Amplifiers, methods of increasing current gain in amplifiers, and imaging devices

A method of manufacturing an amplifier, the method comprising:

• forming a differential stage using a differential pair of transistors of a first conductivity type, the transistors of the differential pair having respective control electrodes, first and second inputs to the amplifier respectively including the control electrodes of the differential pair;
• coupling a current sum stage to the differential stage, using a variable current source in the current sum stage, and arranging the current sum stage to sum current from the variable current source with current from the differential stage that flows to the current sum stage if a voltage at the first input exceeds a voltage at the second input;
• and
• coupling an output stage to the current sum stage.

An amplifier comprising:

• a differential stage including a differential pair of transistors of a first conductivity type, the differential pair having gates, the amplifier having first and second inputs respectively including the gates of the differential pair;
• a current sum branch coupled to the differential stage, the current sum branch including a variable current source and being configured to sum current from the variable current source with current from the differential stage that flows to the current sum branch if a voltage at the first input exceeds a voltage at the second input;
• an output stage coupled to the current sum branch;
• and
• gain control circuitry configured to increase current through the variable current source if a voltage at the first input exceeds a voltage at the second input.

A method of manufacturing an amplifier, the method comprising:

• including in a differential stage a differential pair of transistors of a first conductivity type, the differential pair having gates, first and second inputs to the amplifier respectively including the gates of the differential pair;
• coupling a current sum branch to the differential stage, the current sum branch including a variable current source and being configured to sum current from the variable current source with current from the differential stage that flows to the current sum branch if a voltage at the first input exceeds a voltage at the second input;
• coupling an output stage to the current sum branch;
• and
• increasing current through the variable current source, using gain control circuitry, if a voltage at the first input exceeds a voltage at the second input.

An imaging device, the imaging device comprising:

• a reference voltage generator;
• a plurality of sample and hold capacitors;
• and
• a Class AB CMOS operational amplifier having an input coupled to the reference voltage generator and an output coupled to the plurality of sample and hold capacitors, the amplifier including:
• a differential stage including a differential pair of transistors of a first conductivity type, the differential pair having gates respectively defining first and second inputs to the amplifier;
• a current sum branch coupled to the differential stage, the current sum branch including a variable current source and being configured to sum current from the variable current source with current from the differential stage that flows to the current sum branch if a voltage at the first input exceeds a voltage at the second input;
• an output stage coupled to the current sum branch;
• and
• gain control circuitry configured to increase current through the variable current source if a voltage at the first input exceeds a voltage at the second input.