Amplifier figures of merit

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

Generally, an amplifier is any device that will convert one signal (often with a small amount of energy) such into a another signal (often with a larger amount of energy). In popular use, the term today usually refers to an electronic amplifier, often as in 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 closely related device that emphasizes conversion of signals of one type to another is a sensor.

General characteristics of amplifiers

The quality of an amplifier can be characterized by a number of specifications, enumerated below.

Gain

The gain is the ratio of output power to input power, and is usually measured in decibels . (When measured in decibels it is logarithmically related to the power ratio: G(dB)=10log(Pout/Pin)).

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.

Bandwidth

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.).

A full-range audio amplifier will be essentially flat between twenty hertz to about twenty kilohertz (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 20-20 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.

Rise time

The rise time, t_r, 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 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%). 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.

Overshoot

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

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 amplifiers frequency response when dealing with small signals.

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. It is measured in either decibels or the peak output voltage produced by the amplifier when no signal is applied.

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% with a max efficiency of 25%. 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 tradeoff is the class AB design. Modern Class AB amps are commonly between 35–55% efficient with a theoretical maximum of 78.5%. Commercially available Class D switching 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 amplifiers run much cooler, and often do not need any cooling 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 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 us feedforward, predistortion, postdistortion, EER, LINC, CALLUM, cartesian feedback, etc., in order to avoid the undesired effects of the non-linearities.

Electronic amplifiers

Main article: Electronic amplifier

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.

Electronic signal amplifier types

Electronic amplifiers use two variables: current and voltage. Either can be used as input, and either as output leading to four types of amplifier. In idealized form they are represented by each of the four types of dependent source used in linear analysis, namely:

Input	Output	Dependent source	Amplifier type
I	I	current controlled current source CCCS	current amplifier
I	V	current controlled voltage source CCVS	transresistance amplifier
V	I	voltage controlled current source VCCS	transconductance amplifier
V	V	voltage controlled voltage source VCVS	voltage amplifier

Each type of amplifier in its ideal form has an ideal input and output resistance that is the same as that of the corresponding dependent source:

Amplifier type	Dependent source	Input impedance	Output impedance
Current	CCCS	0	∞
Transresistance	CCVS	0	0
Transconductance	VCCS	∞	∞
Voltage	VCVS	∞	0

Electronic power amplifier classes

Power amplifiers are commonly classified by the conduction angle (sometimes known as 'angle of flow') of the input signal through the amplifying device, which is closely related to the amplifier power efficiency defined above.

Class A

Where efficiency is not a consideration, most small signal linear amplifiers are designed as Class A, which means that the output devices are always in the conduction region. Class A amplifiers are typically more linear and less complex than other types, but are very inefficient. This type of amplifier is most commonly used in small-signal stages or for low-power applications (such as driving headphones).

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

Main article: Switching amplifier

These use switching to achieve a very high power efficiency (more than 90% in modern designs). By allowing each output device to be either fully on or off, losses are minimized. A simple approach such as pulse-width modulation is sometimes still used; however, high-performance switching amplifiers use digital techniques, such as sigma-delta modulation, to achieve superior performance. Formerly used only for subwoofers due to their limited bandwidth and relatively high distortion, the evolution of semiconductor devices has made possible the development of high fidelity, full audio range Class D amplifiers, with S/N and distortion levels similar to their linear counterparts.

Other classes

There are several other amplifier classes, although they are mainly variations of the previous classes. For example, Class H and Class G 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 more complex, and are mainly used for specialized applications, such as very high-power units. Also, Class E and Class F amplifiers are commonly described in literature for radio frequencies applications where efficiency of the traditional classes deviate substantially from their ideal values. These classes use harmonic tuning of their output networks to achieve higher efficiency and can be considered a subset of Class C due to their conduction angle characteristics.

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 and is the amplifier stage that typically requires most attention to power efficiency. For these reasons, a power amplifier is typically any of the above-mentioned classes except Class A.

Vacuum tube (valve) amplifiers

Main article: valve amplifier

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

Main articles: transistor, bipolar junction transistor, and 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.

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)

Main articles: operational amplifier and instrumentation amplifier

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)

Main article: fully differential amplifier

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 etc.. 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

Main article: Distributed Amplifier

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

Main article: Traveling wave tube

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

Main article: Klystron

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 and 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 (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

Main article: 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.[1] See Alexanderson alternator.

Johnsen-Rahbek effect amplifier

The earliest form of audio 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

Main article: Optical amplifier

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

• Attenuator (electronics)
• Electronic amplifier
• Instrument amplifier
• Low noise amplifier
• Preamplifier
• Step response