An amplifier circuit includes: a 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 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 inductor and a first secondary inductor which are magnetically coupled together, the first 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; and a first switch circuit connecting a power-supply potential node or a reference potential node to a midpoint of the first inductor.