Operational amplifier and differential amplifying circuit thereof

An operational amplifier, having a differential input pair and a differential output pair, comprising:

• an output stage amplifying circuit, using a first terminal and a second terminal as an input port thereof and using said differential output pair as an output port thereof;
• a first transistor pair, comprising a first transistor and a second transistor, wherein a first end of said first transistor and a first end of said second transistor are respectively coupled to a first input end and a second input end of said differential input pair, and a second end of said first transistor and a second end of said second transistor are respectively coupled to said first terminal and said second terminal;
• a second transistor pair, comprising a third transistor and a fourth transistor, wherein a first end of said third transistor and a first end of said fourth transistor are respectively coupled to said first input end and said second input end of said differential input pair, and a second end of said third transistor and a second end of said fourth transistor are respectively coupled to said first terminal and said second terminal;
• a first current source, coupled to said first terminal;
• a second current source, coupled to said second terminal;
• a third transistor pair, comprising a fifth transistor and a sixth transistor, wherein a first end of said fifth transistor and a first end of said sixth transistor respectively receive a control signal, and a second end of said fifth transistor and a second end of said sixth transistor are respectively coupled to said first terminal and said second terminal;
• and
• a control circuit, coupled to said differential output pair, for generating said control signal according to voltages of said differential output pair and a common mode voltage.