An amplifier circuit amplifies a differential signal supplied by a pair of input terminals. A phase controller circuit is placed between the emitters of two bipolar transistors and the ground. A feedback circuit is placed across the input and the output of the amplifier circuit for feeding the output of the amplifier circuit back to the input thereof. A phase change amount in the amplifier circuit is determined by the values of two inductors, whereas a phase change amount in the feedback circuit is determined by the values of two resistances and capacitors. The values of these devices are selected so that a phase difference between an input signal and a feedback signal is approximately 180 degrees in a range from the frequency of a fundamental wave of the input signal to the frequency of a second harmonic thereof.