Method and apparatus for providing high common-mode rejection ratio in a single-ended CMOS operational transconductance amplifier

A method for providing high common-mode rejection ratio in a single-ended CMOS operational transconductance amplifier, comprising:

• providing an differential input stage;
• providing a current source to the differential input stage;
• providing a cascode current mirror section coupled to the differential input stage, wherein providing the cascode current mirror section further comprises providing a first current mirror transistor coupled to a drain of a first transistor of the differential input stage and providing a second current mirror transistor coupled to a drain of a second transistor of the differential input stage, the second current mirror transistor being coupled to an output transistor;
• coupling an output transistor to the cascode mirror section to mirror current at the cascode current mirror section;
• compensating for a current change caused by a common-mode level change at the differential input stage to maintain a constant current at an output;
• and
• wherein compensating for a current change further comprises providing a second current source and a common-mode feedback differential pair, a transistor of the common-mode feedback differential pair mirroring current through the first current mirror transistor.

An analog-to-digital converter (ADC), comprising:

• a sample-and-hold circuit for sampling an input analog signal and providing a held sample at an output of the sample-and-hold circuit;
• a gain stage, coupled to the output of the sample-and-hold circuit, for amplifying the held sample and providing an amplified signal at an output of the gain stage;
• a comparator circuit, coupled to the output of the gain stage, the comparator circuit comparing the amplified signal to a reference signal to provide a digital output based upon the comparison;
• wherein at least one of the sample-and-hold circuits, gain stage and comparator circuit includes a CMOS operational transconductance amplifier, the CMOS operational transconductance amplifier comprises:
• a differential input transistor pair providing an input stage for receiving a pair of input voltages;
• a current source, coupled to the differential input transistor pair, for providing current to the differential input transistor pair;
• an output transistor for conducting an output current proportional to a difference between the pair of input voltages applied to the differential input transistor pair;
• and
• a common-mode feedback section, coupled to the differential input transistor pair, wherein the common-mode feedback section absorbs a current change in the current source to maintain a constant current in the output transistor.

A CMOS operational transconductance amplifier, comprising:

• a differential input pair providing an input stage;
• a current source, coupled to the differential input pair, for providing current to the differential input pair;
• an output transistor providing an output current;
• a first current mirror transistor coupled to a drain of a first transistor of the differential input pair;
• a second current mirror transistor coupled to a drain of a second transistor of the differential input pair, the second current mirror transistor being coupled to the output transistor for providing a mirrored current as the output current for the output transistor;
• a second current source;
• and
• a common-mode feedback differential pair, coupled to the second current source and to the first and second current mirror transistors, a transistor of the common-mode feedback differential pair mirroring current through the first current mirror transistor to compensate for a current change in the current source to maintain a constant mirrored current.