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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 first three 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. This metric is known as conduction angle (θ). A class A amplifier is conducting through the entire period of the signal (θ=360°); Class B only for one-half the input period (θ=180°), class C for much less than half the input period (θ<180°). Class D amplifiers operate their output device in a switching manner; the fraction of the time that the device is conducting may be adjusted so a pulse-width modulation output (or other frequency based modulation) can be obtained from the stage.

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70-556: (Redirected from Class-A ) Class A may refer to: Communications technology [ edit ] Class-A amplifier , a category of electronic amplifier Class A network , in Internet technology, a type of large network Class A television service , a system for regulating low power stations in the United States Sports [ edit ] Class A (baseball) ,

140-398: A bipolar junction transistor is shown as the amplifying device. However, the same attributes are found with MOSFETs or vacuum tubes . In a class-A amplifier, 100% of the input signal is used (conduction angle θ = 360°). The active element remains conducting all of the time. Amplifying devices operating in class A conduct over the entire range of the input cycle. A class-A amplifier

210-593: A digital-to-analog converter (DAC) to convert the signal to analog form first. If the signal source is in digital form, such as in a digital media player or computer sound card , the digital circuitry can convert the binary digital signal directly to a pulse-width modulation signal that is applied to the amplifier, simplifying the circuitry considerably and reducing opportunities for noise ingress. A class-D amplifier with moderate output power can be constructed using regular CMOS logic process, making it suitable for integration with other types of digital circuitry. Thus it

280-440: A stellar classification Class A, a retired type of United States Army Service Uniform Class A foams, a type of foam used in firefighting See also [ edit ] A class (disambiguation) Class B (disambiguation) Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title Class A . If an internal link led you here, you may wish to change

350-520: A bandwidth of no higher than 150 Hz, switching speed for the amplifier does not have to be as high as for a full range amplifier, allowing simpler designs. Class-D amplifiers for driving subwoofers are relatively inexpensive in comparison to class-AB amplifiers. The letter D used to designate this amplifier class is simply the next letter after C and, although occasionally used as such, does not stand for digital . Class-D and class-E amplifiers are sometimes mistakenly described as "digital" because

420-468: A class AB with just the 80 V supplies in place of the 40 V supplies, the T1 and T3 transistors would need to be in conduction throughout the 0 V to 80 V signal with the corresponding losses all through the wave period - not just the brief high energy bursts. To achieve this rail tracking control, T2 and T4 act as current amplifiers, each in series with its low voltage counterpart T1 and T3. The purpose of T2 and T3

490-478: A class-C amplifier, less than 50% of the input signal is used (conduction angle θ < 180°). Distortion is high and practical use requires a tuned circuit as load. Efficiency can reach 80% in radio-frequency applications. The usual application for class-C amplifiers is in RF transmitters operating at a single fixed carrier frequency , where the distortion is controlled by a tuned load on the amplifier. The input signal

560-457: A class-D amplifier's lower losses permit the use of smaller heat sinks for the MOSFETs while also reducing the amount of input power required, allowing for a lower-capacity power supply design. Therefore, class-D amplifiers are typically smaller than an equivalent class-AB amplifier. Another advantage of the class-D amplifier is that it can operate from a digital signal source without requiring

630-427: 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. Therefore, there is a minimal overlap between current through the transistors and voltage across the transistors. The sharper

700-475: A continuous fashion, respectively) following the input signal. Wasted heat on the output devices can be reduced as excess voltage is kept to a minimum. The amplifier that is fed with these rails itself can be of any class. These kinds of amplifiers are more complex, and are mainly used for specialized applications, such as very high-power units. Also, class-E and class-F amplifiers are commonly described in literature for radio-frequency applications where efficiency of

770-423: A fixed amplitude, the switching elements (usually MOSFETs , but vacuum tubes and bipolar transistors have also been used) are switched completely on or completely off, rather than operating in linear mode. A MOSFET generally operates with the lowest on-state resistance when fully on and thus (excluding when fully off) has the lowest power dissipation when in that condition. Compared to an equivalent class-AB device,

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840-453: A full description of class-E operation may be found in the 1964 doctoral thesis of Gerald D. Ewing. Interestingly, analytical design equations only recently became known. 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. Theoretically square waves consist of odd harmonics only. In

910-418: A level of American Minor League Baseball Class A (classification) , a Paralympic wheelchair fencing classification Transportation [ edit ] Class A airfield , a type of World War II British military installation Class A surface , in automotive design Milwaukee Road class A , a class of steam locomotives Norfolk and Western Railway class A , a class of steam locomotives Class A,

980-408: A matched temperature coefficient.) Another approach (often used with thermally tracking bias voltages) is to include small value resistors in series with the emitters. Class AB sacrifices some efficiency over class B in favor of linearity, thus is less efficient (below 78.5% for full-amplitude sine waves in transistor amplifiers, typically; much less is common in class-AB vacuum-tube amplifiers). It

1050-466: A new letter symbol is also used by a manufacturer to promote its proprietary design. By December 2010, AB and D classes dominated nearly all of audio amplifier market with the former being favored in portable music players, home audio and cell phone owing to lower cost of class AB chips. Power amplifier circuits (output stages) are classified as A, B, AB and C for linear designs—and class D and E for switching designs. The classes are generally based on

1120-421: A parallel-tuned circuit consisting of an inductor and capacitor in parallel, whose components are chosen to resonate at the frequency of the input signal. Power can be coupled to a load by transformer action with a secondary coil wound on the inductor. The average voltage at the collector is then equal to the supply voltage, and the signal voltage appearing across the tuned circuit varies from near zero to near twice

1190-403: A related modulation technique before being applied to the amplifier. The time average power value of the pulses is directly proportional to the analog signal, so after amplification the signal can be converted back to an analog signal by a passive low-pass filter . The purpose of the output filter is to smooth the pulse stream to an analog signal, removing the high-frequency spectral components of

1260-580: A single number (resistance, capacitance, inductance, respectively). In contrast, a nonlinear element 's behavior is specified by its detailed transfer function , which may be given by a curved line on a graph. So specifying the characteristics of a nonlinear circuit requires more information than is needed for a linear circuit. "Linear" circuits and systems form a separate category within electronic manufacturing. Manufacturers of transistors and integrated circuits often divide their product lines into 'linear' and 'digital' lines. "Linear" here means " analog ";

1330-446: A tuned reactive network between the switch and the load. The circuit obtains high efficiency by only operating the switching element at points of zero current (on to off switching) or zero voltage (off to on switching) which minimizes power lost in the switch, even when the switching time of the devices is long compared to the frequency of operation. The class-E amplifier is frequently cited to have been first reported in 1975. However,

1400-602: A type of commercial driver's license in the United States Class A, an ICAO airspace class Class A, a type of motorhome Other uses [ edit ] Class A (novel) , a 2004 CHERUB novel by Robert Muchamore Class A drug , a classification of drugs controlled by the UK Misuse of Drugs Act Class A office space , a Building Owners and Managers Association category Library of Congress Classification:Class A -- General Works Class A,

1470-403: Is a sound system . The superposition principle, the defining equation of linearity, is equivalent to two properties, additivity and homogeneity , which are sometimes used as an alternate definition That is, a linear circuit is a circuit in which (1) the output when a sum of two signals is applied is equal to the sum of the outputs when the two signals are applied separately, and (2) scaling

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1540-517: Is also sinusoidal with frequency f . A linear circuit with constant component values is called linear time-invariant (LTI). Informally, a linear circuit is one in which the electronic components ' values (such as resistance , capacitance , inductance , gain , etc.) do not change with the level of voltage or current in the circuit. Linear circuits are important because they can amplify and process electronic signals without distortion . An example of an electronic device that uses linear circuits

1610-454: Is because the D1 and D3 diodes which are intended to provide a path for the output voltage back into the upper devices are always reverse biased. They are drawn backwards. In place of these diodes, a voltage amplifier with gain which uses vout as its input would be needed in an actual design. There is another reason for this gain requirement between vout and T2 base in an actual class H design and that

1680-680: Is commonly found in System-on-Chips with integrated audio when the amplifier shares a die with the main processor or DSP. While class-D amplifiers are widely used to control motors , they are also used as power amplifiers. Though if the signal is not already in a pulse modulated format prior to amplification, it must first be converted, which may require additional circuitry. Switching power supplies have even been modified into crude class-D amplifiers (though typically these only reproduce low-frequencies with acceptable accuracy). High-quality class-D audio power amplifiers are readily available on

1750-437: Is distinguished by the output stage devices being biased for class A operation. Subclass A2 is sometimes used to refer to vacuum-tube class-A stages that drive the grid slightly positive on signal peaks for slightly more power than normal class A (A1; where the grid is always negative ). This, however, incurs higher signal distortion . Because transistors biased for class A essentially always have drain current, their efficiency

1820-423: Is done by modulating the supply rails so that the rails are only a few volts larger than the output signal "tracking" it at any given time. The output stage operates at its maximum efficiency all the time. This is due to the circuit ability to keep the rail transistors (T2 and T4) in cutoff until a music voltage peak is of a sufficient magnitude to require the additional voltage from the + and - 80 V supplies. Refer to

1890-487: Is low efficiency and high heat dissipation. In a class-B amplifier, the active device conducts for 180 degrees of the cycle (conduction angle θ = 180°). Because only half the waveform is amplified, significant harmonic distortion is directly present in the output signal. Therefore, class-B amplifiers are generally operated with tuned loading - where harmonics are shorted to ground by a series of resonators. Another method of reducing distortion, especially at audio frequencies,

1960-426: 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, 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

2030-399: Is no signal) makes a large difference to the level of distortion (and to the risk of thermal runaway , which may damage the devices). Often, bias voltage applied to set this quiescent current must be adjusted with the temperature of the output transistors. (For example, in the circuit shown at right, the diodes would be mounted physically close to the output transistors, and specified to have

2100-404: Is not the same as that of straight-line graphs . In the common case of a circuit in which the components' values are constant and don't change with time, an alternate definition of linearity is that when a sinusoidal input voltage or current of frequency f is applied, any steady-state output of the circuit (the current through any component , or the voltage between any two points)

2170-629: Is poor and heat is generated in the transistor. Class-A power amplifier designs have largely been superseded by more efficient designs, though their simplicity makes them popular with some hobbyists. There is a market for expensive high fidelity class-A amps considered a "cult item" among audiophiles mainly for their absence of crossover distortion and reduced odd-harmonic and high-order harmonic distortion . Class A power amplifiers are also used in some "boutique" guitar amplifiers due to their unique tonal quality and for reproducing vintage tones. Some hobbyists who prefer class-A amplifiers also prefer

Class A - Misplaced Pages Continue

2240-407: Is the push–pull stage , such as the very simplified complementary pair arrangement shown at right. Complementary devices are each used for amplifying the opposite halves of the input signal, which is then recombined at the output. This arrangement gives good efficiency, but usually suffers from the drawback that there is a small mismatch in the cross-over region – at the "joins" between

2310-464: Is to allow back-biasing diode D2 when the amplifier output is at a positive peak (above 39.3 V) and back biasing D4 when the output is at negative peak less than -39.3 V. During the musical peaks from 100 to 400 watts, the +/-40 V rails source no current as all the current comes from the +/-80 V rails. This figure is too simplistic, however, as it will not actually control the T2 and T4 transistors at all. This

2380-511: Is to assure that the signal applied to the T2 is always "ahead" of the Vout signal so it can never "catch up" with the rail tracker. The rail tracker amplifier might have a 50 V/μs slew rate while the AB amplifier might have only a 30 V/μs slew rate in order to guarantee this. Linear circuit A linear circuit is an electronic circuit which obeys the superposition principle . This means that

2450-400: Is to use two transistor devices in a push-pull configuration. Each conducts for one half (180°) of the signal cycle, and the device currents are combined so that the load current is continuous. At radio frequency , if the coupling to the load is via a tuned circuit , a single device operating in class B can be used because the stored energy in the tuned circuit supplies the "missing" half of

2520-477: Is typically much more efficient than class A. A vacuum tube amplifier design will sometimes have an additional suffix number for the class, for example, class B1. A suffix 1 indicates that grid current does not flow during any part of the input waveform, where a suffix 2 indicates grid current flows for part of the input waveform. This distinction affects the design of the driver stages for the amplifier. Suffix numbers are not used for semiconductor amplifiers. In

2590-408: Is used to switch the active device, causing pulses of current to flow through a tuned circuit forming part of the load. 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. When

2660-562: The "non-saturated" region), and other "linear" circuit elements . Some examples of nonlinear electronic components are: diodes , transistors , and iron core inductors and transformers when the core is saturated. Some examples of circuits that operate in a nonlinear way are mixers , modulators , rectifiers , radio receiver detectors and digital logic circuits. Linear time-invariant circuits are important because they can process analog signals without introducing intermodulation distortion . This means that separate frequencies in

2730-427: 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 performance, varying from device to device or with temperature). They are sometimes used as medium-power, low-efficiency, and high-cost audio power amplifiers. The power consumption is unrelated to the output power. At idle (no input), the power consumption is essentially the same as at high output volume. The result

2800-454: The active element would pass only an instantaneous current pulse while the voltage across it is zero: it then dissipates no power and 100% efficiency is achieved. However practical devices have a limit to the peak current they can pass, and the pulse must therefore be widened, to around 120 degrees, to obtain a reasonable amount of power, and the efficiency is then 60–70%. Class-D amplifiers use some form of pulse-width modulation to control

2870-432: The amplifier. Such amplifiers have an efficiency around 60%. When Class-B amplifiers amplify the signal with two active devices, each operates over one half of the cycle. Efficiency is much improved over class-A amplifiers. Class-B amplifiers are also favoured in battery-operated devices, such as transistor radios . Class B has a maximum theoretical efficiency of π/4 (≈ 78.5%). A practical circuit using class-B elements

Class A - Misplaced Pages Continue

2940-405: The cutoff can be amplified, 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 degrades, but any overheating is compensated by the lower overall power flowing. Any load mismatch behind

3010-429: The edges, the lower the overlap. While in class D, transistors and the load exist 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. Of course there must be a finite voltage across the transistor to push the current across the on-state resistance. Because the combined current through both transistors

3080-444: 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 current. 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

3150-562: The input signal x ( t ) {\displaystyle x(t)} by a factor h {\displaystyle h} scales the output signal F ( x ( t ) ) {\displaystyle F(x(t))} by the same factor. A linear circuit is one that has no nonlinear electronic components in it. Examples of linear circuits are amplifiers , differentiators , and integrators , linear electronic filters , or any circuit composed exclusively of ideal resistors , capacitors , inductors , op-amps (in

3220-457: The linear line includes integrated circuits designed to process signals linearly, such as op-amps , audio amplifiers , and active filters , as well as a variety of signal processing circuits that implement nonlinear analog functions such as logarithmic amplifiers, analog multipliers , and peak detectors. Nonlinear elements such as transistors tend to behave linearly when small AC signals are applied to them. So in analyzing many circuits where

3290-524: The link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Class_A&oldid=1232174110 " Category : Disambiguation pages Hidden categories: Short description is different from Wikidata All article disambiguation pages All disambiguation pages Class-A amplifier Additional letter classes are defined for special-purpose amplifiers, with additional active elements, power supply improvements, or output tuning; sometimes

3360-438: The market. Dynamic range of 118 dB in a high-end consumer product was seen in the year 2009. Most, however, remain closer to 100 dB dynamic range at this time [2022] due to practical cost considerations. These designs have been said to rival traditional class A and AB amplifiers in terms of quality. An early use of class-D amplifiers was high-power subwoofer amplifiers in cars. Because subwoofers are generally limited to

3430-449: The music signal is between 100 and 400 watts output. The key to understanding this efficiency without churning the actual numbers is that we have a 400-watt-capable amplifier but with the efficiency of a 100-watt amplifier. This is because the waveforms of music contain long periods under 100 watts and contain only brief bursts of up to 400 watts – in other words, the losses at 400 watts are for brief time periods. If this example were drawn as

3500-459: The output devices. The conduction angle of each device is no longer related directly to the input signal but instead varies in pulse width. In the class-D amplifier the active devices (transistors) function as electronic switches instead of linear gain devices; they are either on or off. The analog signal is converted to a stream of pulses that represents the signal by pulse-width modulation , pulse-density modulation , delta-sigma modulation or

3570-405: The output of the circuit F(x) when a linear combination of signals ax 1 (t) + bx 2 (t) is applied to it is equal to the linear combination of the outputs due to the signals x 1 (t) and x 2 (t) applied separately: It is called a linear circuit because the output voltage and current of such a circuit are linear functions of its input voltage and current. This kind of linearity

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3640-434: The output waveform superficially resembles a pulse-train of digital symbols, but a class-D amplifier merely converts an input waveform into a continuously pulse-width modulated analog signal. (A digital waveform would be pulse-code modulated .) Other amplifier classes 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

3710-743: The overlap. There are a variety of amplifier designs that enhance class-AB output stages with more efficient techniques to achieve greater efficiency 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. 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. These amplifiers provide several power rails at different voltages and switch between them as

3780-411: The proper load (e.g., an inductive-capacitive filter plus a load resistor) is used, two things happen. The first is that the output's bias level is clamped with the average output voltage equal to the supply voltage. This is why tuned operation is sometimes called a clamper . This restores the waveform to its proper shape, despite the amplifier having only a one-polarity supply. This is directly related to

3850-405: The proportion of each input cycle (conduction angle) during which an amplifying device passes current. The image of the conduction angle derives from amplifying a sinusoidal signal. If the device is always on, the conducting angle is 360°. If it is on for only half of each cycle, the angle is 180°. The angle of flow is closely related to the amplifier power efficiency . In the illustrations below,

3920-405: The pulses. The frequency of the output pulses is typically ten or more times the highest frequency in the input signal to amplify, so that the filter can adequately reduce the unwanted harmonics and accurately reproduce the input. The main advantage of a class-D amplifier is power efficiency. Efficiency over 90% is achievable with MOSFETs and >80% is fairly common. Because the output pulses have

3990-615: The qualitative behavior of the circuit, characterizing it using terms such as gain , phase shift , resonant frequency , bandwidth , Q factor , poles , and zeros . The analysis of a linear circuit can often be done by hand using a scientific calculator . In contrast, nonlinear circuits usually do not have closed form solutions. They must be analyzed using approximate numerical methods by electronic circuit simulation computer programs such as SPICE , if accurate results are desired. The behavior of such linear circuit elements as resistors, capacitors, and inductors can be specified by

4060-435: The same way as in class B over half the waveform, but also conducts a small amount on the other half. 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. The exact choice of quiescent current (the standing current through both devices when there

4130-471: The schematic figure. The class H amplifier can actually be thought of as two amplifiers in series. In the schematic example shown by the figure, +/- 40 V rail amplifiers can produce about 100 watts continuous into an 8-ohm load. If the output signal is operating below 40 volts, the amplifier only has the losses associated with a 100 W amplifier. This is because the Class H upper devices T2 and T4 are only used when

4200-429: The second phenomenon: the waveform on the center frequency becomes less distorted. The residual distortion 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. The tuned circuit resonates at one frequency, the fixed carrier frequency, and so the unwanted frequencies are suppressed, and

4270-477: The signal levels are small, for example those in TV and radio receivers, nonlinear elements can be replaced with a linear small-signal model , allowing linear analysis techniques to be used. Conversely, all circuit elements, even "linear" elements, show nonlinearity as the signal level is increased. If nothing else, the power supply voltage to the circuit usually puts a limit on the magnitude of voltage output from

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4340-442: 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, however, they do not have the electromagnetic interference effects of class D. Class-H amplifiers create an infinitely variable (analog) supply rail. They are sometimes referred to as rail trackers. This

4410-484: The signal stay separate and do not mix, creating new frequencies ( heterodynes ). They are also easier to understand and analyze. Because they obey the superposition principle , linear circuits are governed by linear differential equations , and can be analyzed with powerful mathematical frequency domain techniques, including Fourier analysis and the Laplace transform . These also give an intuitive understanding of

4480-399: The supply voltage during the RF cycle. The input circuit is biased so that the active element (e.g., transistor) conducts for only a fraction of the RF cycle, usually one-third (120 degrees) or less. The active element conducts only while the collector voltage is passing through its minimum. By this means, power dissipation in the active device is minimised, and efficiency increased. Ideally,

4550-431: The traditional classes is important, yet several aspects 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. The class-E amplifier is a highly efficient tuned switching power amplifier used at radio frequencies. It uses a single-pole switching element and

4620-530: The two active elements conducts more than half of the time. Class AB is widely considered a good compromise for amplifiers, since many types of input signal are nominally quiet enough to stay in the "class-A" region, where they are amplified with good fidelity, and by definition if passing out of this region, will be large enough that the distortion products typical of class B will be relatively small. The crossover distortion can be reduced further by using negative feedback . In class-AB operation, each device operates

4690-424: 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 are not in use. This approach is called class AB operation. In a class-AB amplifier, the conduction angle is intermediate between class A and B (conduction angle θ > 180°); each one of

4760-535: The use of thermionic valve (tube) designs instead of transistors, for several reasons: Transistors are much less expensive than tubes so more elaborate designs that use more parts are still less expensive to manufacture than tube designs. 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 may be used in output stages of op-amps (although

4830-459: The wanted full signal (sine wave) is extracted by the tuned load. The signal bandwidth of the amplifier is limited by the Q-factor of the tuned circuit but this is not a serious limitation. Any residual harmonics can be removed using a further filter. In practical class-C amplifiers a tuned load is invariably used. In one common arrangement the resistor shown in the circuit above is replaced with

4900-443: The waveform. Devices operating in Class B are used in linear amplifiers, so called because the radio frequency output power is proportional to the square of the input excitation voltage. This is more easily understood if stated as "output voltage is proportional to input voltage, thus ouput power is proportional to input power." This characteristic prevents distortion of amplitude-modulated or frequency-modulated signals passing through

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