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Passivity is a property of engineering systems, used in a variety of engineering disciplines, but most commonly found in analog electronics and control systems. A passive component, depending on field, may be either a component that consumes Lenovo S10-2 Battery 5200mAh

(but does not produce) energy (thermodynamic passivity), or a component that is incapable of power gain (incremental passivity).

A component that is not passive is called an active component. An electronic circuit consisting entirely of passive components is called a passive circuit (and has the same properties as a passive component) Lenovo Y730 Battery 5200mAh .

Used without qualifier, the term passive is ambiguous. Typically, analog designers use this term to refer to incrementally passive components and systems, while control systems engineers will use this to refer to thermodynamically passive onesLenovo Y710 Battery 5200mAh .

Thermodynamic passivity

In control systems and circuit network theory, a passive component or circuit is one that consumes energy, but does not produce energy  Lenovo Y530 Battery 5200mAh .

Under this methodology, voltage and current sources are considered active, while resistors, transistors, tunnel diodes, glow tubes, capacitors, metamaterials and other dissipative and energy-neutral components are considered passive. Circuit designers will sometimes refer to this class of components as dissipative, or thermodynamically passive Lenovo Y510 Battery 5200mAh .

While many books give definitions for passivity, many of these contain subtle errors in how initial conditions are treated (and, occasionally, the definitions do not generalize to all types of nonlinear time-varying systems with memory). Below is a correct, formal definition, taken from Wyatt et al Lenovo S10-3 Battery 5200mAh .

(which also explains the problems with many other definitions). Given an n-port R with a state representation S, and initial state x, define available energy EA as:where the notation supx?T?0 indicates that the supremum is taken over all T ? 0 and all admissible pairs {v(·), i(·)} Lenovo G450 Battery 5200mAh

with the fixed initial state x (e.g., all voltage–current trajectories for a given initial condition of the system). A system is considered passive if EA is finite for all initial states x. Otherwise, the system is considered active. Roughly speaking, the inner product \langle v(t),i(t) \rangle is the instantaneous power   Lenovo E30 Battery 5200mAh

(e.g., the product of voltage and current), and EA is the upper bound on the integral of the instantaneous power (i.e., energy). This upper bound (taken over all T ? 0) is the available energy in the system for the particular initial condition x. If, for all possible initial states of the system, the energy available is finite, then the system is called passive  Lenovo E40 Battery 5200mAh.

Incremental passivity

In circuit design, informally, passive components refer to ones that are not capable of power gain. Under this definition, passive components include capacitors, inductors, resistors, diodes,transformers, voltage sources, and current sources  Lenovo E50 Battery 5200mAh .

They exclude devices like transistors, vacuum tubes, relays, tunnel diodes, and glow tubes. Formally, for a memoryless two-terminal element, this means that the current–voltage characteristic is monotonically increasing. For this reason, control systems and circuit network theorists refer to these devices as locally passive Lenovo G460 Battery 5200mAh ,

incrementally passive, increasing, monotone increasing, or monotonic. It is not clear how this definition would be formalized to multiport devices with memory – as a practical matter, circuit designers use this term informally, so it may not be necessary to formalize it  Lenovo Y460 Battery 5200mAh

Systems for which the small signal model is not passive are sometimes called locally active (e.g. transistors and tunnel diodes). Systems that can generate power about a time-variant unperturbed state are often called parametrically active (e.g. certain types of nonlinear capacitors) Lenovo Y450 Battery 5200mAh .

Other definitions of passivity

In some very informal settings, passivity may refer to the simplicity of the device, although this definition is now almost universally considered incorrect  Lenovo N200 Battery 5200mAh .

Here, devices like diodes would be considered active, and only very simple devices like capacitors, inductors, and resistors are considered passive. In some cases, the term "linear element" may be a more appropriate term than "passive device." In other cases, "solid state device" may be a more appropriate term than "active device  Lenovo T410 Battery 5200mAh ."


Passivity, in most cases, can be used to demonstrate that passive circuits will be stable under specific criteria. Note that this only works if only one of the above definitions of passivity is used – if components from the two are mixed, the systems may be unstable under any criteria  Lenovo T510 Battery 5200mAh .

In addition, passive circuits will not necessarily be stable under all stability criteria. For instance, a resonant series LC circuit will have unbounded voltage output for a bounded voltage input, but will be stable in the sense of Lyapunov, and given bounded energy input will have bounded energy output Lenovo SL410 Battery 5200mAh .

Passivity is frequently used in control systems to design stable control systems or to show stability in control systems. This is especially important in the design of large, complex control systems (e.g. stability of airplanes). Passivity is also used in some areas of circuit design, especially filter design  Lenovo SL510 Battery 5200mAh .

Passive filter

A passive filter is a kind of electronic filter that is made only from passive elements – in contrast to an active filter, it does not require an external power source (beyond the signal). Since most filters are linear, in most cases, passive filters are composed of just the four basic linear elements – resistors, capacitors, inductors, and transformers  Lenovo C100 Battery 5200mAh .

More complex passive filters may involve nonlinear elements, or more complex linear elements, such as transmission lines.

A passive filter has several advantages over an active filter  Lenovo T400S Battery 5200mAh :

  • Guaranteed stability
  • Passive filters scale better to large signals (tens of amperes, hundreds of volts), where active devices are often impractical
  • No power consumption  Lenovo Y430 Battery 5200mAh .
  • May be less expensive in discrete designs (unless large coils are required)
  • For linear filters, may be, more linear than filters including active (and therefore non-linear) elements, depending on components required  Ibm ThinkPad G40 Battery 5200mAh .

They are commonly used in speaker crossover design (due to the moderately large voltages and currents, and the lack of easy access to power), filters in power distribution networks (due to the large voltages and currents), power supply bypassing (due to low cost, and in some cases, power requirements) Ibm ThinkPad X200T Battery 5200mAh ,

as well as a variety of discrete and home brew circuits (for low-cost and simplicity). Passive filters are uncommon in monolithic integrated circuit design, where active devices are inexpensive compared to resistors and capacitors, and inductors are prohibitively expensive  Apple A1280 battery 5200mAh .

Passive filters are still found, however, in hybrid integrated circuits. Indeed, it may be the desire to incorporate a passive filter that leads the designer to use the hybrid format.

Generally, an amplifier or simply amp, is a device for increasing the power of a signal Apple A1281 battery 5200mAh .

In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systemsto make the human voice louder or play recorded musicSamsung R522 battery 5200mAh .

Amplifiers may be classified according to the input (source) they are designed to amplify (such as a guitar amplifier, to perform with an electric guitar), the device they are intended to drive (such as a headphone amplifier), the frequency range of the signals (Audio, IF, RF, and VHF amplifiers, for example)  Samsung P467 battery 5200mAh ,

whether they invert the signal (inverting amplifiers and non-inverting amplifiers), or the type of device used in the amplification (valve or tube amplifiers, FET amplifiers, etc.).

A related device that emphasizes conversion of signals of one type to another Hp Pavilion dm3 Battery 5400mAh

(for example, a light signal in photons to a DC signal in amperes) is a transducer, a transformer, or a sensor. However, none of these amplify power.

he quality of an amplifier can be characterized by a number of specifications, listed below  Hp Pavilion dm4 Battery 5200mAh .


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(Pout /(Pin))  Hp Pavilion cq62 Battery 5200mAh .

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 (since the amplifier's input impedance will often be much higher than the source impedance, and the load impedance higher than the amplifier's output impedance) Hp Pavilion cq42 Battery 5200mAh .

  • Example: an audio amplifier with a gain given as 20 dB will have a voltage gain of ten (but a power gain of 100 would only occur in the unlikely event the input and output impedances were identical)  Hp Pavilion cq72 Battery 5200mAh .

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.


The bandwidth of an amplifier is the range of frequencies for which the amplifier gives "satisfactory performance" Hp cq20 Battery 2600mAh .

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 curveHp 2230 Battery 2600mAh .

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.) Dell Inspiron Mini 12 battery 7800mAh

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)  Dell XPS M1730 battery 7800mAh .

In ultra high fidelity amplifier design, the amp's frequency response should extend considerably beyond this (one or more octaves either side) and might 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 Dell XPS M2010 battery 7800mAh ;

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  Dell Studio XPS 1340 battery 5200mAh/7800mAh .


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% fordirect coupling of the output. Inductive coupling of the output can raise their efficiency to a maximum of 50%  Dell Inspiron 9100 battery 7800mAh .

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 efficienies between 30–55% in audio systems and 50-70% in radio frequency systems with a theoretical maximum of 78.5% Dell Inspiron N4010 battery 5200mAh .

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  Dell Inspiron N3610 battery 5200mAh .

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  Dell Inspiron N5010 battery 5200mAh .

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  Dell Inspiron N7010 battery 5200mAh .

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.


An ideal amplifier would be a totally linear device, but real amplifiers are only linear within limits Dell Inspiron 1464 battery 5200mAh .

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  Dell Inspiron 1564 battery 5200mAh ;

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  Dell Inspiron 1764 battery 5200mAh .

Sometimes this nonlinearity is deliberately designed in to reduce the audible unpleasantness of hard clipping under overload.

Ill effects of nonlinearity can be reduced with negative feedback Dell Inspiron 1320 battery 5200mAh .

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


This is a measure of how much noise is introduced in the amplification process  Dell Inspiron 1320n battery 5200mAh .

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 Dell Vostro 1710 battery 5200mAh.

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  Dell Vostro 1720 battery 5200mAh .

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.

In many switched mode amplifiers, dynamic range is limited by the minimum output step size  Dell Studio XPS 1640 battery 5200mAh .

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)  Dell Vostro A840 battery 5200mAh ,

which sometimes limits the full power bandwidth to frequencies well below the amplifier's small-signal frequency response.

Rise time

The rise time, tr, 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 Dell Vostro A860 battery 5200mAh.
tr * BW = 0.35, where tr 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 Dell Studio 1535 battery  7800mAh .

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.


In response to a step input, the overshoot is the amount the output exceeds its final, steady-state value Dell Studio 1735 battery  7800mAh .


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 Dell Vostro 1310 battery 5200mAh .

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 Dell Latitude E6400 battery,10400mAh .

Electronic amplifiers

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  Dell Latitude E6400 Battery,10400mAh .

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  Dell Latitude E6400 Battery,10400mAh .

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 Dell Latitude E6500 battery,10400mAh .

See Alexanderson alternator.

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  Dell Latitude E6500 battery,10400mAh .

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 Dell Latitude E6500 battery,10400mAh .

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  Dell Latitude E6500 battery,10400mAh .

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.

The Electrostatic drum amplifier used a band wrapped partway around a rotating drum, and fixed at its anchored end to a spring  HP Pavilion DV6-1210SA battery -  8800mAh .

The other end connected to a speaker cone. The input signal was transformed up to high voltage, and added to a high voltage dc supply line. This voltage was connected between drum and belt. Thus the input signal varied the electric field between belt and drum, and thus the friction between them, and thus the amount of lateral movement of the belt and thus speaker cone HP Pavilion DV7 battery - 7200mAh .

Other variations on the theme also existed at one time.

An electronic amplifier is a device for increasing the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a largeramplitude Ibm THINKPAD X200 Battery 8800mAh .

In this sense, an amplifier may be considered as modulating the output of the power supply.

Types of amplifier

Amplifiers can be specified according to their input and output properties. They have some kind of gain, or multiplication factor relating the magnitude of the output signal to the input signal  Ibm THINKPAD X200 Battery 8800mAh .

The gain may be specified as the ratio of output voltage to input voltage (voltage gain), output power to input power (power gain), or some combination of current, voltage and power. In many cases, with input and output in the same units, gain will be unitless (although often expressed in decibels) Ibm THINKPAD X200 Battery 8800mAh ;

for others this is not necessarily so. For example, a transconductance amplifier has a gain with units ofconductance (output current per input voltage). The power gain of an amplifier depends on the source and load impedances used as well as its voltage gain SONY VAIO VGN-FW21E Battery ;

while an RF amplifier may have its impedances optimized for power transfer, audio and instrumentation amplifiers are normally employed with amplifier input and output impedances optimized for least loading and highest quality  Asus M52N Battery .

So an amplifier that is said to have a gain of 20 dB might have a voltage gain of ten times and an available power gain of much more than 20 dB (100 times power ratio), yet be delivering a much lower power gain if, for example, the input is a 600 ohm microphone and the output is a 47 kilohm power amplifier's input socket Dell Latitude E6400 ATG Battery .

In most cases an amplifier should be linear; that is, the gain should be constant for any combination of input and output signal. If the gain is not constant, e.g., by clipping the output signal at the limits of its capabilities, the output signal will be distorted. There are however cases where variable gain is useful  SONY VAIO VGN-FW21J Battery .

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  SONY VAIO VGN-FW21L Battery .

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  SONY VAIO VGN-FW21M Battery .

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: seepower amplifier classes   SONY VAIO VGN-FW21Z Battery .

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)  SONY VAIO VGN-FW25T/B Battery .

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)  SONY VAIO VGN-FW26T/B Battery .

Valves/tube amplifiers also have niche uses in other areas, such as

  • Electric guitar amplification
  • in Russian military aircraft, for their EMP tolerance  SONY VAIO VGN-FW27/B Battery
  • niche audio for their sound qualities (recording, and audiophile equipment)

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  SONY VAIO VGN-FW27/W Battery .

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  SONY VAIO VGN-FW31ZJ Battery .

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 SONY VAIO VGN-FW27T/H Battery .

Each configuration has different characteristic (gain, impedance...).

Operational amplifiers (op-amps)

An operational amplifier is an amplifier circuit with very high open loop gain and differential inputs which employs external feedback for control of its transfer function or gain  SONY VAIO VGN-FW29/B Battery .

Although the term is today commonly applied to integrated circuits, the original operational amplifier design was implemented with valves.

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  SONY VAIO VGN-FW290JRB Battery .

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. SONY VAIO VGN-FW290JTB Battery

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 SONY VAIO VGN-FW290JTH Battery .

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 difficultSONY VAIO VGN-FW33G/E1 Battery .

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  SONY VAIO VGN-FW33G/W Battery .

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  SONY VAIO VGN-FW33GW Battery .

Switched mode amplifiers

These nonlinear amplifiers have much higher efficiencies than linear amps, and are used where the power saving justifies the extra complexity.

Negative resistance devices

Negative resistances can be used as amplifiers, such as the tunnel diode amplifier Dell XPS M1210 Batteryhttp://www.hdd-shop.co.uk .

There are many alternative classifications that address different aspects of amplifier designs, and they all express some particular perspective relating the design parameters to the objectives of the circuit. Amplifier design is always a compromise of numerous factors, such as cost  Dell KM958 battery ,

power consumption, real-world device imperfections, and a multitude of performance specifications. Below are several different approaches to classification:

Input and output variables

Electronic amplifiers use two variables: current and voltage. Either can be used as input, and either as output leading to four types of amplifiers  Dell Studio 1555 battery .

In idealized form they are represented by each of the four types of dependent source used in linear analysis, as shown in the figure, namely:

In practice the ideal impedances are only approximated. For any particular circuit, a small-signal analysis is often used to find the impedance actually achieved  Sony VGP-BPS13 battery .

A small-signal AC test current Ix is applied to the input or output node, all external sources are set to AC zero, and the corresponding alternating voltage Vx across the test current source determines the impedance seen at that node asR = Vx / ISony VGP-BPS13/B battery .

Amplifiers designed to attach to a transmission line at input and/or output, especially RF amplifiers, do not fit into this classification approach. Rather than dealing with voltage or current individually, they ideally couple with an input and/or output impedance matched to the transmission line impedance, that is, match ratios of voltage to current Sony VGP-BPS13/S battery .

Many real RF amplifiers come close to this ideal. Although, for a given appropriate source and load impedance, RF amplifiers can be characterized as amplifying voltage or current, they fundamentally are amplifying power Sony VGP-BPS13A/B battery .

Common terminal

One set of classifications for amplifiers is based on which device terminal is common to both the input and the output circuit. In the case of bipolar junction transistors, the three classes are common emitter, common base, and common collector Sony VGP-BPS13B/B battery .

For field-effect transistors, the corresponding configurations are common source, common gate, and common drain; for triode vacuum devices, common cathodecommon grid, and common plate. The output voltage of a common plate amplifier is the same as the input  Sony VGP-BPL9 battery

(this arrangement is used as the input presents a high impedance and does not load the signal source, although it does not amplify the voltage), i.e., the output at the cathode follows the input at the grid; consequently it was commonly called a cathode follower. By analogy the termsemitter follower and source follower are sometimes used Sony VGP-BPS13B/B battery .

Unilateral or bilateral

When an amplifier has an output that exhibits no feedback to its input side, it is called unilateral. The input impedance of a unilateral amplifier is independent of the load, and the output impedance is independent of the signal source impedance Sony VGP-BPL15 battery .

If feedback connects part of the output back to the input of the amplifier it is called a bilateral amplifier. The input impedance of a bilateral amplifier is dependent upon the load, and the output impedance is dependent upon the signal source impedance Dell Inspiron E1505 battery .

All amplifiers are bilateral to some degree; however they may often be modeled as unilateral under operating conditions where feedback is small enough to neglect for most purposes, simplifying analysis (see the common base article for an example) HP Pavilion dv6000 Battery .

Negative feedback is often applied deliberately to tailor amplifier behavior. Some feedback, which may be positive or negative, is unavoidable and often undesirable, introduced, for example, by parasitic elements such as the inherent capacitance between input and output of a device such as a transistor and capacitative coupling due to external wiring HP Pavilion dv8000 battery .

Excessive frequency-dependent positive feedback may cause what is supposed to be an amplifier to become an oscillator.

Linear unilateral and bilateral amplifiers can be represented as two-port networks  HP PAVILION DV6000 Battery,  8800mAh .

Inverting or non-inverting

Another way to classify amps is the phase relationship of the input signal to the output signal. An inverting amplifier produces an output 180 degrees out of phase with the input signal HP PAVILION DV8000 Battery,  8800mAh

(that is, a polarity inversion or mirror image of the input as seen on an oscilloscope). A non-inverting amplifier maintains the phase of the input signal waveforms. An emitter follower is a type of non-inverting amplifier, indicating that the signal at the emitter of a transistor is following (that is, matching with unity gain but perhaps an offset) the input signal SONY VAIO VGN-FZ Battery .

This description can apply to a single stage of an amplifier, or to a complete amplifier system.


Other amplifiers may be classified by their function or output characteristics. These functional descriptions usually apply to complete amplifier systems or sub-systems and rarely to individual stages HP Pavilion DV6000 battery - 8800mAh .

  • servo amplifier indicates an integrated feedback loop to actively control the output at some desired level. A DC servoindicates use at frequencies down to DC levels, where the rapid fluctuations of an audio or RF signal do not occur HP Pavilion DV8000 battery - 7800mAh .
  • These are often used in mechanical actuators, or devices such as DC motors that must maintain a constant speed or torque. An AC servo amp can do this for some ac motors.
  • linear amplifier responds to different frequency components independently, and does not generate harmonic distortion or Intermodulation distortion HP Pavilion DV6000 battery - 8800mAh .
  • nonlinear amplifier does generate distortion (e.g. the output is a current to a lamp that must be either fully on or off, but the input is continuously variable; or the amplifier is used in an analog computer where a special transfer function, such as logarithmic, is desired HP Pavilion DV8000 battery - 6600mAh ;
  • or a following tuned circuit will remove the harmonics generated by a non-linear RF amplifier).
  • wideband amplifier has a precise amplification factor over a wide range of frequencies, and is often used to boost signals for relay in communications systems  SONY VAIO VGN-FZ18 Battery .
  • narrowband amp is made to amplify only a specific narrow range of frequencies, to the exclusion of other frequencies.
  • An RF amplifier refers to an amplifier designed for use in the radio frequency range of the electromagnetic spectrum, and is often used to increase the sensitivity of a receiver or the output power of atransmitter SONY VAIO VGN-FZ21E Battery .
  • An audio amplifier is designed for use in reproducing audio frequencies. This category subdivides into small signal amplification, and power amps which are optimised for driving speakers, sometimes with multiple amps grouped together as separate or bridgeable channels to accommodate different audio reproduction requirements SONY VAIO VGN-FW31J Battery 10400mAh .
  • Frequently used terms within audio amplifiers include:
    • preamplifier (preamp), that may include phono or gramophone preamp with equalization for RIAA LP recordings, or tape head preamps with CCIR equalisation filters; they may include filters ortone control circuitry SONY VAIO VGN-FW31J Battery 10400mAh .
    • power amplifier (normally assumed to drive loudspeakers), headphone amplifiers, and public address amplifiers.
    • stereo amplifiers imply two channels of output (left and right), although the term simply means "solid" sound (referring to three-dimensional) - so quadraphonic stereo was used for amplifiers with 4 channels SONY VAIO VGN-FW21E Battery;
    • 5.1 and 7.1 systems refer to Home theatre systems with 5 or 7 normal spacial channels, plus a subwoofer channel (that is not very directional).
  • Buffer amplifiers, that may include emitter followers  SONY VAIO VGN-NR11S/S Battery ,
  • provide a high impedance input for a device (perhaps another amplifier, or perhaps an energy-hungry load such as lights) that would otherwise draw too much current from the source. Line drivers are a type of buffer intended to feed long or interference-prone interconnect cables, possibly with differential outputs if driving twisted pairs of cables SONY VAIO VGN-NR11M/S Battery .
  • A special type of amplifier is widely used in instruments and for signal processing, among many other varied uses. These are known as operational amplifiers or op-amps. This is because this type of amplifier is used in circuits that perform mathematical algorithmic functions SONY VAIO VGN-NR11Z/S Battery ,
  • or "operations" on input signals to obtain specific types of output signals. A typical modern op-amp has differential inputs (one "inverting", one "non-inverting") and one output. An idealised op-amp has the following characteristics:
The performance of an op-amp with these characteristics would be entirely defined by the (usually passive) components forming a negative feedback loop around it, that is, the amplifier itself has no effect on the output   Sony VAIO VGN-FW21E Battery .
Today, op-amps are usually provided as integrated circuits, rather than constructed from discrete components. All real-world op-amps fall short of the idealised specification above – but some modern components have remarkable performance and come close in some respects  Sony VAIO VGN-NR11S/S Battery .

Interstage coupling method

Amplifiers are sometimes classified by the coupling method of the signal at the input, output, or between stages. Different types of these include  Sony VAIO VGN-NR11Z/S Battery :

Resistive-capacitive (RC) coupled amplifier, using a network of resistors and capacitors
By design these amplifiers cannot amplify DC signals as the capacitors block the DC component of the input signal. RC-coupled amplifiers were used very often in circuits with vacuum tubes or discrete transistors  Sony VAIO VGN-NR11M/S Battery .
In the days of the integrated circuit a few more transistors on a chip are much cheaper and smaller than a capacitor.
Inductive-capacitive (LC) coupled amplifier, using a network of inductors and capacitors
This kind of amplifier is most often used in selective radio-frequency circuits Sony VAIO VGN-NR11Z/T Battery .
Transformer coupled amplifier, using a transformer to match impedances or to decouple parts of the circuits
Quite often LC-coupled and transformer-coupled amplifiers cannot be distinguished as a transformer is some kind of inductor.
Direct coupled amplifier, using no impedance and bias matching components SONY VAIO VGN-FZ180E Battery
This class of amplifier was very uncommon in the vacuum tube days when the anode (output) voltage was at greater than several hundred volts and the grid (input) voltage at a few volts minus. So they were only used if the gain was specified down to DC (e.g., in an oscilloscope) SONY VAIO VGN-FZ220E Battery .
In the context of modern electronics developers are encouraged to use directly coupled amplifiers whenever possible.

Frequency range

Depending on the frequency range and other properties amplifiers are designed according to different principles SONY VAIO VGN-FZ340E Battery .

  • Frequency ranges down to DC are only used when this property is needed. DC amplification leads to specific complications that are avoided if possible; DC-blocking capacitors are added to remove DC and sub-sonic frequencies from audio amplifiersSONY VAIO VGN-FZ430E Battery .
  • Depending on the frequency range specified different design principles must be used. Up to the MHz range only "discrete" properties need be considered; e.g., a terminal has an input impedance SONY VAIO VGN-FZ460E Battery .
  • As soon as any connection within the circuit gets longer than perhaps 1% of the wavelength of the highest specified frequency (e.g., at 100 MHz the wavelength is 3 m, so the critical connection length is approx. 3 cm) design properties radically changeSONY VAIO VGN-FZ480E Battery .
  • For example, a specified length and width of a PCB trace can be used as a selective or impedance-matching entity.
  • Above a few 100 MHz, it gets difficult to use discrete elements, especially inductors. In most cases PCB traces of very closely defined shapes are used instead  SONY VAIO VGN-FZ4000 Battery .

The frequency range handled by an amplifier might be specified in terms of bandwidth (normally implying a response that is 3 dB down when the frequency reaches the specified bandwidth), or by specifying a frequency response that is within a certain number of deciBels between a lower and an upper frequency (e.g. "20 Hz to 20 kHz plus or minus 1 dB") SONY VAIO VGN-FZ31E Battery .

Amplifiers are implemented using active elements of different kinds:

  • The first active elements were relays. They were for example used in transcontinental telegraph lines: a weak current was used to switch the voltage of a battery to the outgoing line SONY VAIO VGN-FZ31B Battery
  • For transmitting audio, carbon microphones were used as the active element. This was used to modulate a radio-frequency source in one of the first AM audio transmissions, by Reginald Fessendenon Dec. 24, 1906.
  • In the 1960s, the transistor started to take over. These days, discrete transistors are still used in high-power amplifiers and in specialist audio devices SONY VAIO VGN-FZ31J Battery .
  • Up to the early 1970s, most amplifiers used vacuum tubes ("valves" in the UK). Today, tubes are used for specialist audio applications such as guitar amplifiers and audiophile amplifiers. Many broadcast transmitters still use vacuum tubes SONY VAIO VGN-FZ31M Battery .
  • Beginning in the 1970s, more and more transistors were connected on a single chip therefore creating the integrated circuit. Nearly all amplifiers commercially available today are based on integrated circuits SONY VAIO VGN-FZ31Z Battery .

For exotic purposes, other active elements have been used. For example, in the early days of the communication satellite parametric amplifiers were used. The core circuit was a diode whose capacity was changed by an RF signal created locally SONY VAIO VGN-FZ38M Battery .

Under certain conditions, this RF signal provided energy that was modulated by the extremely weak satellite signal received at the earth station. The operating principle of a parametric

amplifier is somewhat similar to the principle by which children keep their swings in motion: as long as the swing moves you only need to change a parameter of the swinging entity SONY VGP-BPS8 Battery ;

e.g., you must move your center of gravity up and down. In our case, the capacity of the diode is changed periodically.

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  SONY VGP-BPS13/S Battery .
The analog output is created by pulse-width modulation; i.e., the active element is switched on for shorter or longer intervals instead of modifying its resistance. There are more complicated switching schemes likesigma-delta modulation, to improve some performance aspects like lower distortions or better efficiency SONY VGP-BPS13A/B Battery .
Additional classes
There are several other amplifier classes, although they are mainly variations of the previous classes. For example, class G and class H amplifiers are marked by variation of the supply rails (in discrete steps or in a continuous fashion, respectively) following the input signal  SONY VGP-BPS13B/B Battery .
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  SONY VGP-BPS13A/S Battery .
Also, class E and class F amplifiers are commonly described in literature for radio frequencies applications where efficiency of the traditional classes in are important, yet several aspects not covered elsewhere  SONY VGP-BPS13AS Battery
(e.g.: amplifiers often simply said to have a gain of x dB - so what power gain?) 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 Dell Inspiron 1320n Battery .

The classes can be most easily understood using the diagrams in each section below. For the sake of illustration, a bipolar junction transistor is shown as the amplifying device, but in practice this could be a MOSFET or vacuum tube device  Dell Studio 1450 Battery.

In an analog amplifier (the most common kind), the signal is applied to the input terminal of the device (base, gate or grid), and this causes a proportional output drive current to flow out of the output terminal. The output drive current comes from the power supply  Dell Studio 1457 Battery .

Class A

Amplifying devices operating in class A conduct over the whole of the input cycle such that the output signal is an exact scaled-up replica of the input with no clipping. A class A amplifier is distinguished by the output stage being biased into class A (see definition above) Dell Latitude D610 Battery .

Advantages of class A amplifiers

  • Class A designs are simpler than other classes; for example class AB and B designs require two devices (push-pull output) to handle both halves of the waveform; class A can use a single device single-ended  Toshiba NB100 Battery .
  • The amplifying element is biased so the device is always conducting to some extent, normally implying the quiescent (small-signal) collector current (fortransistors; drain current for FETs or anode/plate current for vacuum tubes) is close to the most linear portion of its transconductance curve  Toshiba Satellite M65 battery .
  • Because the device is never shut off completely there is no "turn on" time, little problem with charge storage, and generally better high frequency performance and feedback loop stability (and usually fewer high-order harmonics)  Toshiba Satellite M60 battery .
  • The point at which the device comes closest to being cut off is not close to zero signal, so the problem of crossover distortion associated with class AB and B designs is avoided   Dell Latitude D830 Battery .

Disadvantage of class A amplifiers

  • They are very inefficient; a theoretical maximum of 50% is obtainable with inductive output coupling and only 25% with capacitive coupling, unless square law output stages are used   Dell Latitude D620 Battery .
  • In a power amplifier this not only wastes power and limits battery operation, it may place restrictions on the output devices that can be used (for example: ruling out some audio triodes if modern low-efficiencyloudspeakers are to be used), and will increase costs  Dell Inspiron Mini 10 Battery .
  • Inefficiency comes not just from the fact that the device is always conducting to some extent (that happens even with class AB, yet its efficiency can be close to that of class B); it is that the standing current is roughly half the maximum output current (although this can be less with square law output stage)  Sony VGN-FW11S Battery ,
  • together with the problem that a large part of the power supply voltage is developed across the output device at low signal levels (as with classes AB and B, but unlike output stages such as class D). If high output powers are needed from a class A circuit, the power waste (and the accompanying heat) will become significant  Sony VGN-FW11M Battery .
  • For every watt delivered to the load, the amplifier itself will, at best, dissipate another watt. For large powers this means very large and expensive power supplies and heat sinking  Sony VGN-FW139E/H battery .

Class A designs have largely been superseded by the more efficient designs for power amplifiers, though they remain popular with some hobbyists, mostly for their simplicity  Dell Latitude E5400 Battery .

Also, many audiophilesbelieve that class A gives the best sound quality (for their absence of crossover distortion and reduced odd-harmonic and high-order harmonic distortion) which provides a small market for expensivehigh fidelity class A amps  Dell Latitude E4200 Battery .

Single-ended and triode class A amplifiers

Some aficionados who prefer class A amplifiers also prefer the use of thermionic valve (or "tube") designs instead of transistors, especially in Single-ended triode output configurations for several claimed reasons  Dell Inspiron 300M Battery :

  • Single-ended output stages (be they tube or transistor) have an asymmetrical transfer function, meaning that even harmonics in the created distortion tend not to be canceled (as they are in push-pull output stages)  Dell Studio 1737 battery ;
  • by using tubes OR FETs most of the distortion is from the square law transfer characteristic and so second-order, which some consider to be "warmer" and more pleasant.
  • For those who prefer low distortion figures, the use of tubes with class A (generating little odd-harmonic distortion Dell Inspiron E1505 battery ,
  • as mentioned above) together with symmetrical circuits (such as push-pull output stages, or balanced low-level stages) results in the cancellation of most of the even distortion harmonics, hence the removal of most of the distortion Dell RM791 battery .
  • Though good amplifier design can reduce harmonic distortion patterns to almost nothing, distortion is essential to the sound of electric guitar amplifiers, for example, and is held by recording engineers to offer more flattering microphones and to enhance "clinical-sounding" digital technology  Toshiba Satellite P10 Battery .
  • Historically, valve amplifiers often used a class A power amplifier simply because valves are large and expensive; many class A designs use only a single device.

Transistors are much cheaper, and so more elaborate designs that give greater efficiency but use more parts are still cost-effectiveAcer Aspire One battery 10400mAh .

A classic application for a pair of class A devices is the long-tailed pair, which is exceptionally linear, and forms the basis of many more complex circuits, including many audio amplifiers and almost all op-amps. Class A amplifiers are often used in output stages of high quality op-amps Dell Precision M70 Battery

(although the accuracy of the bias in low cost op-amps such as the 741 may result in class A or class AB or class B, varying from device to device or with temperature). They are sometimes used as medium-power, low-efficiency, and high-cost audio amplifiers. The power consumption is unrelated to the output power  . SONY VGN -FZ210CE Battery

At idle (no input), the power consumption is essentially the same as at high output volume. The result is low efficiency and high heat dissipation.

Class B and AB

Class B or AB push–pull circuits are the most common design type found in audio power amplifiers Toshiba Satellite L305 Battery .

Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the "class A" region, where it is amplified with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small  Toshiba Satellite T4900 Battery .

The crossover distortion can be reduced further by using negative feedback. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well. Class B amplifiers are also favored in battery-operated devices, such as transistor radios Toshiba PA3399U-2BRS battery .

Class B

Class B amplifiers only amplify half of the input wave cycle, thus creating a large amount of distortion, but their efficiency is greatly improved and is much better than class A. Class B has a maximum theoretical efficiency of 78.5% (i.e., ?/4) Sony VGP-BPS10 battery 10400mAh .

This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power. A single class B element is rarely found in practice, though it has been used for driving the loudspeaker in the early IBM Personal Computers with beeps, and it can be used in RF power amplifier where the distortion levels are less important Sony VGP-BPS9 battery 10400mAh .

However, class C is more commonly used for this.

A practical circuit using class B elements is the push-pull stage, such as the very simplified complementary pair arrangement shown below  Sony VGP-BPS11 battery 10400mAh .

Here, complementary or quasi-complementary devices are each used for amplifying the opposite halves of the input signal, which is then recombined at the output. This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small mismatch in the cross-over region Sony VGP-BPS22 battery 10400mAh

- at the "joins" between the two halves of the signal, as one output device has to take over supplying power exactly as the other finishes. This is called crossover distortion. An improvement is to bias the devices so they are not completely off when they're not in use. This approach is called class AB operation Sony VGP-BPL8 battery  10400mAh .

Digital class B

A limited power output class B amplifier with a single-ended supply rail of 5±0.5 V.

Class AB

In class AB operation, each device operates the same way as in class B over half the waveform, but also conducts a small amount on the other half Sony VGP-BPS14 battery 10400mAh .

As a result, the region where both devices simultaneously are nearly off (the "dead zone") is reduced. The result is that when the waveforms from the two devices are combined, the crossover is greatly minimised or eliminated altogether Sony VGP-BPS21 battery   10400mAh .

The exact choice of quiescent current, the standing current through both devices when there is no signal, makes a large difference to the level of distortion (and to the risk of thermal runaway, that may damage the devices); often the bias voltage applied to set this quiescent current has to be adjusted with the temperature of the output transistors Toshiba Satellite A200 Battery

(for example in the circuit at the beginning of the article the diodes would be mounted physically close to the output transistors, and chosen to have a matched temperature coefficient). Another approach (often used as well as thermally-tracking bias voltages) is to include small value resistors in series with the emitters  Toshiba Satellite 1200 Battery .

Class AB sacrifices some efficiency over class B in favor of linearity, thus is less efficient (below 78.5% for full-amplitude sinewaves in transistor amplifiers, typically; much less is common in class AB vacuum tube amplifiers). It is typically much more efficient than class A  Toshiba Satellite M300 Battery .

Class C

Class C amplifiers conduct less than 50% of the input signal and the distortion at the output is high, but high efficiencies (up to 90%) are possible. Some applications (for example, megaphones) can tolerate the distortion   SONY VGP-BPS13A/Q Battery .

A much more common application for class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by using tuned loads on the amplifier stage. The input signal is used to roughly switch the amplifying device on and off, which causes pulses of current to flow through a tuned circuit  SONY VGP-BPS13A/Q Battery .

The class C amplifier has two modes of operation: tuned and untuned. The diagram shows a waveform from a simple class C circuit without the tuned load. This is called untuned operation, and the analysis of the waveforms shows the massive distortion that appears in the signal   SONY VGP-BPS13B/Q Battery .

When the proper load (e.g., a pure inductive-capacitive filter) is used, two things happen. The first is that the output's bias level is clamped, so that the output variation is centered at one-half of the supply voltage. This is why tuned operation is sometimes called a clamper   SONY VGP-BPS13B/Q Battery .

This action of elevating bias level allows the waveform to be restored to its proper shape, allowing a complete waveform to be re-established despite having only a one-polarity supply. This is directly related to the second phenomenon: the waveform on the center frequency becomes much less distorted  SONY VGP-BPS13/Q Battery .

The distortion that is present is dependent upon the bandwidth of the tuned load, with the center frequency seeing very little distortion, but greater attenuation the farther from the tuned frequency that the signal gets  SONY VGP-BPS13/Q Battery .

The tuned circuit will only resonate at particular frequencies, and so the unwanted frequencies are dramatically suppressed, and the wanted full signal (sine wave) will be extracted by the tuned load (e.g., a high-quality bell will ring at a particular frequency when it is hit periodically with a hammer)  SONY VGP-BPS21A/B Battery .

Provided the transmitter is not required to operate over a very wide band of frequencies, this arrangement works extremely well. Other residual harmonics can be removed using a filter.

Class D

In the class D amplifier the input signal is converted to a sequence of higher voltage output pulses  SONY VGP-BPS21/S Battery .

The averaged-over-time power values of these pulses are directly proportional to the instantaneous amplitude of the input signal. The frequency of the output pulses is typically ten or more times the highest frequency in the input signal to be amplified   SONY VGP-BPS21B Battery .

The output pulses contain inaccurate spectral components (that is, the pulse frequency and its harmonics) which must be removed by a low-pass passive filter. The resulting filtered signal is then an amplified replica of the input SONY VGP-BPS21A Battery .

These amplifiers use pulse width modulation, pulse density modulation (sometimes referred to as pulse frequency modulation) or more advanced form of modulation such as Delta-sigma modulation (for example, in the Analog Devices AD1990 class D audio power amplifier)  SONY VGP-BPS21 Battery .

Output stages such as those used in pulse generators are examples of class D amplifiers. The termclass D is usually applied to devices intended to reproduce signals with a bandwidth well below the switching frequency.

Class D amplifiers can be controlled by either analog or digital circuits  Sony VGP-BPS21A/B Battery .

The digital control introduces additional distortion called quantization error caused by its conversion of the input signal to a digital value.

The main advantage of a class D amplifier is power efficiency  Sony VGP-BPS21/S Battery .

Because the output pulses have a fixed amplitude, the switching elements (usuallyMOSFETs, but valves and bipolar transistors were once used) are switched either completely on or completely off, rather than operated in linear mode   Sony VGP-BPS21A Battery .

A MOSFET operates with the lowest resistance when fully-on and thus has the lowest power dissipation when in that condition, except when fully off. When operated in a linear mode the MOSFET has variable amounts of resistance that vary linearly with the input voltage and the resistance is something other than the minimum possible Sony VGP-BPS21 Battery ,

therefore more electrical energy is dissipated as heat. Compared to class A/B operation, class D's lower losses permit the use of a smaller heat sink for the MOSFETS while also reducing the amount of AC power supply power required. Thus, class D amplifiers do not need as large or as heavy power supply transformers or heatsinks Sony Vaio PCG-5G2L Battery ,

so they are smaller and more compact in size than an equivalent class AB amplifier.

Class D amplifiers have been widely used to control motors, and almost exclusively for small DC motors, but they are now also used as audio amplifiers  Sony Vaio PCG-5G3L Battery ,

with some extra circuitry to allow analogue to be converted to a much higher frequency pulse width modulated signal. The relative difficulty of achieving good audio quality means that nearly all are used in applications where quality is not a factor, such as modestly-priced bookshelf audio systems and "DVD-receivers" in mid-price home theater systems Sony Vaio PCG-5J1L Battery .

High quality class D audio amplifiers have now appeared in the market and these revised designs have been said to rival good traditional AB amplifiers in terms of quality. Before these higher quality designs existed an earlier use of class D amplifiers and prolific area of application was high-powered, subwoofer amplifiers in cars  Sony Vaio PCG-5K2L Battery .

Because subwoofers are generally limited to a bandwidth of no higher than 150 Hz, the switching speed for the amplifier does not have to be as high as for a full range amplifier. Class D amplifiers for driving subwoofers are relatively inexpensive, in comparison to class AB amplifiers  Sony Vaio PCG-5J2L Battery .

The letter D used to designate this amplifier class is simply the next letter after C, and does not stand for digital. Class D and class E amplifiers are sometimes mistakenly described as "digital" because the output waveform superficially resembles a pulse-train of digital symbols  Sony Vaio PCG-5K1L Battery ,

but a class D amplifier merely converts an input waveform into a continuously pulse-width modulated (square wave) analog signal. (A digital waveform would be pulse-code modulated.)

Class E

The class E/F amplifier is a highly efficient switching power amplifier, typically used at such high frequencies that the switching time becomes comparable to the duty time  Sony Vaio PCG-6S2L Battery .

As said in the class D amplifier, the transistor is connected via a serial LC circuit to the load, and connected via a large L (inductor) to the supply voltage. The supply voltage is connected to ground via a large capacitor to prevent any RF signals leaking into the supplySony Vaio PCG-6S3L Battery .

The class E amplifier adds a C (capacitor) between the transistor and ground and uses a defined L1 to connect to the supply voltage.

The following description ignores DC, which can be added easily afterwards. The above mentioned C and L are in effect a parallel LC circuit to ground   Sony Vaio PCG-6V1L Battery .

When the transistor is on, it pushes through the serial LC circuit into the load and some current begins to flow to the parallel LC circuit to ground. Then the serial LC circuit swings back and compensates the current into the parallel LC circuit  Sony Vaio PCG-6W1L Battery .

At this point the current through the transistor is zero and it is switched off. Both LC circuits are now filled with energy in C and L0. The whole circuit performs a damped oscillation. The damping by the load has been adjusted so that some time later the energy from the Ls is gone into the load  Sony Vaio PCG-6W2L Battery ,

but the energy in both C0 peaks at the original value to in turn restore the original voltage so that the voltage across the transistor is zero again and it can be switched on.

With load, frequency, and duty cycle (0.5) as given parameters and the constraint that the voltage is not only restored  Sony Vaio PCG-6W3L Battery ,

but peaks at the original voltage, the four parameters (L, L0, C and C0) are determined. The class E amplifier takes the finite on resistance into account and tries to make the current touch the bottom at zero. This means that the voltage and the current at the transistor are symmetric with respect to time  Sony Vaio PCG-7111L Battery .

The Fourier transform allows an elegant formulation to generate the complicated LC networks and says that the first harmonic is passed into the load, all even harmonics are shorted and all higher odd harmonics are open.

Class E uses a significant amount of second-harmonic voltage   Sony Vaio PCG-7112L Battery .

The second harmonic can be used to reduce the overlap with edges with finite sharpness. For this to work, energy on the second harmonic has to flow from the load into the transistor, and no source for this is visible in the circuit diagram    Sony Vaio PCG-7113L Battery .

In reality, the impedance is mostly reactive and the only reason for it is that class E is a class F (see below) amplifier with a much simplified load network and thus has to deal with imperfections.

In many amateur simulations of class E amplifiers  Sony Vaio PCG-7133L Battery ,

sharp current edges are assumed nullifying the very motivation for class E and measurements near the transit frequency of the transistors show very symmetric curves, which look much similar to class F simulations.

The class E amplifier was invented in 1972 by Nathan O. Sokal and Alan D. Sokal, and details were first published in 1975  Sony Vaio PCG-7Z2L Battery .

Some earlier reports on this operating class have been published in Russian.

Class F

In push-pull amplifiers and in CMOS, the even harmonics of both transistors just cancel. Experiment shows that a square wave can be generated by those amplifiers and theory shows that square waves do consist of odd harmonics only Sony Vaio PCG-8Y1L Battery .

In a class D amplifier, the output filter blocks all harmonics; i.e., the harmonics see an open load. So even small currents in the harmonics suffice to generate a voltage square wave. The current is in phase with the voltage applied to the filter, but the voltage across the transistors is out of phase   Sony Vaio PCG-8Y2L Battery .

Therefore, there is a minimal overlap between current through the transistors and voltage across the transistors. The sharper the edges, the lower the overlap.

While class D sees the transistors and the load as two separate modules, class F admits imperfections like the parasitics of the transistor and tries to optimise the global system to have a high impedance at the harmonics  Sony Vaio PCG-8Z1L Battery .

Of course there has to be a finite voltage across the transistor to push the current across the on-state resistance. Because the combined current through both transistors is mostly in the first harmonic, it looks like a sine. That means that in the middle of the square the maximum of current has to flow  Sony Vaio PCG-8Z2L Battery ,

so it may make sense to have a dip in the square or in other words to allow some overswing of the voltage square wave. A class F load network by definition has to transmit below a cutoff frequency and reflect above.

Any frequency lying below the cutoff and having its second harmonic above the cutoff can be amplified SONY VAIO VGN-FW31J Battery ,

that is an octave bandwidth. On the other hand, an inductive-capacitive series circuit with a large inductance and a tunable capacitance may be simpler to implement. By reducing the duty cycle below 0.5, the output amplitude can be modulated. The voltage square waveform will degrade  SONY VAIO VGN-FW31E Battery ,

but any overheating is compensated by the lower overall power flowing. Any load mismatch behind the filter can only act on the first harmonic current waveform, clearly only a purely resistive load makes sense, then the lower the resistance, the higher the currentSONY VAIO VGN-FW31B Battery .

Class F can be driven by sine or by a square wave, for a sine the input can be tuned by an inductor to increase gain. If class F is implemented with a single transistor, the filter is complicated to short the even harmonics. All previous designs use sharp edges to minimise the overlap  SONY VAIO VGN-FW31M Battery .

Classes G and H

There are a variety of amplifier designs that enhance class AB output stages with more efficient techniques to achieve greater efficiencies with low distortion. These designs are common in large audio amplifiers since the heatsinks and power transformers would be prohibitively large (and costly) without the efficiency increases   SONY VAIO VGN-FW31Z Battery .

The terms "class G" and "class H" are used interchangeably to refer to different designs, varying in definition from one manufacturer or paper to another.

Class G amplifiers (which use "rail switching" to decrease power consumption and increase efficiency) are more efficient than class AB amplifiers  SONY VAIO VGN-FW38M Battery .

These amplifiers provide several power rails at different voltages and switch between them as the signal output approaches each level. Thus, the amplifier increases efficiency by reducing the wasted power at the output transistors. Class G amplifiers are more efficient than class AB but less efficient when compared to class D, without the negative EMI effects of class D SONY VAIO VGN-AW41JF Battery .

Class H amplifiers take the idea of class G one step further creating an infinitely variable supply rail. This is done by modulating the supply rails so that the rails are only a few volts larger than the output signal at any given time. The output stage operates at its maximum efficiency all the time  SONY VAIO VGN-AW41JF/H Battery .

Switched-mode power supplies can be used to create the tracking rails. Significant efficiency gains can be achieved but with the drawback of more complicated supply design and reduced THD performance.

The voltage signal shown is thus a larger version of the input, but has been changed in sign (inverted) by the amplification SONY VAIO VGN-AW41MF Battery .

Other arrangements of amplifying device are possible, but that given (that is, common emitter, common source or common cathode) is the easiest to understand and employ in practice. If the amplifying element is linear, then the output will be faithful copy of the input, only larger and inverted  SONY VAIO VGN-AW41MF/H Battery .

In practice, transistors are not linear, and the output will only approximate the input. Non-linearity from any of several sources is the origin of distortion within an amplifier. Which class of amplifier (A, B, AB or C) depends on how the amplifying device is biased — in the diagrams the bias circuits are omitted for clarity SONY VAIO VGN-FW30B Battery .

Any real amplifier is an imperfect realization of an ideal amplifier. One important limitation of a real amplifier is that the output it can generate is ultimately limited by the power available from the power supply. An amplifier will saturate and clip the output if the input signal becomes too large for the amplifier to reproduce or if operational limits for a device are exceeded SONY VAIO VGN-FW50B Battery .

For additional information on class H: Efficiency Class H

Doherty amplifiers

A hybrid configuration receiving new attention is the Doherty amplifier, invented in 1934 by William H. Doherty for Bell Laboratories (whose sister company, Western Electric, was then an important manufacturer of radio transmitters)  SONY VAIO VGN-FW51B/W Battery .

The Doherty amplifier consists of a class B primary or carrier stage in parallel with a class C auxiliary or peak stage. The input signal is split to drive the two amplifiers and a combining network sums the two output signals. Phase shifting networks are employed in the inputs and the outputs  SONY VAIO VGN-FW51MF Battery .

During periods of low signal level, the class B amplifier efficiently operates on the signal and the class C amplifier is cutoff and consumes little power. During periods of high signal level, the class B amplifier delivers its maximum power and the class C amplifier delivers up to its maximum power  SONY VAIO VGN-FW51MF/H Battery .

The efficiency of previous AM transmitter designs was proportional to modulation but, with average modulation typically around 20%, transmitters were limited to less than 50% efficiency. In Doherty's design, even with zero modulation, a transmitter could achieve at least 60% efficiency  SONY VAIO VGN-FW51ZF Battery .

As a successor to Western Electric for broadcast transmitters, the Doherty concept was considerably refined by Continental Electronics Manufacturing Company of Dallas, TX. Perhaps, the ultimate refinement was the screen-grid modulation scheme invented by Joseph B. Sainton  SONY VAIO VGN-FW51ZF/H Battery .

The Sainton amplifier consists of a class C primary or carrier stage in parallel with a class C auxiliary or peak stage. The stages are split and combined through 90-degree phase shifting networks as in the Doherty amplifier. The unmodulated radio frequency carrier is applied to the control grids of both tubes  SONY VAIO VGN-FW52JB Battery .

Carrier modulation is applied to the screen grids of both tubes. The bias point of the carrier and peak tubes is different, and is established such that the peak tube is cutoff when modulation is absent (and the amplifier is producing rated unmodulated carrier power) SONY VAIO VGN-FW54FB Battery

whereas both tubes contribute twice the rated carrier power during 100% modulation (as four times the carrier power is required to achieve 100% modulation). As both tubes operate in class C, a significant improvement in efficiency is thereby achieved in the final stage  SONY VAIO VGN-FW70DB Battery .

In addition, as the tetrode carrier and peak tubes require very little drive power, a significant improvement in efficiency within the driver stage is achieved as well (317C, et al.). The released version of the Sainton amplifier employs a cathode-follower modulator, not a push-pull modulator  SONY VAIO VGN-FW71DB/W Battery .

Previous Continental Electronics designs, by James O. Weldon and others, retained most of the characteristics of the Doherty amplifier but added screen-grid modulation of the driver (317B, et al.).

The Doherty amplifier remains in use in very-high-power AM transmitters, but for lower-power AM transmitters SONY VAIO VGN-FW72JGB Battery ,

vacuum-tube amplifiers in general were eclipsed in the 1980s by arrays of solid-state amplifiers, which could be switched on and off with much finer granularity in response to the requirements of the input audio. However, interest in the Doherty configuration has been revived by SONY VAIO VGN-FW73JGB Battery

cellular-telephone and wireless-Internet applications where the sum of several constant-envelope users creates an aggregate AM result. The main challenge of the Doherty amplifier for digital transmission modes is in aligning the two stages and getting the class-C amplifier to turn on and off very quickly  SONY VAIO VGN-FW74FB Battery .

Recently, Doherty amplifiers have found widespread use in cellular base station transmitters for GHz frequencies. Implementations for transmitters in mobile devices have also been demonstrated.

Special classes

Several audio amplifier manufacturers have started "inventing" new classes as a way to differentiate themselves SONY VAIO VGN-FW81HS Battery .

These class names usually do not reflect any revolutionary amplification technique, and are used mostly for marketing purposes. This can easily be determined by the fact that the class name is trademarked or copyrighted. For example, Crown's K and I-Tech Series as well as several other models utilise Crown's patented class I (or BCA) technology SONY VAIO VGN-FW81NS Battery .

Lab.gruppen use a form of class D amplifier called class TD or tracked class D which tracks the waveform to more accurately amplify it without the drawbacks of traditional class D amplifiers.

"Class T" was a trademark of TriPath company which manufactures audio amplifier ICs  SONY VAIO VGN-FW81S Battery .

This new class T is a revision of the common class D amplifier, but with changes to ensure fidelity over the full audio spectrum, unlike traditional class D designs. It operates at different frequencies depending on the power output, with values ranging from as low as 200 kHz to 1.2 MHz, using a proprietary modulator  SONY VAIO VGN-FW82DS Battery .

Tripath ceased operations in 2007, its patents acquired by Cirrus Logic for their Mixed-Signal Audio division. Some Kenwood Recorder use Class W amplifier  "Class Z" is a trademark of Zetex Semiconductors (now part of Diodes Inc. of Dallas, TX) and is a direct-digital-feedback technology  SONY VAIO VGN-FW82JS Battery .

Zetex-patented circuits are being utilised in the latest power amplifiers by NAD Electronics of Canada.

Amplifier circuit

The practical amplifier circuit to the right could be the basis for a moderate-power audio amplifier   SONY VAIO VGN-FW82XS Battery .

It features a typical (though substantially simplified) design as found in modern amplifiers, with a class AB push–pull output stage, and uses some overall negative feedback. Bipolar transistors are shown, but this design would also be realizable with FETs or valvesSONY VAIO VGN-FW83DS Battery .

The input signal is coupled through capacitor C1 to the base of transistor Q1. The capacitor allows the AC signal to pass, but blocks theDC bias voltage established by resistors R1 and R2 so that any preceding circuit is not affected by it. Q1 and Q2 form a differential amplifier SONY VAIO VGN-FW83JS Battery .

(an amplifier that multiplies the difference between two inputs by some constant), in an arrangement known as a long-tailed pair. This arrangement is used to conveniently allow the use of negative feedback, which is fed from the output to Q2 via R7 and R8  SONY VAIO VGN-FW83XS Battery .

The negative feedback into the difference amplifier allows the amplifier to compare the input to the actual output. The amplified signal from Q1 is directly fed to the second stage, Q3, which is a common emitter stage that provides further amplification of the signal and the DC bias for the output stages, Q4 and Q5  SONY VAIO VGN-FW90NS Battery .

R6 provides the load for Q3 (A better design would probably use some form of active load here, such as a constant-current sink). So far, all of the amplifier is operating in class A. The output pair are arranged in class AB push–pull, also called a complementary pairSONY VAIO VGN-FW37J/B Battery .

They provide the majority of the current amplification (while consuming low quiescent current) and directly drive the load, connected via DC-blocking capacitor C2. The diodes D1 and D2 provide a small amount of constant voltage bias for the output pair, just biasing them into the conducting state so that crossover distortion is minimized SONY VAIO VGN-FW37J Battery .

That is, the diodes push the output stage firmly into class-AB mode (assuming that the base-emitter drop of the output transistors is reduced by heat dissipation).

This design is simple, but a good basis for a practical design because it automatically stabilises its operating point SONY VAIO VGN-FW378J/B Battery ,

since feedback internally operates from DC up through the audio range and beyond. Further circuit elements would probably be found in a real design that would roll off the frequency response above the needed range to prevent the possibility of unwanted oscillation  SONY VAIO VGN-FW378DH Battery .

Also, the use of fixed diode bias as shown here can cause problems if the diodes are not both electrically and thermally matched to the output transistors — if the output transistors turn on too much, they can easily overheat and destroy themselves, as the full current from the power supply is not limited at this stage  SONY VAIO VGN-FW378J/B Battery .

A common solution to help stabilise the output devices is to include some emitter resistors, typically an ohm or so. Calculating the values of the circuit's resistors and capacitors is done based on the components employed and the intended use of the amp.

For the basics of radio frequency amplifiers using valves, see Valved RF amplifiers SONY VAIO VGN-FW37J Battery .

Notes on implementation

Real world amplifiers are imperfect.

  • One consequence is that the power supply itself may influence the output, and must itself be considered when designing the amplifier SONY VAIO VGN-FW41E/H Battery
  • The amplifier circuit has an "open loop" performance, that can be described as various parameters (gain, slew rate, output impedance, distortion, bandwidth, signal to noise ratio, etc.)
  • Many modern amplifiers use negative feedback techniques to hold the gain at the desired value SONY VAIO VGN-FW41J/H Battery .

Different methods of supplying power result in many different methods of bias. Bias is a technique by which the active devices are set up to operate in a particular regime, or by which the DC component of the output signal is set to the midpoint between the maximum voltages available from the power supply  SONY VAIO VGN-FW41M/H Battery .

Most amplifiers use several devices at each stage; they are typically matched in specifications except for polarity. Matched inverted polarity devices are called complementary pairs. Class A amplifiers generally use only one device, unless the power supply is set to provide both positive and negative voltages, in which case a dual device symmetrical design may be used SONY VAIO VGN-FWZJ/H Battery .

Class C amplifiers, by definition, use a single polarity supply.

Amplifiers often have multiple stages in cascade to increase gain  Dell Inspiron 6400 AC Adapter .

Each stage of these designs may be a different type of amp to suit the needs of that stage. For instance, the first stage might be a class A stage, feeding a class AB push–pull second stage, which then drives a class G final output stage, taking advantage of the strengths of each type, while minimizing their weaknesses  Dell LATITUDE D800 battery .

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