Patent ID: 8487698

Claim:
An amplifier with pass-through mode comprising: a ground terminal maintained in a ground potential; a power supply terminal applied with a power supply voltage; signal input terminal to which an input signal is fed from a previous-stage circuit via a DC cut-off capacitor; a signal output terminal through which an output signal is fed to a subsequent-stage circuit via a DC cut-off capacitor; an amplification circuit including a load impedance element and a first amplification transistor, the first amplification transistor being configured such that the input signal with a first bias voltage superposed thereon is fed to a control terminal thereof, one main terminal thereof is connected to the power supply terminal via the load impedance element, the other main terminal thereof is directly or indirectly connected to the ground terminal, and the load impedance element being configured such that the output signal derived by inverting and amplifying the input signal is taken out through an end at the one main terminal of the first amplification transistor side thereof; a DC cut-off unit which cuts off a DC component of the output signal taken out through the end at the one main terminal of the first amplification transistor side of the load impedance element and feeds the output signal to the output terminal; a first bias circuit which receives as an input a mode control signal indicating an amplification mode or a pass-through mode, generates the first bias voltage for turning ON the first amplification transistor in the amplification mode, and generates the first bias voltage for turning OFF the first amplification transistor in the pass-through mode; a signal bypass circuit including a signal transmission transistor, the signal transmission transistor being applied at a control terminal thereof with a power supply voltage or a ground potential, turned OFF in the amplification mode, turned ON in the pass-through mode, one main terminal of the signal transmission transistor being connected to the output terminal and the input signal with the first bias voltage superposed thereon being fed to the other main terminal of the signal transmission transistor; and a second bias circuit including a bias control transistor and a high impedance element, the bias control transistor being applied at a control terminal thereof with a power supply voltage or a ground potential, turned OFF in the amplification mode, turned ON in the pass-through mode, one main terminal of the bias control transistor being connected to the output terminal, and the other main terminal of the bias control transistor being connected to the ground terminal via the high impedance element.