Amplifier figures of merit

Last updated September 05, 2023

In electronics, the figures of merit of an amplifier are numerical measures that characterize its properties and performance. Figures of merit can be given as a list of specifications that include properties such as gain, bandwidth, noise and linearity, among others listed in this article. Figures of merit are important for determining the suitability of a particular amplifier for an intended use.

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). RF amplifiers are often specified in terms of the maximum power gain obtainable, while the voltage gain of audio amplifiers and instrumentation amplifiers will be more often specified. For example, an audio amplifier with a gain given as 20 dB will have a voltage gain of ten.

The use of voltage gain figure is appropriate when the amplifier's input impedance is much higher than the source impedance, and the load impedance higher than the amplifier's output impedance.

If two equivalent amplifiers are being compared, the amplifier with higher gain settings would be more sensitive as it would take less input signal to produce a given amount of power.^[1]^{[ unreliable source? ]}

Bandwidth

The bandwidth of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance". The definition of "satisfactory performance" may be different for different applications. However, a common and well-accepted metric is the half-power points (i.e. frequency where the power goes down by half its peak value) on the output 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 (otherwise called "frequency responses") for other response tolerances are sometimes quoted (−1 dB, −6 dB etc.) or "plus or minus 1dB" (roughly the sound level difference people usually can detect).

The gain of a good quality full-range audio amplifier will be essentially flat between 20 Hz to about 20 kHz (the range of normal human hearing). In ultra-high-fidelity amplifier design, the amplifier's frequency response should extend considerably beyond this (one or more octaves either side) and might have −3 dB points < 10 Hz 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.

The range of frequency over which the gain is equal to or greater than 70.7% of its maximum gain is termed as bandwidth.^[2]

Efficiency

Efficiency is a measure of how much of the power source 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% for direct coupling of the output. Inductive coupling of the output can raise their efficiency to a maximum of 50%.

Drain efficiency is the ratio of output RF power to input DC power when primary input DC power has been fed to the drain of a field-effect transistor. Based on this definition, the drain efficiency cannot exceed 25% for a class A amplifier that is supplied drain bias current through resistors (because RF signal has its zero level at about 50% of the input DC). Manufacturers specify much higher drain efficiencies, and designers are able to obtain higher efficiencies by providing current to the drain of the transistor through an inductor or a transformer winding. In this case the RF zero level is near the DC rail and will swing both above and below the rail during operation. While the voltage level is above the DC rail current is supplied by the inductor.

Class B amplifiers have a very high efficiency but are impractical for audio work 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 amplifiers commonly have peak efficiencies between 30 and 55% in audio systems and 50-70% in radio frequency systems with a theoretical maximum of 78.5%.

Commercially available Class D switching amplifiers have reported efficiencies as high as 90%. Amplifiers of Class C-F are usually known to be very high-efficiency amplifiers. RCA manufactured an AM broadcast transmitter employing a single class-C low-mu triode with an RF efficiency in the 90% range.

More efficient amplifiers run 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.

In RF linear Power Amplifiers, such as cellular base stations and broadcast transmitters, special design techniques can be used to improve efficiency. Doherty designs, which use a second output stage as a "peak" amplifier, can lift efficiency from the typical 15% up to 30-35% in a narrow bandwidth. Envelope Tracking designs are able to achieve efficiencies of up to 60%, by modulating the supply voltage to the amplifier in line with the envelope of the signal.

Linearity

An ideal amplifier would be a totally linear device, but real amplifiers are only linear within 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.

In most amplifiers a reduction in gain takes place before hard clipping occurs; the result is a compression effect, which (if the amplifier is an audio amplifier) sounds 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. Sometimes this non linearity is deliberately designed in to reduce the audible unpleasantness of hard clipping under overload.

Ill effects of non-linearity can be reduced with negative feedback.

Linearization is an emergent field, and there are many techniques, such as feed forward, predistortion, postdistortion, 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; also, much noise results from intentional economies of manufacture and design time. The metric for noise performance of a circuit is noise figure or noise factor. Noise figure is a comparison between the output signal to noise ratio and the thermal noise of the input 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.^[3]

In many switched mode amplifiers, dynamic range is limited by the minimum output step size.

Slew rate

Slew rate is the maximum rate of change of the output, 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 sometimes limits the full power bandwidth to frequencies well below the amplifier's small-signal frequency response.

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

The time taken for the output to settle to within a certain percentage of the final value (for instance 0.1%) is called the settling time, and is usually specified for oscilloscope vertical amplifiers and high-accuracy measurement systems. Ringing refers to an output variation that cycles above and below an amplifier's final value and leads to a delay in reaching a stable output. Ringing is the result of overshoot caused by an underdamped circuit.

Overshoot

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

Stability

Stability is an issue in all amplifiers with feedback, whether that feedback is added intentionally or results unintentionally. It is especially an issue when applied over multiple amplifying stages.

Stability is a major concern in RF and microwave amplifiers. The degree of an amplifier's 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.

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An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is defined as a circuit that has a power gain greater than one.

An operational amplifier is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential that is typically 100,000 times larger than the potential difference between its input terminals. The operational amplifier traces its origin and name to analog computers, where they were used to perform mathematical operations in linear, non-linear, and frequency-dependent circuits.

A negative-feedback amplifier is an electronic amplifier that subtracts a fraction of its output from its input, so that negative feedback opposes the original signal. The applied negative feedback can improve its performance and reduces sensitivity to parameter variations due to manufacturing or environment. Because of these advantages, many amplifiers and control systems use negative feedback.

A low-noise amplifier (LNA) is an electronic component that amplifies a very low-power signal without significantly degrading its signal-to-noise ratio (SNR). Any electronic amplifier will increase the power of both the signal and the noise present at its input, but the amplifier will also introduce some additional noise. LNAs are designed to minimize that additional noise, by choosing special components, operating points, and circuit topologies. Minimizing additional noise must balance with other design goals such as power gain and impedance matching.

Audio system measurements are a means of quantifying system performance. These measurements are made for several purposes. Designers take measurements so that they can specify the performance of a piece of equipment. Maintenance engineers make them to ensure equipment is still working to specification, or to ensure that the cumulative defects of an audio path are within limits considered acceptable. Audio system measurements often accommodate psychoacoustic principles to measure the system in a way that relates to human hearing.

A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers in the 1960s and 1970s. Valve amplifiers can be used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, high quality stereo amplifiers, military applications and very high power radio and UHF television transmitters.

In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as a voltage amplifier. It offers high current gain, medium input resistance and a high output resistance. The output of a common emitter amplifier is 180 degrees out of phase to the input signal.

The Williamson amplifier is a four-stage, push-pull, Class A triode-output valve audio power amplifier designed by D. T. N. Williamson during World War II. The original circuit, published in 1947 and addressed to the worldwide do it yourself community, set the standard of high fidelity sound reproduction and served as a benchmark or reference amplifier design throughout the 1950s. The original circuit was copied by hundreds of thousands amateurs worldwide. It was an absolute favourite on the DIY scene of the 1950s, and in the beginning of the decade also dominated British and North American markets for factory-assembled amplifiers.

A class-D amplifier or switching amplifier is an electronic amplifier in which the amplifying devices operate as electronic switches, and not as linear gain devices as in other amplifiers. They operate by rapidly switching back and forth between the supply rails, using pulse-width modulation, pulse-density modulation, or related techniques to produce a pulse train output. This passes through a simple low-pass filter which blocks the high-frequency pulses and provides analog output current and voltage. Because they are always either in fully on or fully off modes, little energy is dissipated in the transistors and efficiency can exceed 90%.

<span class="mw-page-title-main">Linear amplifier</span> Electronic circuit

A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency (RF) power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio. Other types of linear amplifier are used in audio and laboratory equipment. Linearity refers to the ability of the amplifier to produce signals that are accurate copies of the input. A linear amplifier responds to different frequency components independently, and tends not to generate harmonic distortion or intermodulation distortion. No amplifier can provide perfect linearity however, because the amplifying devices—transistors or vacuum tubes—follow nonlinear transfer function and rely on circuitry techniques to reduce those effects. There are a number of amplifier classes providing various trade-offs between implementation cost, efficiency, and signal accuracy.

The cascode is a two-stage amplifier that consists of a common-emitter stage feeding into a common-base stage.

<span class="mw-page-title-main">Single-ended triode</span> Vacuum tube electronic amplifier that uses a single triode to produce an output

A single-ended triode (SET) is a vacuum tube electronic amplifier that uses a single triode to produce an output, in contrast to a push-pull amplifier which uses a pair of devices with antiphase inputs to generate an output with the wanted signals added and the distortion components subtracted. Single-ended amplifiers normally operate in Class A; push-pull amplifiers can also operate in Classes AB or B without excessive net distortion, due to cancellation.

A radio transmitter or just transmitter is an electronic device which produces radio waves with an antenna. Radio waves are electromagnetic waves with frequencies between about 30 Hz and 300 GHz. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating current, the antenna radiates radio waves. Transmitters are necessary parts of all systems that use radio: radio and television broadcasting, cell phones, wireless networks, radar, two way radios like walkie talkies, radio navigation systems like GPS, remote entry systems, among numerous other uses.

A fully differential amplifier (FDA) is a DC-coupled high-gain electronic voltage amplifier with differential inputs and differential outputs. In its ordinary usage, the output of the FDA is controlled by two feedback paths which, because of the amplifier's high gain, almost completely determine the output voltage for any given input.

A valve RF amplifier or tube amplifier (U.S.) is a device for electrically amplifying the power of an electrical radio frequency signal.

Technical specifications and detailed information on the valve audio amplifier, including its development history.

<span class="mw-page-title-main">Tube sound</span> Characteristic quality of sounds from vacuum tube amplifiers

Tube sound is the characteristic sound associated with a vacuum tube amplifier, a vacuum tube-based audio amplifier. At first, the concept of tube sound did not exist, because practically all electronic amplification of audio signals was done with vacuum tubes and other comparable methods were not known or used. After introduction of solid state amplifiers, tube sound appeared as the logical complement of transistor sound, which had some negative connotations due to crossover distortion in early transistor amplifiers. However, solid state amplifiers have been developed to be flawless and the sound is later regarded neutral compared to tube amplifiers. Thus the tube sound now means 'euphonic distortion.' The audible significance of tube amplification on audio signals is a subject of continuing debate among audio enthusiasts.

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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.

The NE5532, also sold as SA5532, SE5532 and NG5532 is a dual monolithic, bipolar, internally compensated operational amplifier for audio applications introduced by Signetics in 1979. The 5532 and the contemporary TL072 were the first operational amplifiers that outperformed discrete class A circuits in professional audio applications. Due to low noise and very low distortion, the 5532 became the industry standard for professional audio. According to Douglas Self, "there is probably no music on the planet that has not passed through a hundred or more 5532s on its way to the consumer". The performance of the 5532 remained best in class for almost thirty years, until the introduction of the LM4562 in 2007. As of 2021, the 5532 remains in mass production as a generic product.

References

↑ "Amplifier Gain Controls" . Retrieved 2017-11-11.
↑ KUMAR, GANGULY, PARTHA (2015-09-16). PRINCIPLES OF ELECTRONICS. PHI Learning Pvt. Ltd. ISBN 9788120351240.{{cite book}}: CS1 maint: multiple names: authors list (link)
↑ Verhoeven CJM, van Staveren A, Monna GLE, Kouwenhoven MHL, Yildiz E (2003). Structured electronic design: negative feedback amplifiers. Boston/Dordrecht: Kluwer Academic. p. 10. ISBN 1-4020-7590-1.

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[1] "Amplifier Gain Controls" . Retrieved 2017-11-11.

[2] KUMAR, GANGULY, PARTHA (2015-09-16). PRINCIPLES OF ELECTRONICS. PHI Learning Pvt. Ltd. ISBN 9788120351240.{{cite book}}: CS1 maint: multiple names: authors list (link)

[Verhoeven-3] Verhoeven CJM, van Staveren A, Monna GLE, Kouwenhoven MHL, Yildiz E (2003). Structured electronic design: negative feedback amplifiers. Boston/Dordrecht: Kluwer Academic. p. 10. ISBN 1-4020-7590-1.

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