Amplifier

For the British rock band of the same name, see Amplifier (band)

An amplifier can be considered to be any device that uses a small amount of energy to control a source of a larger amount of energy, although the term today usually refers to an electronic amplifier. The relationship of the input to the output of an amplifier — usually expressed as a function of the input frequency — is called the transfer function of the amplifier, and the magnitude of the transfer function is termed the gain.

Contents

General characteristics of amplifiers

Most amplifiers can be characterised by a number of parameters.

Gain

How much an amplifier increases the signal level is called the gain. This is usually measured in decibels (dB). Mathematically speaking, the gain is equal to the output level divided by the input level. (for power gain in decibels gain is computed by the relation G(dB)=10log(Pout/Pin)(Electrical)).

Output This is the range usually quoted in dB between the lowest useful output and the largest useful output level. Since the lowest useful level is limited by output noise, this is quoted as the amplifier dynamic range.

Bandwidth and rise time

The bandwidth (BW) of an amplifier is usually defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB BW. Bandwidths for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.).

As an example, a good audio amplifier will have a −3 dB BW from around twenty hertz to about twenty kilohertz (the range of normal human hearing).

The rise time of an amplifier is the time taken for the output to change from 10% to 90% of its final level when driven by a step input. For a Gaussian response system (or a simple RC roll off), the rise time is given by:

Tr = BW/0.35, where BW is in Hz and Tr is in seconds.

Settling time and aberrations

Time taken for output to settle to within a certain percentage of the final value (say 0.1%). This is usually specified for oscilloscope vertical amplifiers and high accuracy measurement systems.

Slew rate

Slew rate is the maximum rate of change of output variable, usually quoted in volts per second (or microsecond).

Sine wave distortion

The properties of amplifier circuits distort the signal. This distortion comes in several forms including harmonic distortion and intermodulation distortion.

Noise

How much noise is introduced by the amplification process? This is an undesirable but inevitable result of the electronic devices and components. It is measured in either decibels or the peak output voltage produced by the amp when no signal is applied.

Efficiency

How much of the input power is usefully applied to the amplifier's output? Class A amplifiers are very inefficient, in the range of 10–20% with a max efficiency of 25%. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%. Commercially available class D amplifiers have reported efficiencies as high as 97%. The efficiency of the amplifier limits the amount of total power output that is usefully available. Note that more efficient amps run much cooler, and often do not need any fans even in multi-kilowatt designs.

Linearity

An ideal amplifier would be a totally linear device, but real amplifiers are only linear within certain practical limits. When the signal drive to the amplifier is increased, the output also increases until a point is reached where some part of the amplifier becomes saturated and cannot produce any more output; this is called clipping, and results in distortion.

Some amplifiers are designed to handle this in a controlled way which causes a reduction in gain to take place instead of excessive distortion; the result is a compression effect, which (if the amplifier is an audio amplifier) will sound much less unpleasant to the ear. For these amplifiers, the 1dB compression point is defined as the input power (or output power), where the gain is 1dB less than the small signal gain.

Linearization is an emergent field, and there are many techniques, such us feedforward, predistortion, postdistortion, EER, LINC, CALLUM, cartesian feedback... in order to avoid the undesired effects of the non-linearities.

Electronic amplifiers

There are many types of electronic amplifiers for different applications.

One common type of amplifier is the electronic amplifier, commonly used in radio and television transmitters and receivers, high-fidelity ("hi-fi") stereo equipment, microcomputers and other electronic digital equipment, and guitar and other instrument amplifiers. Its critical components are active devices, such as vacuum tubes or transistors.

Power amplifier classes

Amplifiers are commonly classified by the conduction angle (sometimes known as 'angle of flow') of the input signal through the amplifying device; see electronic amplifier.

Class A: Where efficiency is not a consideration, most small signal linear amplifiers are designed as Class A, which means that one active device amplifies all portions (360deg) of the input signal.

Class B: In Class B, there are two output devices (or sets of output devices), each of which conducts alternately for exactly 180 deg (or half cycle) of the input signal.

Class AB: Class AB amplifiers are a compromise between Class A and B, which improves small signal output linearity; conduction angles vary from 180 degrees upwards, selected by the amplifier designer. Usually found in low frequency amplifiers (such as audio and hi-fi) owing to their relatively high efficiency, or other designs where both linearity and efficiency are important (cell phones, cell towers, TV transmitters).

Class C: Popular for high power RF amplifiers, Class C is defined by conduction for less than 180° of the input signal. Linearity is not good, but this is of no significance for single frequency power amplifiers. The signal is restored to near sinusoidal shape by a tuned circuit, and efficiency is much higher than A, AB, or B classes of amplification.

Class D: Class D amplifiers are usually dedicated to audio, and based on PWM (Pulse Width Modulation) techniques to achieve a very high power efficiency (more than 90% in modern designs). Formerly used for subwoofers only due to their limited bandwidth and relatively high distortion, the evolution of semiconductor devices has made possible the development of very high fidelity, full audio range Class D amplifiers, with S/N and distortion levels even better than their linear counterparts.

Other classes: There are several other amplifier classes, although they are mainly combinations of the previous classes. For example, Class H and Class I amplifiers are marked by variation of the supply rails (in discrete steps or in a continuous fashion, respectively) following the input signal. Wasted heat on the output devices can be reduced as excess voltage is kept to a minimum. The amplifier that is fed with these rails itself can be of any class. These kinds of amplifiers are complicated and have some problems that limit their applications.

Vacuum tube (valve) amplifiers

According to Symons, while semiconductor amplifiers have largely displaced valve amplifiers for low power applications, valve amplifiers are much more cost effective in high power applications such as "radar, countermeasures equipment, or communications equipment" (p. 56). Many microwave amplifiers are specially designed valves, such as the klystron, gyrotron, traveling wave tube, and crossed-field amplifier, and these microwave valves provide much greater single-device power output at microwave frequencies than solid-state devices (p. 59).

Valve amplifiers are widely, but not always correctly, associated with the valve sound. Some claim this sound has more to do with the circuit topology and circuit design of the amplifier, than to the use of valves rather than transistors as the active gain devices. In the earlier years of audio, vacuum tubes filled the active device role.

Transistor amplifiers

Main articles: transistor, bipolar junction transistor, MOSFET

The essential role of this active element is to magnify an input signal to yield a significantly larger output signal. The amount of magnification (the "forward gain") is determined by the external circuit design as well as the active device.

Many common active devices in transistor amplifiers are bipolar junction transistors (BJTs) and metal oxide semiconductor field-effect transistors (MOSFETs).

Applications are numerous, some common examples are audio amplifiers in a home stereo or PA system, RF high power generation for semiconductor equipment, to RF and Microwave applications such as radio transmitters.

Operational amplifiers (op-amps)

Main articles: operational amplifier, instrumentation amplifier

An operational amplifier is a solid state integrated circuit amplifier which employs external feedback for control of its transfer function or gain.

Video amplifiers

These deal with video signals and have bandwidths of about 5 MHz. Certain requirements for step response and overshoot are necessary in order for acceptable TV images to be presented. The design of high bandwidth video amplifiers is a difficult undertaking.

Oscilloscope vertical amplifiers

These are used to deal with video signals to drive an oscilloscope display tube and can have bandwidths of about 500 MHz. The specifications on step response, rise time, overshoot and aberrations can make the design of these amplifiers extremely difficult. One of the pioneers in high bandwidth vertical amplifiers was the Tektronix company.

Distributed amplifiers

These use a transmission lines to temporally split the signal and amplify each portion separately in order to achieve higher bandwidth than can be obtained from a single amplifying device. The outputs of each stage are combined in the output transmission line. This type of amplifier was commonly used on oscilloscopes as the final vertical amplifier. The transmission lines were often housed inside the display tube glass envelope.

Microwave amplifiers

Travelling wave tube (TWT) amplifiers

Used for high power amplification at low microwave frequencies. They typically can amplify across a broad spectrum of frequencies; however, they are usually not as tunable as klystrons.

Klystrons

Very similar to TWT amplifiers, but more powerful and with a specific frequency "sweet spot". They generally are also much heavier than TWT amplifiers, and are therefore ill-suited for light-weight mobile applications. Klystrons are tunable, offering selective output within their specified frequency range.

Musical instrument (audio) amplifiers

Main articles: instrument amplifier, audio amplifier

An audio amplifier is usually used to amplify signals such as music or speech.

Other amplifier types

Carbon microphone

One of the first devices used to amplify signals was the carbon microphone. By channeling a large electric current through the compressed carbon granules in the microphone, a small sound signal could produce a much larger electric signal. The carbon microphone was extremely important in early telecommunications, analog telephones in fact work without the use of any other amplifier. Before the invention of electronic amplifiers, mechanically coupled carbon microphones where also used as amplifiers in telephone repeaters for long distance service.

Magnetic amplifier

A magnetic amplifier is a transformer-like device that makes use of the saturation of magnetic materials to produce amplification. It is a non-electronic electrical amplifier with no moving parts. The bandwidth of magnetic amplifiers extends to the hundreds of kilohertz.

An Amplidyne or Rototrol is a rotating machine like an electrical generator that provides amplification of electrical signals by the conversion of mechanical energy to electrical energy.

Optical amplifiers

Optical amplifiers amplify light through the process of stimulated emission.

Miscellaneous types

There are also mechanical amplifiers, such as the automotive servo used in braking.
Relays can be included under the above definition of amplifiers, although their transfer function is not linear (that is, they are either open or closed).
Another type of amplifier is the fluidic amplifier, based on the fluidic triode.

References

External links

[Commlinx Circuits] Large database of amplifier schematics.
[60 Watt RF Amplifier] Solid state RF power amplifier using IRF840. Simple and easy to construct. IRF840 can handle a maximum power output of 125 Watts.
[The Audio Circuit] — Information on and user reviews of loudspeakers, headphones, amplifiers, and playback equipment.
[Car Audio] — Talk about Car Audio amplifiers and other mobile electronics.

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