(1) Field of the Invention
The present invention relates to an operational amplifier circuit comprising a push-pull output stage having a stable operating point even when fluctuations occur in a power supply voltage.
(2) Description of the Related Art
An operational amplifier circuit is widely used for various analog operators. For example, a known prior art operational amplifier circuit comprises a differential amplifier circuit for generating an output voltage in response to a difference in potential between two input signals, a level shift circuit for shifting the output voltage of the differential amplifier circuit, and a push-pull output circuit which operates in response to the output voltages of the differential amplifier circuit and the level shift circuit (see: Japanese Unexamined Patent Publication (Kokai) No. 59-156012). In this prior art circuit, however, when fluctuations occur in a power supply voltage, fluctuations of the operating point of the operational amplifier circuit, i.e., the operating point of the push-pull output circuit, occur, thus increasing the fluctuation current, the fluctuation of the unity gain frequency characteristics, the fluctuation of the DC gain characteristics, and the like, which will be later explained in detail. This large fluctuation generated by the fluctuation of the power supply voltage is not advantageous in the design of operational amplifier circuits, since the scope of a design choice thereof is reduced thereby.
Therefore, an object of the present invention is to provide an operational amplifier circuit having a stable operating point, to thereby reduce the fluctuation of the current, the fluctuation of the unity gain frequency characteristics, the fluctuation of the DC gain characteristics, and the like.
According to the present invention, an operational amplifier circuit comprises a differential amplifier circuit for generating an output voltage in response to a difference in potential between two input signals, a level shift circuit for shifting the output voltage of the differential amplifier circuit, a push-pull output circuit which operates in response to the output voltages of the differential amplifier circuit and the level shift circuit, and a bias circuit for generating a bias voltage in response to a power supply voltage to control the level shift circuit. The output voltage of the level shift circuit is not affected fluctuations of the power supply voltage.