Patent ID: 7408402

Claim:
An operational amplifier, comprising: a differential pair section; a load section configured to function as active load of said differential pair section, said load section comprises: a first pair of P-channel MOS transistors connected to a higher power supply voltage; a second pair of P-channel MOS transistors; a first load switch section configured to switch connections between said P-channel MOS transistors of said first pair and said P-channel MOS transistors of said second pair; a third pair of N-channel MOS transistors connected to a lower power supply voltage; a fourth pair of N-channel MOS transistors; a second load switch section configured to switch connections between said N-channel MOS transistors of said third pair and said N-channel MOS transistors of said fourth pair; a floating constant current source provided between one of said P-channel MOS transistors of said second pair and one of said N-channel MOS transistors of said fourth pair to supply a constant current; and an output buffer amplifier connected to provided between the other of said P-channel MOS transistors of said second pair and the other of said N-channel MOS transistors of said fourth pair; and a switch section configured to switch supply of a differential input signal to said differential pair section and to switch connection of outputs of said differential pair section to said load section, wherein said switch section is operated to cancel an offset voltage of said operational amplifier.