General structure of an opamp:

• Two-stage circuit architecture
  Two gain stages (unity-gain third stage)
  Most popular approach for both bipolar and CMOS opamps

• First block:
  Input differential amplifier
• High input impedance
• Large CMRR
• Large PSRR
• Low offset voltage
• Low noise
• High gain
  
• Second block:
  Level shifting
        Needed for dc voltage change compensation occurring in the input stage
  Added gain
  Differential-to-single-ended conversion

• Third block:
  

  Present when resistive load need to be driven

  Not necessary for the capacitive load

  
  Output buffer
• Low input impedance
• Larger output current to drive the load of the opamp
• No contribution to the voltage gain