what does the acronym “h.e.l.p.” stand for?
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Which power amplifier has highest efficiency?
The amplifier that has the highest efficiency is a class D amplifier.
It has the highest power efficiency compared to other analogue classes such as A, B, AB, and C amplifiers. The D amplifier basically makes use of non-linear switching technology and the output devices can be either on or off. As a result, Class-D amplifiers can even reach up to 100% efficiency. Class-D amplifiers make use of some form of pulse-width modulation to control the output devices. Additionally, the switching elements also have the lowest resistance when fully on resulting in lowest power dissipation.
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).
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
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 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 also used in electricity substations
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.
If you want to amplify a fluctuating signal
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.
As we’ve already seen
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.
Power amplifier classes
In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier’s characteristics and performance.
The classes are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input.
A class A amplifier is conducting through all the period of the signal; Class B only for one-half the input period, class C for much less than half the input period.
A Class D amplifier operates its output device in a switching manner; the fraction of the time that the device is conducting is adjusted so a pulse-width modulation output is obtained from the stage.
Additional letter classes are defined for special purpose amplifiers,
with additional active elements or particular power supply improvements; sometimes a new letter symbol is used by a manufacturer to promote its proprietary design.
Efficiency is the ratio of total output power to input power, expressed in percent. … If a power supply with 50% efficiency is required to provide a 50W power to a load, it will draw 100 W from the wall. The other 50% gets wasted as heat and other losses.