How Amplifiers Work

In the last section, we saw that an amplifier's job is to take a weak audio signal and boost it to generate a signal that is powerful enough to drive a speaker. This is an accurate description when you consider the amplifier as a whole, but the process inside the amplifier is a little more complex.

In actuality, the amplifier generates a completely new output signal based on the input signal. You can understand these signals as two separate circuits. The output circuit is generated by the amplifier's power supply, which draws energy from a battery or power outlet. If the amplifier is powered by household alternating current, where the flow of charge changes directions, the power supply will convert it into direct current, where the charge always flows in the same direction. The power supply also smoothes out the current to generate an absolutely even, uninterrupted signal. The output circuit's load (the work it does) is moving the speaker cone.

The input circuit is the electrical audio signal recorded on tape or running in from a microphone. Its load is modifying the output circuit. It applies a varying resistance to the output circuit to re-create the voltage fluctuations of the original audio signal.

In most amplifiers, this load is too much work for the original audio signal. For this reason, the signal is first boosted by a pre-amplifier, which sends a stronger output signal to the power amplifier. The pre-amplifier works the same basic way as the amplifier: The input circuit applies varying resistance to an output circuit generated by the power supply. Some amplifier systems use several pre-amplifiers to gradually build up to a high-voltage output signal.

So how does the amplifier do this? If you look inside an amplifier for an answer, you'll only find a complex mass of wires and circuitry components. The amplifier needs this elaborate setup to make sure each part of the audio signal is represented correctly and accurately. Hi-fidelity output requires very precise control.

All of the pieces in an amplifier are important, but you certainly don't need to examine each one to understand how an amplifier works. There are only a few elements that are crucial to the amplifier's functioning. In the next section, we'll see how these elements come together in a very basic amplifier design.

The component at the heart of most amplifiers is the transistor. The main elements in a transistor are semiconductors, materials with varying ability to conduct electric current. Typically, a semiconductor is made of a poor conductor, such as silicon, that has had impurities (atoms of another material) added to it. The process of adding impurities is called doping.

In pure silicon, all of the silicon atoms bond perfectly to their neighbors, leaving no free electrons to conduct electric current. In doped silicon, additional atoms change the balance, either adding free electrons or creating holes where electrons can go. Electrical charge moves when electrons move from hole to hole, so either one of these additions will make the material more conductive. (See How Semiconductors Work for a full explanation.)

N-type semiconductors are characterized by extra electrons (which have a negative charge). P-type semiconductors have an abundance of extra holes (which have a positive charge).

Let's look at an amplifier built around a basic bipolar-junction transistor. This sort of transistor consists of three semiconductor layers -- in this case, a p-type semiconductor sandwiched between two n-type semiconductors. This structure is best represented as a bar, as shown in the diagram below (the actual design of modern transistors is a little different).

The first n-type layer is called the emitter, the p-type layer is called the base and the second n-type layer is called the collector. The output circuit (the circuit that drives the speaker) is connected to electrodes at the transistor's emitter and collector. The input circuit connects to the emitter and the base.

The free electrons in the n-type layers naturally want to fill the holes in the p-type layer. There are many more free electrons than holes, so the holes fill up very quickly. This creates depletion zones at the boundaries between n-type material and p-type material. In a depletion zone, the semiconductor material is returned to its original insulating state -- all the holes are filled, so there are no free electrons or empty spaces for electrons, and charge can't flow. When the depletion zones are thick, very little charge can move from the emitter to the collector, even though there is a strong voltage difference between the two electrodes.

In the next section, we'll see what can be done to change this situation.

When depletion zones are thick, you can boost the voltage on the base electrode. The voltage at this electrode is directly controlled by the input current. When the input current is flowing, the base electrode has a relative positive charge, so it draws electrons toward it from the emitter. This frees up some of the holes, which shrinks the depletion zones. As the depletion zones are reduced, charge can move from the emitter to the collector more easily -- the transistor becomes more conductive. The size of the depletion zones, and therefore the conductivity of the transistor, is determined by the voltage at the base electrode. In this way, the fluctuating input current at the base electrode varies the current output at the collector electrode. This output drives the speaker.

A single transistor like this represents one "stage" of an amplifier. A typical amplifier will have several boosting stages, with the final stage driving the speaker.

In a small amplifier -- the amplifier in a speaker phone, for example -- the final stage might produce only half a watt of power. In a home stereo amplifier, the final stage might produce hundreds of watts. The amplifiers used in outdoor concerts can produce thousands of watts.

The goal of a good amplifier is to cause as little distortion as possible. The final signal driving the speakers should mimic the original input signal as closely as possible, even though it has been boosted several times.

This basic approach can be used to amplify all kinds of things, not just audio signals. Anything that can be carried by an electrical current -- radio and video signals, for example -- can be amplified by similar means. Audio amplifiers seem to catch people's attention more than anything else, however. Sound enthusiasts are fascinated with variations in design that affect power rating, impedance and fidelity, among other specifications.

For much more information on amplifiers, check out the links on the next page.