A modulation transformer is an audio-frequency transformer that forms a major part of most AM transmitters. The primary winding of a modulation transformer is fed by an audio amplifier that has about 1/2 of the rated input power of the transmitter's final amplifier stage. The secondary winding is in series with the power supply of that final radio-frequency amplifier stage, thereby lowering and rising the operating voltage of the power amplifier (PA) tube or transistor. Considering that the PA device is operated as a class-C amplifier, i.e. as a switch, the modulation transformer is responsible for the amplitude modulation (AM) of the transmitter.