Patent ID: 8552799

Claim:
A power amplifying circuit, comprising: a first operational amplifier connected to a first signal input terminal at a non-inverting input terminal thereof; a first feedback resistor connected between an output terminal of the first operational amplifier and an inverting input terminal of the first operational amplifier; a second feedback resistor connected to the inverting input terminal of the first operational amplifier at a first end thereof; a second operational amplifier connected to a second signal input terminal at a non-inverting input terminal thereof; a third feedback resistor connected to an inverting input terminal of the second operational amplifier at a first end thereof and to a second end of the second feedback resistor at a second end thereof; a fourth feedback resistor connected between an output terminal of the second operational amplifier and an inverting input terminal of the second operational amplifier; a fully differential operational amplifier that is connected to the output terminal of the first operational amplifier at a non-inverting input terminal thereof, to the output terminal of the second operational amplifier at an inverting input terminal thereof, to a first signal output terminal at a non-inverting output terminal thereof, and to a second signal output terminal at an inverting output terminal thereof and maintains a constant differential gain; a switching circuit connected between the second end of the second feedback resistor and the ground; a first input resistor connected between the first signal input terminal and a reference terminal to which a reference voltage is applied; a second input resistor connected between the second signal input terminal and the reference terminal; and a midpoint potential controlling circuit that monitors a power supply voltage and controls the switching circuit based on the value of the power supply voltage, wherein the midpoint potential controlling circuit turns on the switching circuit when the power supply voltage is equal to or higher than a preset switching threshold, and turns off the switching circuit when the power supply voltage is lower than the switching threshold.