Patent ID: 8305143

Claim:
An amplifier circuit comprising: a first field-effect transistor including a first gate, a first source and a first drain, the first gate receiving one of differential signals, and the first source being connected to a reference potential node; a second field-effect transistor including a second gate, a second source and a second drain, the second gate receiving the other of the differential signals, and the second source being connected to a reference potential node; a first transformer including a first primary inductor and a first secondary inductor which are magnetically coupled together, the first primary inductor being connected between the first drain of the first field-effect transistor and the second drain of the second field-effect transistor, and the first secondary inductor being connected between an output terminal and a reference potential node; a first switch circuit connecting a power-supply potential node or a reference potential node to a midpoint of the first primary inductor; a first inductor connected between the first gate of the first field-effect transistor and a gate bias node; a second inductor connected between the second gate of the second field-effect transistor and the gate bias node; and a bias switch circuit connecting a first bias node or a second bias node to the gate bias node.