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energy, class-D amplifiers return it to the power supply which should somehow be able to store it. In addition, half-bridge class-D amplifiers transfer energy from one supply rail (e.g. the positive rail) to the other (e.g. the negative) depending on the sign of the output current. This happens with both resistive and reactive loads. The supply should either have enough capacitive storage on both rails, or be able to transfer this energy to the other rail.
138:
218:, delivering a constant DC voltage into a variable load, and can only source current, a class-D amplifier delivers a constantly changing voltage into a fixed load. A switching amplifier may use any type of power supply (e.g., a car battery or an internal SMPS), but the defining characteristic is that the amplification process itself operates by switching.
341:. Dead times need to be as short as possible to maintain an accurate low-distortion output signal, but dead times that are too short cause the MOSFET that is switching on to start conducting before the MOSFET that is switching off has stopped conducting and the MOSFETs effectively short the output power supply through themselves in a condition known as
22:
300:) either use a counter to time the pulse length or implement a digital equivalent of the triangle-based modulator. In either case, the time resolution afforded by practical clock frequencies is only a few hundredths of a switching period, which is not enough to ensure low noise. In effect, the pulse length gets
348:
The controlling circuitry also needs to switch the MOSFETs as quickly as possible to minimize the amount of time a MOSFET is in linear mode—the state between cut-off mode and saturation mode where the MOSFET is neither fully on nor fully off and conducts current with significant resistance, creating
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Mitigating the same issues in an amplifier without feedback requires addressing each separately at the source. Power supply modulation can be partially canceled by measuring the supply voltage to adjust signal gain as part of PWM conversion. Distortion can be reduced by switching faster. The output
202:
represents the amplitude variations of the analog audio input signal. In some implementations, the pulses are synchronized with an incoming digital audio signal removing the necessity to convert the signal to analog. The output of the modulator is then used to turn the output transistors on and off
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With fixed-frequency PWM modulation, as the (peak) output voltage approaches either of the supply rails, the pulse width can get so narrow as to challenge the ability of the driver circuit and the MOSFET to respond. These pulses can be as short as a few nanoseconds and can result in shoot through
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The actual output of the amplifier is not just dependent on the content of the modulated PWM signal. A number of sources may introduce errors. Any variation in power supply voltage directly amplitude-modulates the output voltage. Dead time errors make the output impedance non-linear. The output
361:
Class-D amplifiers place an additional requirement on their power supply, namely that it be able to sink energy returning from the load. Reactive (capacitive or inductive) loads store energy during part of a cycle and release some of this energy back later. Linear amplifiers will dissipate this
472:. For very high power amplification the power loss of class-AB amplifiers is unacceptable. Amplifiers with several kilowatts of output power are available as class D. Class-D power amplifiers are available that are rated at 3000 W total output, yet weigh only 3.6 kg (8 lb).
230:, it would have the full supply voltage across it but no leakage current flowing through it, and again no power would be dissipated. Real-world power MOSFETs are not ideal switches, but practical efficiencies well over 90% are common for class-D amplifiers. By contrast, linear
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alternately. Since the transistors are either fully on or fully off, they dissipate very little power. A simple low-pass filter consisting of an inductor and a capacitor provides a path for the low frequencies of the audio signal, leaving the high-frequency pulses behind.
59:
may be used to attenuate their high-frequency content to provide analog output current and voltage. Little energy is dissipated in the amplifying transistors because they are always either fully on or fully off, so efficiency can exceed 90%.
116:. There were subsequently rapid developments in MOSFET technology between 1979 and 1985. The availability of low-cost, fast-switching MOSFETs led to class-D amplifiers becoming successful in the mid-1980s. The first class-D amplifier based
293:). This produces an amplified replica of the comparator's PWM signal. The output filter removes the high-frequency switching components of the PWM signal and reconstructs audio information that the speaker can use.
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systems are almost universally equipped with class-D amplifiers. On account of modest performance requirements and straightforward design, direct conversion from digital audio to PWM without feedback is most
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and heating due to linear mode operation. Other modulation techniques such as pulse-density modulation can achieve higher peak output voltages, as well as greater efficiency compared to fixed-frequency PWM.
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The theoretical power efficiency of class-D amplifiers is 100%. That is to say, all of the power supplied to it is delivered to the load and none is turned to heat. This is because an ideal switch in its
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is directly proportional with the instantaneous value of the audio signal. The comparator then drives a MOS gate driver which in turn drives a pair of high-power switching transistors (usually
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The active devices in a class-D amplifier need only act as controllable switches and need not have a particularly linear response to the control input. MOSFETs are usually used.
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The generic analysis of sliding mode control is quite math heavy. The specific case of 2-state self-oscillating class-D amplifiers is much more intuitive and can be found in
451:. The miniature loudspeaker (known as the receiver) is directly driven by a class-D amplifier to maximize battery life and can provide levels of 130 dB SPL or more.
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The major advantage of a class-D amplifier is that it can be more efficient than a linear amplifier by dissipating less power as heat in the active devices. Given that large
47:) operate as electronic switches, and not as linear gain devices as in other amplifiers. They operate by rapidly switching back and forth between the supply rails, using
96:
available at the time. As a result, these early class-D amplifiers were impractical and unsuccessful. Practical class-D amplifiers were enabled by the development of
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significant heat. Failures that allow shoot-through or too much linear mode operation result in excessive losses and sometimes catastrophic failure of the MOSFETs.
594:
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Two significant design challenges for MOSFET driver circuits in class-D amplifiers are keeping dead times and linear mode operation as short as possible.
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state would encounter no resistance and conduct all the current with no voltage drop across it, hence no power would be dissipated as heat. And when it is
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is used, sometimes with additional integrating terms. The need to feed the actual output signal back into the modulator makes the direct generation of
607:"IRAUDAMP7S, 25W-500W Scalable Output Power Class D Audio Power Amplifier Reference Design, Using the IRS2092S Protected Digital Audio Driver"
285:" in the block diagram above) that compares a high-frequency triangular wave with the audio input. This generates a series of pulses of which the
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are not required, class-D amplifiers are much lighter weight than class-A, -B, or -AB amplifiers, an important consideration with portable
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may use class D or other switch-mode classes to provide high-efficiency RF power amplification in communications systems.
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Class-D amplifiers work by generating a train of rectangular pulses of fixed amplitude but varying width and separation. This
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amplifiers are always operated with both current flowing through and voltage standing across the power devices. An ideal
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386:. A feedback loop including the output stage can be made using a simple integrator. To include the output filter, a
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is generally conservative with regards to adopting new technologies but class-D amplifiers have made an appearance
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is the period during a switching transition when both output MOSFETs are driven into cut-off mode and both are
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647:"Real-Time Power Supply Feedback Reduces Power Conversion Requirements For Digital Class D Amplifiers"
324:
Boss Audio mono amp. The output stage is top left, the output chokes are the two yellow toroids underneath.
88:. The Sinclair X-20 in 1966 produced 20 watts but suffered from the inconsistencies and limitations of the
744: (archived 2014-03-08) – an article on basic digital RF amplifier design intended for
445:. The internal loudspeaker is driven by up to 1 W. Class D is used to preserve battery lifetime.
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271:
255:
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665:"Group review of "high end" class D offerings and round-table discussion with amplifier designers"
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Sandler et al., Ultra-Low
Distortion Digital Power Amplification, Presented at the 91st AES convention
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DSP-based amplifiers that generate a PWM signal directly from a digital audio signal (e. g.
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in the 1950s and was first called by that name in 1955. The first commercial product was a
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Globally
Modulated Self-Oscillating Amplifier with Improved Linearity, 37th AES Conference
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The structure of a class-D power stage is comparable to that of a synchronously rectified
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308:. In both cases, negative feedback is applied inside the digital domain, forming a
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Analytical and numerical analysis of dead-time distortion in power inverters
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Putzeys et al. All
Amplifiers etc., Presented at the AES 120th convention
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104:(metal–oxide–semiconductor field-effect transistor) technology. In 1978,
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733: – some IC-oriented Class D design considerations
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An effective way to combat errors, regardless of their source, is
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55:, or related techniques to produce a pulse train output. A simple
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The
Handiman's Guide to MOSFET "Switched Mode" Amplifiers, Part 1
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A simple means of creating the PWM signal is to use a high-speed
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The first class-D amplifier was invented by
British scientist
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which results in lower noise in the audible frequency range.
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impedance cannot be controlled other than through feedback.
242:(purely linear, with the devices always at least partially
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filter has a strongly load-dependent frequency response.
27:
Note: For clarity, signal periods are not shown to scale.
108:
introduced the TA-N88, the first class-D unit to employ
262:(sometimes referred to as pulse frequency modulation),
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in which the amplifying devices (transistors, usually
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in 1964. However, it had an output power of only 2.5
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214:(SMPS). Whereas buck converters usually function as
270:.) or discrete-time forms of modulation such as
719:"Class-D Audio Amplifiers - Theory and Design"
238:has a theoretical maximum efficiency of 78%.
8:
166:. Unsourced material may be challenged and
25:Block diagram of a basic class-D amplifier.
748:but applicable to audio class-D amplifiers
679:"Behringer | Product | NX3000D"
186:Learn how and when to remove this message
651:Journal of the Audio Engineering Society
539:"Class-D Audio: The Power and the Glory"
20:
515:High Performance Audio Power Amplifiers
505:
254:The 2-level waveform is derived using
7:
731:Designing With class-D amplifier ICs
717:Sánchez Moreno, Sergio (June 2005).
572:AD1990 class-D audio power amplifier
164:adding citations to reliable sources
124:in 1996, and it saw widespread use.
645:Boudreaux, Randy (September 2005).
16:Audio amplifier based on switching
14:
136:
616:. October 28, 2009. p. 26.
693:Switchmode RF Power Amplifiers
1:
476:Bass instrument amplification
517:. Newnes. pp. 147–148.
94:bipolar junction transistors
470:Sound reinforcement systems
268:self-oscillating modulation
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481:Radio frequency amplifiers
434:systems. These economical
416:sound reinforcement system
212:switched-mode power supply
114:switched-mode power supply
210:, a type of non-isolated
260:pulse-density modulation
53:pulse-density modulation
494:Power amplifier classes
366:Active device selection
306:quantization distortion
398:source unattractive.
325:
272:delta-sigma modulation
266:(more commonly called
256:pulse-width modulation
49:pulse-width modulation
28:
762:Electronic amplifiers
432:Home theater in a box
323:
24:
767:Switching amplifiers
513:Duncan, Ben (1996).
264:sliding mode control
160:improve this section
41:electronic amplifier
746:ham radio operators
545:. 30 December 2010.
357:Power supply design
37:switching amplifier
632:2011-07-24 at the
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240:Class-A amplifiers
216:voltage regulators
118:integrated circuit
82:Sinclair Radionics
76:module called the
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384:negative feedback
316:Design challenges
250:Signal modulation
236:class-B amplifier
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33:class-D amplifier
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772:Audio amplifiers
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721:. Archived from
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80:released by
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457:and active
436:home cinema
70:Alec Reeves
756:Categories
703:, page vii
701:0080550649
500:References
459:subwoofers
412:heat sinks
406:Advantages
287:duty cycle
279:comparator
200:modulation
335:Dead time
302:quantized
176:June 2023
147:does not
90:germanium
630:Archived
488:See also
232:AB-class
740:at the
614:irf.com
439:common.
394:from a
291:MOSFETs
258:(PWM),
168:removed
153:sources
122:Tripath
100:-based
98:silicon
92:-based
64:History
45:MOSFETs
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521:
112:and a
102:MOSFET
39:is an
610:(PDF)
396:SPDIF
298:SPDIF
86:watts
697:ISBN
567:The
519:ISBN
426:Uses
151:any
149:cite
106:Sony
78:X-10
392:PWM
339:off
228:off
162:by
74:kit
35:or
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