Rail-to-rail class-AB operational amplifier

A rail-to-rail class-AB operational amplifier, comprising:

• a first differential pair unit, for receiving a pair of differential signals and generating a first control signal;
• a second differential pair unit, for receiving the pair of differential signals and generating a second control signal;
• and
• an output stage, for receiving the first control signal and the second control signal and then generating an output voltage, wherein:
• the first differential pair unit includes:
• a first active load, having one terminal connected to an operation voltage;
• a first transistor differential pair, having gates for respectively receiving the pair of differential signals, drains connected to the first active load and sources connected to each other;
• and
• a first current source, having one terminal connected to the sources of the first transistor differential pair, and the other terminal grounded;
• the second differential pair unit includes:
• a second current source, having one terminal connected to the operation voltage;
• a second transistor differential pair, having gates for receiving the differential signals, and sources connected to each other and to the other terminal of the second current source;
• and
• a second active load, having one terminal connected to the second transistor differential pair, and the other terminal grounded;
• and
• the output stage includes:
• a third current source, having a first terminal connected to the operation voltage and a second terminal for receiving the first control signal;
• a fourth current source, having a first terminal for receiving the second control signal, and a second terminal grounded;
• a parallel transistor pair, which is disposed between the second terminal of the third current source and the first terminal of the fourth current source, and has gates for respectively receiving a pair of bias voltages;
• a first output transistor, having a source connected to the operation voltage, a gate for receiving the first control signal and a drain for generating the output voltage;
• and
• a second output transistor, having a source grounded, a gate for receiving the second control signal and a drain connected to the drain of the first output transistor.