Rail-to-rail type of operational amplifier with a low offset voltage achieved by mixed compensation

An operational amplifier for generating an output in response to a first input voltage and a second input voltage, comprising:

• a first level-shifting input circuit, receiving said first input voltage, for dividing said first input voltage into a first subvoltage and a second subvoltage;
• a second level-shifting input circuit, receiving said second input voltage, for dividing said second input voltage into a third subvoltage and a fourth subvoltage;
• a first differential amplifier having a positive input end coupled to receive said first subvoltage from said first level-shifting input circuit and a negative input end coupled to receive said third subvoltage from said second level-shifting input circuit, said first differential amplifier generating a first differential current and a second differential current in response to said first and third subvoltages;
• a second differential amplifier having a positive input end coupled to receive said second subvoltage from said first level-shifting input circuit and a negative input end coupled to receive said fourth subvoltage from said second level-shifting input circuit, said second differential amplifier generating a third differential current and a fourth differential current in response to said second and fourth subvoltages;
• a first current-summing circuit for summing said first differential current and said second differential current from said first differential amplifier to thereby generate a first output potential;
• a second current-summing circuit for summing said third differential current and said fourth differential current from said second differential amplifier to thereby generate a second output potential;
• a bias circuit, taking the difference between said first output potential from said first current-summing circuit and said second output potential from said second current-summing circuit as input, for generating a bias voltage;
• an output circuit, coupled to said bias circuit, for generating the output voltage of said operational amplifier;
• a first compensation circuit for performing a cascaded-Miller compensation on the output voltage of said operational amplifier and then feeding back the compensated voltage to said first differential current;
• a second compensation circuit for performing a cascaded-Miller compensation on the output voltage of said operational amplifier and then feeding back the compensated voltage to said fourth differential current;
• a third compensation circuit for performing a mixed compensation on the output voltage of said operational amplifier and then feeding back the compensated voltage to the negative input of said first differential amplifier;
• and
• a fourth compensation circuit for performing a mixed compensation on the output voltage of said operational amplifier and then feeding back the compensated voltage to the negative input of said second differential amplifier.