Operational amplifier with class AB output

A multi-stage operational amplifier, comprising:

• a first stage configured to amplify an input voltage;
• a second stage configured to drive an output terminal of the multi-stage operational amplifier, wherein the second stage is coupled to an output of the first stage;
• a first auxiliary amplifier coupled to the output of the input stage and to a first input of the second stage, the first auxiliary amplifier configured to:
• generate a first bias voltage and apply the first bias voltage to the second stage for class AB operation;
• and
• isolate the first stage from the first bias voltage;
• and
• a second auxiliary amplifier coupled to the output of the input stage and to a second input of the second stage, the second auxiliary amplifier configured to:
• generate a second bias voltage and apply the second bias voltage to the second stage for class AB operation;
• and
• isolate the first stage from the second bias voltage,
• wherein each of the first auxiliary amplifier and the second auxiliary amplifier comprises a first input transistor and a second input transistor coupled as a differential pair, and wherein in each of the first auxiliary amplifier and the second auxiliary amplifier an input terminal of the first input transistor is coupled to the output of the first stage, and an input terminal of the second input transistor is connected to a predetermined DC voltage.

A class AB output operational amplifier, comprising:

• an output stage comprising a high-side output transistor and a low-side output transistor;
• an input stage configured to drive the output stage;
• a first auxiliary amplifier coupled to an output of the input stage and an input of the low-side output transistor, the first auxiliary amplifier configured to bias the low-side output transistor for class AB operation and to isolate the input stage from a bias voltage applied to the low-side output transistor, the first auxiliary amplifier comprising:
• a first transistor and a second transistor coupled as a differential pair, wherein an input terminal of the first transistor is coupled to the output of the input stage, and an input terminal of the second transistor is connected to a predetermined DC voltage;
• and
• a second auxiliary amplifier coupled to the output of the input stage and an input of the high-side output transistor, the second auxiliary amplifier configured to bias the high-side output transistor for class AB operation and to isolate the input stage from a bias voltage applied to the high-side output transistor, the second auxiliary amplifier comprising:
• a third transistor and a fourth transistor coupled as a differential pair, wherein an input terminal of the third transistor is coupled to the output of the input stage, and an input terminal of the fourth transistor is connected to the predetermined DC voltage,
• wherein isolation of the input stage from the output stage by the first auxiliary amplifier and the second auxiliary amplifier provides the input stage with headroom of approximately one-half of the power supply voltage that provides supply voltage to the operational amplifier.