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Valve amplifier from 1949, home built to a design by D T N Williamson.


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: A home-made Williamson vacuum tube (valve) amplifier from around 1949, built from a magazine article to a design by D T N Williamson. Photo by courtesy of the Science Museum, London, published on Wikimedia Commons under a Creative Commons Licence (CC BY-SA 2.0).

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  1. What is an amplifier?
  2. Distortion and feedback
  3. How does an amplifier work?
  4. Do amplifiers make energy?
  5. Types of amplifiers
  6. Who invented amplifiers?
  7. Find out more

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).

Inductive amplifier testing telephone equipment.

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 Tony Harp courtesy of US Air National Guard and DVIDS.

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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.)

Two charts comparing a linear amplifier response and a clipped response.

Charts: Top: Linear amplification: If the gain of an amplifier is always the same, no matter what the input signal, we call that a linear response. Here you can see that the output (vertical axis) is always ten times greater than the input (horizontal axis). Bottom: Clipping: In practice, no amplifier will have a perfectly linear response: it can only amplify so much. If the input signal is too large, the response will be linear up to a point and then "clipped" beyond it: even if you increase the input, the output doesn't increase as much.

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 Jimi Hendrix and Nirvana.

How amplifiers make feedback.

Artwork: How feedback occurs: Top: When the microphone (left) picks up sound (red arrow), the amplifier (not shown) boosts the signal and the loudspeaker (right) creates a louder version of the same sound. Bottom: If enough amplified sound from the loudspeaker reaches the microphone (blue arrow), it's amplified once again producing an even bigger sound (large blue arrow)—and so on. This produces the deafening sound of feedback.

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. In all these everyday cases, transformers are turning large voltages into smaller ones, (they're "step-down" transformers), but we can also use them the opposite way (as "step-up" devices) to boost smaller voltages into bigger ones.

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.

An electromagnetic relay photographed from the side, showing the spring contacts and electromagnet.

Photo: In a relay, a small current flowing through one circuit activates an electromagnet that allows a bigger current to flow through a second circuit. Relays can therefore work as amplifiers, but they tend to be noisy, their contacts can vibrate, and they can't switch fast enough for some applications. In this photo, you can see the spring contacts of the output circuit (left) and the electromagnet that pulls them together (right) when a current flows through the input circuit.

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 generally use a transistor-based amplifier. 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.

A FET transistor on a printed circuit board.

Photo: A typical transistor mounted on a circuit board. Technically, it's an A1048 designed for use in audio-frequency amplifiers. 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.

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.

So how come something like an electric guitar amplifier puts out more sound than it takes in? Isn't that creating energy? No! An amplifier almost always uses an external power supply of some sort and that accounts for the difference between the energy you get out and the energy you put in: the "extra" energy is coming from the power supply.

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.

An amplifier doesn't make energy. The total energy you get out is equal to the total energy you feed in.

Artwork: Amplifiers don't make energy. Like everything else in the universe, they obey the law of conservation of energy. With a guitar amplifier, the total energy it gives out (2) is equal to any energy it gets from your guitar (1) plus whatever energy it gets from its power supply (3), minus any energy it wastes as heat. So although it looks like you're changing a small amount of energy (1) into a bigger amount (2), the energy you supply from (3) explains the discrepancy; energy books always balance.

Alas, there's no such thing as energy for free—"extra" energy always has to come from somewhere. Even with Shockley's mule, energy isn't being created out of thin air: the difference in energy between the match you strike and the bucking mule comes from the food the mule must have consumed beforehand!

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Types of amplifiers

A airman uses giant ear trumpet amplifiers listening out for approaching airplanes at Bolling Field in 1921.

Photo: Amplifiers aren't always electrical. Before the invention of electronic hearing aids, ear trumpets were widely used by the hard of hearing to amplify sounds. They collect sound energy over a large area and funnel it into a much smaller area, so making it sound louder—that's simple, mechanical amplification. Exactly the same idea was used by the military to listen out for approaching enemy aircraft. In this photo from 1921, you can see an airman (red) at Bolling Field airbase using two gigantic ear trumpets (blue) pointed at the horizon. These days we use radar instead. Photo by National Photo Company courtesy of US Library of Congress.

If amplifiers have only one simple job to do—making a signal bigger while distorting it as little as possible—you might think one type of amplifier would be plenty. After all, how many different ways can you make something bigger? In fact, as a quick online search will reveal, there are zillions of different kinds of amplifier, they come in all shapes and sizes (from single transistors used in hearing aids right up to gigantic audio amps used to power loudspeakers at rock concerts) and we can classify them in many different ways. We could, for example, sort them by what they do for us (boosting radio signals, perhaps, or making the signals from a record-player pickup loud enough to push a loudspeaker back and forth) or how they do it (how their circuits are wired up inside); whether they work in an analog way or using digital circuits; whether they're used alone or in sequence with other amplifiers; how much gain they give to our signal or how efficiently they use power; and even by what sorts of components they're built from (vacuum tubes, transistors, or integrated circuits). With so many different factors to think about, amplifiers can be very confusing when you first encounter them; let's try to make sense of them all the same.

Classed by voltage, current, and power

Every amplifier takes in some kind of input signal (a certain current and voltage, which, together multiply to give a certain power level) and produces a bigger output signal (which may have a different current, voltage, or power). One very basic classification we can make is between voltage and power amplifiers. In a voltage amplifier, the output voltage is always bigger than the input voltage (so there's a voltage gain), although that doesn't necessarily mean there's also a gain in power (because the current could be reduced at the same time). In a power amplifier, the output power is always bigger than the input power because the product of the output voltage and output current (the output power) is bigger than the product of the input voltage and input current (the input power).

Amplifiers aren't always designed to turn a small voltage or power level into a bigger one; sometimes it's the current we're interested in instead. With conventional amplifiers, the gain we're interested in is defined as the ratio of the output voltage to the input voltage (or the output current to the input current). Sometimes, however, we want an amplifier to produce an output current that's proportional to our input voltage; for that job, we'd use what's called a transconductance amplifier. A transresistance amplifier does the opposite job (producing an output voltage proportional to the input current).

Classed by frequency

From crackly radio signals zapping through the air to scratchy sounds scraped from the face of an LP, amplifiers usually boost not a constant voltage or current but a fluctuating signal of some kind. By fluctuating, we mean that it changes at a certain frequency (so many times per second, measured as so many hertz, Hz). Audio signals (ones we can hear), for example, change in the broad frequency range from about 20 Hz to 20,000 kHz (the sound range young, keen human ears can detect); radio signals fluctuate thousands of times faster (in the range from kilohertz to megahertz); and video signals (used in TV broadcasting) cover a wide band of frequencies, equivalent to running from the very low audio (a few hertz) right up to the very high radio (many megahertz). Because of the way amplifiers are designed, they invariably work better at some frequencies than others. That means that an amplifier designed to faithfully boost audio signals is unlikely to work as effectively with radio or video signals—and vice-versa.

The electronic components inside a pocket-style analog hearing aid.

Photo: This electronic hearing aid from the 1950s–1960s (shown here with its case open) is essentially a pocket audio amplifier: the four black transistors on the left do the amplifying. All we're interested in amplifying here are audible sound frequencies. Early hearing aids amplified all frequencies by the same amount; modern aids can be tuned to give more selective amplification of particular frequencies to match a person's precise pattern of hearing loss.

Classes A–D

No amplifier is perfect in every respect or perfectly suited for every application; there are many different applications for amplifiers and many different types available. When you choose an amplifier for a particular application, you're always compromising on something—either gain (how much of a boost you get), linearity (how closely the output signal resembles the input, often informally referred to as "fidelity," especially for audio amplifiers), or efficiency (how much power you waste during the amplification process). Apart from being classified by things like voltage, power, current, and frequency, or their end-use, amplifiers are often also classified using letters of the alphabet (typically A to D), which, broadly speaking, tell you whether a certain amplifier is optimized for linearity, efficiency, or a compromise between them both.

Class A amplifiers generally provide the best output quality (the best linearity), but tend to be large, hot, heavy, power-hungry, and inefficient. Class B offer poorer linearity but are cheaper, run cooler, and are much more efficient. Class AB are a compromise solution, aiming for the output quality of class A and the efficiency of class B. Class C amplifiers have much higher efficiency but much poorer output quality. You'll also sometimes see other amplifier classes (D, F, G, H, I, S, and T), though we'll not go into those any further here.

Guitar amplifier app by AmpKit, simulating a Peavey ValveKing on an iPod Touch.

Photo: And, yes, if you want a guitar amp but you can't afford one, there's an app for it. This one's AmpKit, a "virtual amplifier" that runs on your iPhone. Simply plug in your guitar, download your choice of amps, pedals, cabinets, and mikes, and off you go. Here I'm simulating a Peavey ValveKing on an iPod Touch, but this app offers lots of others to choose from.

Classed by construction

Does it really matter how an amplifier is made if it boosts your signal? To many people, the answer is a very resounding "yes." Hi-fi buffs often swear by old-style vacuum-tube ("valve") amplifiers, which they insist give a "warm," hi-fidelity sound, though inevitably much comes down to personal preference. Vacuum tubes might be great if your objective is to drive a pair of classy speakers with an expensive turntable, but they're no use whatsoever if you're deaf and you want a small, discreet, but still highly accurate hearing-aid. Back in the 1950s and 1960s, transistors revolutionized hearing-aid technology but did little more than boost signals from a microphone (amplifying all the input equally, so making background noise worse as well as the sounds you really wanted to hear); from the 1980s onward, digital hearing aids used much more complex integrated circuits to provide more selective amplification of signals, without noise, tailored precisely to each person's specific pattern of hearing loss. In other words, it matters very much how an amplifier's constructed.

Other special amplifiers

Amplifiers aren't always designed to carry out simple, straightforward signal boosting. Operational amplifiers ("op-amps"), for example, are common components in electronic circuits that can be used for all kinds of things, from basic computing operations like addition and subtraction to signal filtering and oscillation.

Who invented amplifiers?

Although he liked to refer to himself as the "father of radio," American physicist and electronics engineer Lee de Forest might be better described as the "father of the amplifier."

Lee de Forest pictured c.1920–25

Photo: Lee de Forest (1873–1961) pictured in the 1920s. Photo by Bain News Service courtesy of US Library of Congress.

In the early years of the 20th century, he was working in radio ("wireless telegraphy" as it was then often called) and had filed a couple of dozen patents on improved antennas and receivers. On October 25, 1906, he filed a patent for a "device for amplifying feeble electrical currents": a compact, electron vacuum tube called the Audion, later known as the triode (because, in its final version, it had three key electrical components inside). Although de Forest originally imagined the triode as a telephone-circuit amplifier, it eventually became an essential component of radio receivers. Despite inventing the Audion, de Forest neither perfected it nor ever really understood how it worked (the two things may well have been connected); it was left to others (notably Irving Langmuir of General Electric and Edward Armstrong, inventor of FM radio) to turn the idea into a practical device and explain the physics behind it.

Lee de Forest's sketch of his audion triode vacuum tube amplifier from his US Patent 879,532.

Artwork: An original sketch of Lee de Forest's Audion (triode) vacuum tube from one of his later patents, US Patent 879,532: Space Telegraphy, filed January 29, 1907 and granted on February 18, 1908. from which this artwork is taken, courtesy of the US Patent and Trademark Office.

How, then, does it work? The sealed glass vacuum tube (shaded gray and labeled D, from which all the air has been pumped out) has three key components inside, which I've colored red, blue, and green. On the left, there's a wire filament (red, F) heated by a battery. This is the cathode or negative terminal. On the right, there's a platinum plate (green, b), which is the positive terminal. In between them, there's a grid of platinum wire (blue, a). When the filament is heated, negatively charged electrons boil off it and are pulled toward the positively charged plate, making an electric current flow. A negative voltage is also applied to the grid, effectively "braking" the flow of electrons. Because the grid is so close to the filament, even tiny changes to its voltage will make a huge difference to the current that flows from the filament to the plate. If we consider the grid to be the amplifier's input, the cathode-plate circuit is its output; a small signal applied to the grid can become a much larger, amplified signal at the plate.

Vacuum tubes based on the triode made splendid amplifiers, but they were large, unreliable, and power-hungry. When John Bardeen, Walter Brattain, and William Shockley developed their "solid-state" transistor in 1947, they solved all three problems at a stroke, making possible small, portable, highly reliable amplifiers for such things as hearing aids and transistor radios. When integrated circuits were invented, in the late 1950s, they led to smaller, more complex amplifier circuits packaged as single chips (as op-amps typically are now).

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