Amplifiers

Last updated: September 21, 2009.

William Shockley, Nobel-Prize winning co-inventor of the transistor (a revolutionary electronic amplifier dating from the 1940s) had a vivid way of explaining it: "If you take a bale of hay and tie it to the tail of a mule and then strike a match and set the bale of hay on fire, and if you then compare the energy expended shortly thereafter by the mule with the energy expended by yourself in the striking of the match, you will understand the concept of amplification."

Amplifiers are the tiny components in hearing aids that make voices sound louder. They're also the gadgets in radios that boost faraway signals and the devices in stereo equipment that drive your loudspeakers and the huge black boxes you plug into electric guitars to make them raise the roof. What are amplifiers? How do they work? Let's take a closer look!

Photo: An amplifier mixing console (also called a mixing board) used to control the output from a public address system. Photo by Esperanza Berrios courtesy of US Air Force and Defense Imagery.

What is an amplifier?

An amplifier (often loosely called an "amp") is an electromagnetic or electronic component that boosts an electric current. If you wear a hearing aid, you'll know it uses a microphone to pick up sounds from the world around you and convert them into a fluctuating electric current (a signal) that constantly changes in strength. A transistor-based amplifier takes the signal (the input) and boosts it many times before feeding it into a tiny loudspeaker placed inside your ear canal so you hear a much-magnified version of the original sounds (the output).

It's easy to calculate how much difference an amplifier makes: it's the ratio of the output signal to the input signal, a measurement called the gain of an amplifier (or sometimes the gain factor or amplification factor). So an amplifier that doubles the size of the original signal has a gain of 2. For audio (sound) amplifiers, the gain is often expressed in decibels (specifically, it's ten times the logarithm of the output power divided by the input power).

Photo: Testing telephone circuits with an inductive amplifier. It's a type of probe that can test a circuit without direct electrical contact and works through electromagnetic induction, a bit like induction chargers. Photo by Denise Rayder courtesy of US Air Force and Defense Imagery.

Distortion and feedback

Now the key thing about an amplifier is not just that it boosts an electric current. That's the easy bit. The hard bit is that it must faithfully reproduce the quality of the input signal even when that signal is constantly (and sometimes dramatically) varying in both frequency and amplitude (for an audio amplifier, that means volume).

An audio amplifier might work better with some sound frequencies than others; the range of frequencies over which it works satisfactorily is called its bandwidth. Ideally, it has to produce a reasonably flat response or linear response with a wide range of different input signals (so the gain is pretty much constant across a range of frequencies). If the amplifier doesn't faithfully reproduce input frequencies in its output, it suffers from what's called a frequency response, which means it boosts some frequencies more than others. (Sometimes this effect is deliberate. Small earbud headphones are often designed this way so they give extra bass.)

Amplifiers also have to work across a wide range of amplitudes (typically that means sound volumes), which leads to another problem. As the input amplitude increases, the amplifier will struggle to produce a corresponding increase in output, because there's a limit to how much power it can make. That means any further increases in the input will simply produce the same level of output—a phenomenon known as clipping—and increasing amounts of distortion.

Another problem amplifiers have is called feedback—and people who use microphones on stage are very familiar with it. If a microphone is turned up too much or placed too near to a loudspeaker, it picks up not only the sound of a person's voice or an instrument (as it's supposed to), but also the amplified sound of the voice or instrument coming from the speaker slightly after, which is then re-amplified—only to pass through the speaker once more and be amplified yet again. The result is the horribly deafening whistle we call feedback. Many rock stars and groups have made feedback effects a deliberate part of their sound, including Jim Hendrix and Nirvana.

How does an amplifier work?

An amplifier's job is to turn a small electric current into a larger one, and there are various different ways to achieve this depending on exactly what you're trying to do.

If you want to boost a reasonably constant electric voltage, you can use an electromagnetic device called a transformer. Most of us have a house full of transformers without realizing it. They're widely used to drive low-voltage appliances such as MP3 players and laptop computers from higher-voltage household power outlets, They're also used in electricity substations to convert very high-voltage electricity from power plants to the much lower voltages that homes and offices require.

If the input current is simply a brief pulse of electricity designed to switch something on or off, you can use an electromagnetic relay to amplify it. A relay uses electromagnets to couple two electric circuits together so that when a small current flows through one of the circuits, a much larger current flows through the other. Using a relay, a tiny electric current can power something that would normally need a much larger current to operate it. For example, you might have a photoelectric cell ("magic eye") set up to receive a beam of invisible infrared light in an intruder alarm. When someone breaks the beam, a tiny current is sent to a relay that snaps into action and turns on a much larger current that rings the alarm bell on the side of a house. The tiny output current from a photoelectric cell would be far too small to power a bell all by itself.

Photo: A typical transistor mounted on a circuit board. It has three connections, the base, collector, and emitter (though it's hard to tell which is which from this photo). Hundreds, thousands, or even millions of these are built into tiny chips called integrated circuits.

If you want to amplify a fluctuating signal, such as a radio or TV signal, the sound of someone's voice coming down a telephone line, or the input from a microphone in a hearing aid, you'd use a transistor-based amplifier instead. A transistor has three wire connections called a base, an emitter, and a collector. When you feed a small input current between the base and the emitter, you get a much larger output current flowing between the emitter and the collector. So in something like a hearing aid, very broadly speaking, you'd feed the output from the microphone to the base and use the output from the collector to drive the loudspeaker. Before transistors were invented in 1947, much larger electronic amplifiers called vacuum tubes (popularly known as "valves" in the UK) were used in such things as TVs and radios.

As we've already seen, there's a limit to how much an amplifier will boost a signal without clipping or distortion. One way to get around this is to connect more than one amplifier together so the output from one feeds into the next one's input—and so on, in a chain, until you get as much of a boost as you need. Devices that work like this are called multistage amplifiers.

Some types of audio equipment use two separate amplifiers—a pre-amplifier ("pre-amp") and a main amplifier. The pre-amplifier takes the original signal and boosts it to the minimum input level that the main amplifier can handle. The main amplifier then boosts the signal enough to power loudspeakers. Such things as record-player turntables and MP3 players (played through big stereo equipment) typically need pre-amplifiers.

Do amplifiers make energy?

Whichever kind of amplifier you use, you never get out more energy than you put in. It's true that the output current or voltage may be many times bigger than the input signal, but that doesn't mean you're generating extra energy for free—a basic law of physics called the conservation of energy doesn't allow such things. An amplifier almost always uses an external powerful supply of some sort and that accounts for the difference between the energy you get out and the energy you put in. Think about a typical hearing aid. There's much more sound energy coming out of its loudspeaker than there is going into its microphone, but that doesn't mean it's making energy out of thin air. The transistor (or integrated circuit chip) that's amplifying the input signal has to be powered by batteries, and that's where the extra energy is coming from. Similarly, with an electric guitar: you have to plug the amplifier into an electrical outlet before you hear any sound. Alas, there's no such thing as energy for free—"extra" energy always has to come from somewhere.