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Transistor
A transistor is a three-terminal component made of three treated silicon slivers, pressed together, forming two connections between them. Each terminal has a distinct function in the device; the three are named the base, the collector and the emitter. A current flowing through the base and emitter terminals controls the flow between the emitter and collector. When no current flows through the base-emitter pair, the emitter-collector pair also has no flow; this condition is called cutoff. When maximum current flows through the transistor, it is in saturation. The two extreme states represent what a switch does, turning current fully on or off. Unlike a switch, a transistor also operates between the on and off states, controlling current in varying amounts.
Power and Efficiency
Transistors make more efficient switches than amplifiers. When it is fully on or off, the device itself uses little power. On the other hand, a single-transistor amplifier, called a Class A amplifier, uses more power than it delivers. For example, a Class A audio amplifier consumes 10 watts of power from a battery. Of that 10 watts, it sends only 2 watts to a loudspeaker. It uses up the other 8 watts acting as a current control, turning the power into heat and warming the transistor.
Bias and Pairing
The transistor controls current, but only direct current, or current moving in one direction. This is usually fine for switches, but it creates a problem for amplifying signals. Signals are AC, or alternating current -- they flow back and forth in two directions. To solve this problem, circuit designers can take one of two basic approaches. They bias the transistor̵7;s base and emitter terminals with a voltage so the back-and-forth flow stays either positive or negative all the time and never crosses the zero mark. Alternatively, a design uses multiple transistors, some of which amplify the positive half of a signal, the others amplifying the negative half. While more complicated than a single-transistor amplifier, this turns out to be more efficient.
Class D Amplifier
Some modern transistorized audio amplifiers use the transistor in its most efficient switching mode; these are called Class D. Normally, doing this results in extreme distortion, but these amplifiers employ a trick: they switch at speeds far higher than the human ear can hear, and the switching frequency has audio superimposed on it. While complex, these designs have good energy efficiency, running cool while sending hundreds of watts to the speakers.