An amplifier takes a weak audio signal and boosts it to a stronger signal that can drive a speaker. It does this through a process of multiple boosting stages within the amplifier. At the heart of most amplifiers is a transistor, which uses variations in its electrical resistance controlled by a small input signal to modify a larger output current supplied by the amplifier's power source. This allows it to accurately reproduce the original audio signal but at a higher volume level suitable for the speaker.