transistors.