An amplifier circuit comprises a variable degeneration circuit connected to emitter terminals of transistors, and a variable negative capacitance circuit connected to differential output signal terminals. The variable degeneration circuit includes a variable capacitor and a resistor. The variable negative capacitance circuit, which is a variable current source, includes a transistor, a capacitor, and a variable current source. The variable negative capacitance circuit includes transistors, a capacitor, and variable current sources.