Class AB amplifier and imagers and systems using same

An amplifier, comprising:

• an input stage receiving an input signal, the input stage comprising:
• a first input transistor having a gate coupled to receive a first input signal;
• a second input transistor having a gate coupled to receive a second input signal;
• a common mode feedback circuit coupled to the first and second input transistors, the common mode feedback circuit comprising:
• a first feedback transistor having a source coupled to a voltage node;
• a first feedback resistor coupled between a gate of the first feedback transistor and a drain of the first input transistor;
• a second feedback transistor having a source coupled to a voltage node and a gate coupled to the gate of the first feedback transistor;
• a second feedback resistor coupled between a gate of the second feedback transistor and a drain of the second input transistor;
• a first cascode transistor coupled between a drain of the first feedback transistor and the drain of the first input transistor;
• a second cascode transistor coupled between a drain of the second feedback transistor and the drain of the second input transistor;
• and
• an output stage coupled to the input stage, the output stage having differential first and second input terminals, the first input terminal coupled to the drain of the first input transistor and the second input terminal coupled to the drain of the second input transistor.

A CMOS imager pixel, comprising:

• a photosensor;
• a pixel circuit coupled to the photosensor, the pixel circuit being operable to apply a signal indicative of light received by the photosensor to a column line;
• a capacitor having first and second terminals, the first terminal of the capacitor being coupled by a transistor to the column line;
• a reference voltage generator structured to produce a reference voltage at an output terminal;
• an amplifier having an output terminal coupled to the second terminal of the capacitor, a first input terminal coupled to the output terminal of the reference voltage generator, and a second input terminal coupled to the output terminal of the amplifier, the amplifier comprising:
• an input stage receiving an input signal, the input stage having a first output node on which a first voltage is generated that varies directly with the magnitude of the input signal and a second output node on which a second voltage is generated that varies inversely with the magnitude of the input signal;
• an output stage coupled to the input stage, the output stage comprising:
• a first transistor and a second transistor connected in series with each other between first and second voltage nodes, the second transistor being complementary to the first transistor, the first transistor having a gate that is coupled to one of the output nodes of the input stage;
• a third transistor of the same type as the first transistor and a fourth transistor of the same type as the second transistor connected in series with each other between third and fourth voltage nodes, the third transistor having a gate that is coupled to the other of the output nodes of the input stage, the fourth transistor being connected to the second transistor as a current mirror;
• a first cascode transistor coupled between the third transistor and the fourth transistor, the first cascode transistor being of the same type as the third transistor, and
• a second cascode transistor coupled between the fourth transistor and the third transistor, the second cascode transistor being of the same type as the fourth transistor.