OPERATIONAL AMPLIFIER

PURPOSE:To approach the amplitude of an output voltage to a power voltage as near as possible by adopting a same channel structure of an output transistor(TR) as that of a 1st TR used to generate a bias current, and selecting the direction of the dispersion of a threshold level in the same direction. CONSTITUTION:When a bias current ID41 flows, a current ID42 equal to the bias current ID41 flows to an output TR Q21. Suppose that the threshold voltage of the TR Q21 reaches a high level, a drain-source voltage VDS of the TR Q21 reaches a high level in response to the threshold voltage. Moreover, a TR Q41 of a bias current generating circuit 4 is a PMOS of the same channel structure as that of the TR Q21. Thus, even when the threshold voltage is high, the drain-source voltage VDS 21 of the TR Q21 is kept to a small level. As a result, the reduction in the amplitude of an output voltage VOUT is prevented and the amplitude of the output voltage VOUT is made close as near as possible to the power voltage VDD.