A CMOS power operational amplifier with large output voltage swing and high noise rejection is obtained by coupling a folded cascode type differential input stage and an output stage comprising an intermediate signal shifting amplifier and two common source output stages. Constant current generators inject into the drain of grounded gate MOS transistors pairs of said folded cascode type stage and of said intermediately signal shifting amplifier, respectively, a current which is pulled out of the source of the same grounded gate transistors by other constant current generators for increasing the effective transconductance of said grounded gate transistors pairs.