A power supply circuit supplies a variable voltage to a power amplifier that amplifies a radio-frequency signal, and includes a transistor and a current detecting resistor. The transistor includes a collector or drain that is supplied with a fixed voltage from a fixed voltage source, a base or gate that receives an envelope signal tracking an envelope of the radio-frequency signal, and an emitter or source that outputs the variable voltage that is based on the envelope signal. The current detecting resistor is electrically connected between the fixed voltage source and the collector or drain of the transistor.