Active pixel sensor with bootstrap amplification

A method of operating an active pixel sensor having a photosensor, a reset transistor, a readout transistor, and a row select transistor, said readout transistor having a gate coupled to said photosensor, a drain coupled to a switchable supply voltage, and a source, and said row select transistor having a gate coupled to a row select line, a drain coupled to said source of said readout transistor, and a source, comprising the steps of:

• applying a reset signal at a first logic level to said gate of said reset transistor during a reset period to set said photosensor at a reset potential;
• applying a reset signal at a second logic level to said gate of said reset transistor during an integration period to begin capture of photocharge;
• applying a first supply voltage to said drain of said readout transistor during said integration period to bias said readout transistor to have a first gate capacitance;
• applying a row select signal at said gate of said row select transistor during a readout period to output a signal corresponding to said photocharge;
• and
• applying a second supply voltage to said drain of said readout transistor during said readout period to bias said readout transistor to have a second gate capacitance less than said first gate capacitance, said second supply voltage being different from said first supply voltage and operable to provide a compression gain.

A method of operating an active pixel sensor having a photosensor, a reset transistor, a transfer transistor, a readout transistor, and a row select transistor, said transfer transistor having a first source/drain coupled to said photosensor, a gate coupled to a transfer line, and a second source/drain, said readout transistor having a gate coupled to said second source/drain of said transfer transistor, a drain coupled to a switchable supply voltage, and a source, and said row select transistor having a gate coupled to a row select line, a drain coupled to said source of said readout transistor, and a source, comprising the steps of:

• applying a reset signal at first logic level to said gate of said reset transistor during a reset period to set said photosensor to a reset potential;
• applying a transfer signal at a first logic level to said gate of said transfer transistor to transfer charge between said photosensor and said gate of said readout transistor;
• applying a reset signal at a second logic level to said gate of said reset transistor during an integration period to begin capture of photocharge;
• applying a transfer signal at a second logic level to said gate of said transfer transistor to end capture of photocharge;
• applying a first supply voltage to said drain of said readout transistor during said integration period to bias said readout transistor to have a first gate capacitance;
• applying a row select signal at said gain of said row select transistor during a readout period to output a signal corresponding to said photocharge;
• and
• applying a second supply voltage to said drain of said readout transistor during said readout period to bias said readout transistor to have a second gate capacitance less than said first gate capacitance, said second supply voltage different from said first supply voltage and operable to provide a compression gain.