Amplifier

In general, an amplifier (or simply amp) is a device that changes, usually increases, the amplitude of a signal. The "signal" is usually in the form of a voltage or current. In popular use, the term today usually refers to an electronic amplifier, often for audio applications. 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.

A related device that emphasizes conversion of signals of one type to another (for example, conversion of a light signal in photons to a DC signal in amperes) is called a transducer or sensor. However, a transducer does not amplify power.

Parameters

The quality of an amplifier can be characterized by a number of parameters, noted below.

Bandwidth

The bandwidth (BW) of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance." The "satisfactory performance" may be different for different applications. However, a common and well-accepted metric are the half power points (that is, frequency where the power goes down by half its peak value) on the power vs. frequency curve. Therefore bandwidth can be defined as the difference between the lower and upper half power points. This is therefore also known as the −3 dB bandwidth. Bandwidths for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.).

A full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing.) In minimalist amplifier design, the amp's usable frequency response needs to extend considerably beyond this (one or more octaves either side) and typically a good minimalist amplifier will have −3 dB points < 10 and > 65 kHz. Professional touring amplifiers often have input and/or output filtering to sharply limit frequency response beyond 20 Hz-20 kHz; too much of the amplifier's potential output power would otherwise be wasted on infrasonic and ultrasonic frequencies, and the danger of AM radio interference would increase. Modern switching amplifiers need steep low pass filtering at the output to get rid of high frequency switching noise and harmonics.

Efficiency

Efficiency is a measure of 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 percent with a max efficiency of 25 percent. Class B amplifiers have a very high efficiency but are impractical because of high levels of distortion (See: Crossover distortion). In practical design, the result of a trade off is the class AB design. Modern Class AB amps are commonly between 35–55 percent efficient with a theoretical maximum of 78.5 percent. Commercially available Class D switching amplifiers have reported efficiencies as high as 97 percent. Amplifiers of Class C-F are usually known to be very high efficiency amplifiers. The efficiency of the amplifier limits the amount of total power output that is usefully available. Note that more efficient amplifiers run much cooler, and often do not need any cooling fans even in multi-kilowatt designs. The reason for this is that the loss of efficiency produces heat as a by-product of the energy lost during the conversion of power. In more efficient amplifiers there is less loss of energy, so in turn less heat.

Gain

The gain of an amplifier is the ratio of output to input power or amplitude, and is usually measured in decibels. (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10 log(P_out/P_in)).

Linearity

An ideal amplifier would be a totally linear device, but real amplifiers are linear only 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 1 dB compression point is defined as the input power (or output power) where the gain is 1 dB less than the small signal gain.

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

Noise

This is a measure of how much noise is introduced in the amplification process. Noise is an undesirable but inevitable product of the electronic devices and components. The metric for noise performance of a circuit is Noise Factor. Noise Factor is the ratio of Signal to Noise Ratio of input signal to that of the output signal.

Output dynamic range

Output dynamic range is the range, usually given in dB, between the smallest and largest useful output levels. The lowest useful level is limited by output noise, while the largest is limited most often by distortion. The ratio of these two is quoted as the amplifier dynamic range. More precisely, if S = maximal allowed signal power and N = noise power, the dynamic range DR is DR = (S + N ) /N.

Overshoot

In response to a step input, the overshoot is the amount the output exceeds its final, steady-state value.

Rise time

The rise time, t_r, of an amplifier is the time taken for the output to change from 10 percent to 90 percent 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 approximated by:

t_r * BW = 0.35

where t_r is rise time in seconds and BW is bandwidth in Hz.

Settling time and ringing

Time taken for output to settle to within a certain percentage of the final value (say 0.1 percent). This is usually specified for oscilloscope vertical amplifiers and high accuracy measurement systems. Ringing refers to an output that cycles above and below its final value, leading to a delay in reaching final value quantified by the settling time above.

Slew rate

Slew rate is the maximum rate of change of output variable, usually quoted in volts per second (or microsecond). Many amplifiers are ultimately slew rate limited (typically by the impedance of a drive current having to overcome capacitive effects at some point in the circuit), which may limit the full power bandwidth to frequencies well below the amplifier's small-signal frequency response.

Stability factor

Stability is a major concern in RF and microwave amplifiers. The degree of an amplifiers stability can be quantified by a so-called stability factor. There are several different stability factors, such as the Stern stability factor and the Linvil stability factor, which specify a condition that must be met for the absolute stability of an amplifier in terms of its two-port parameters.

Electronic amplifiers

There are many types of electronic amplifiers, 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. Critical components include active devices, such as vacuum tubes or transistors. A brief introduction to the many types of electronic amplifier follows.

Power amplifier

The term "power amplifier" is a relative term with respect to the amount of power delivered to the load and/or sourced by the supply circuit. In general a power amplifier is designated as the last amplifier in a transmission chain (the output stage) and is the amplifier stage that typically requires most attention to power efficiency. Efficiency considerations lead to various classes of power amplifier.

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).

Transistor amplifiers

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.

Transistor-based amplifier can be realized using various configurations: for example with a bipolar junction transistor we can realize common base, common collector or common emitter amplifier; using a MOSFET we can realize common gate, common source or common drain amplifier. Each configuration has different characteristic (gain, impedance…).

Operational amplifiers (op-amps)

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

Fully differential amplifiers (FDA)

A fully differential amplifier is a solid state integrated circuit amplifier which employs external feedback for control of its transfer function or gain. It is similar to the operational amplifier but it also has differential output pins.

Video amplifiers

These deal with video signals and have varying bandwidths depending on whether the video signal is for SDTV, EDTV, HDTV 720p or 1080i/p and so on. The specification of the bandwidth itself depends on what kind of filter is used and which point (-1 dB or -3 dB for example) the bandwidth is measured. Certain requirements for step response and overshoot are necessary in order for acceptable TV images to be presented.

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 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

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 (effectively a sound-controlled variable resistor). 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 were 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.

Rotating electrical machinery amplifier

A Ward Leonard control is a rotating machine like an electrical generator that provides amplification of electrical signals by the conversion of mechanical energy to electrical energy. Changes in generator field current result in larger changes in the output current of the generator, providing gain. This class of device was used for smooth control of large motors, primarily for elevators and naval guns.

Field modulation of a very high speed AC generator was also used for some early AM radio transmissions.

Johnsen-Rahbek effect amplifier

The earliest form of audio power amplifier was Edison's "electromotograph" loud-speaking telephone, which used a wetted rotating chalk cylinder in contact with a stationary contact. The friction between cylinder and contact varied with the current, providing gain. Edison discovered this effect in 1874, but the theory behind the Johnsen-Rahbek effect was not understood until the semiconductor era.

Mechanical amplifiers

Mechanical amplifiers were used in the pre-electronic era in specialized applications. Early autopilot units designed by Elmer Ambrose Sperry incorporated a mechanical amplifier using belts wrapped around rotating drums; a slight increase in the tension of the belt caused the drum to move the belt. A paired, opposing set of such drives made up a single amplifier. This amplified small gyro errors into signals large enough to move aircraft control surfaces. A similar mechanism was used in the Vannevar Bush differential analyzer.

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).
• Also purely mechanical manifestations of such digital amplifiers can be built (for theoretical, didactical purposes, or for entertainment), see e.g. domino computer.
• Another type of amplifier is the fluidic amplifier, based on the fluidic triode.

See also

• Acoustics
• Electric power
• Electricity
• Music
• Photon
• Sensor
• Signal (electrical engineering)
• Sound
• Sound recording and reproduction

References

ISBN links support NWE through referral fees

• Cripps, Steve C. 2006. RF Power Amplifiers for Wireless Communications, 2nd edition. Boston: Artech House. ISBN 978-1596930186.
• Jones, Morgan. 2004. Building Valve Amplifiers. Burlington, MA: Newnes. ISBN 0750656956.
• Pittman, Aspen. 2003. The Tube Amp Book. San Francisco: Backbeat. ISBN 0879307676.
• Verhoeven, C. J. 2003. Structured Electronic Design: Negative-Feedback Amplifiers. Boston: Kluwer Academic Publishers. ISBN 1402075901.

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