Patent ID: 7375589

Claim:
A power amplifier system comprising: a power terminal; a ground terminal; an output terminal; a ripple terminal; a control terminal to which a control signal for start-up is supplied from outside; first and second MOS transistors that configure a power amplifier circuit connected between the power terminal and the ground terminal, the first and second MOS transistors configuring a push-pull circuit, the first and second MOS transistors having the connection node thereof connected to the output terminal; a negative potential detection circuit being connected to the output terminal, which detects the potential of the output terminal becomes negative; a bias circuit which supplies a bias voltage to the power amplifier circuit, the bias circuit having its output node for outputting the bias voltage connected to the ripple terminal; a bias start-up circuit to which the control signal supplied to the control terminal, a detection signal supplied from the negative potential detection circuit, and the potential of the ripple terminal are supplied, the bias start-up circuit controlling the start-up operation of the bias circuit in accordance with the control signal, the detection signal, and the potential of the ripple terminal; and a ripple filter capacitor connected to the ripple terminal.