Switching amplifier
Encyclopedia
Electronic amplifier
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 larger amplitude...
where all power devices (usually MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
s) are operated as binary switches. They are either fully on or fully off. Ideally, zero time is spent transitioning between those two states.
Output stages such as those used in pulse generator
Pulse generator
A pulse generator is either an electronic circuit or a piece of electronic test equipment used to generate rectangular pulses. This article describes the test equipment.-Bench pulse generators:...
s are examples of class D amplifiers. However, the term mostly applies to power amplifiers intended to reproduce signals with a bandwidth well below the switching frequency.
Basic operation
Class D amplifiers work by generating a square wave of which the low-frequency portion of the spectrum is essentially the wanted output signal, and of which the high-frequency portion serves no purpose other than to make the wave-form binary so it can be amplified by switching the power devices.A passive low-pass filter removes the unwanted high-frequency components, i.e., smooths the pulses out and recovers the desired low-frequency signal. To maintain high efficiency, the filter is made with purely reactive components (inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
s and capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
s), which store the excess energy until it is needed instead of converting some of it into heat. The switching frequency is typically chosen to be ten or more times the highest frequency of interest in the input signal. This eases the requirements placed on the output filter. The low pass filter is sometimes not implemented because most loudspeakers do not make sound in response to the switching frequency (though they may increase in heat) and the human ear cannot identify such high frequencies.
The structure of a class D power stage is essentially identical to that of a synchronously rectified buck converter
Buck converter
A buck converter is a step-down DC to DC converter. Its design is similar to the step-up boost converter, and like the boost converter it is a switched-mode power supply that uses two switches , an inductor and a capacitor....
, a type of non-isolated switched-mode power supply
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
. Whereas buck converters usually function as voltage regulator
Voltage regulator
A voltage regulator is an electrical regulator designed to automatically maintain a constant voltage level. A voltage regulator may be a simple "feed-forward" design or may include negative feedback control loops. It may use an electromechanical mechanism, or electronic components...
s, delivering a constant DC voltage into a variable load and can only source current (one-quadrant operation), a class D amplifier delivers a constantly changing voltage into a fixed load, where current and voltage can independently change sign (four-quadrant operation). A switching amplifier must not be confused with any amplifier that uses an SMPS
Switched-mode power supply
A switched-mode power supply is an electronic power supply that incorporates a switching regulator in order to be highly efficient in the conversion of electrical power...
. A switching amplifier may use any type of power supply but the amplification process itself operates by switching.
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, none is turned to heat. This is because an ideal switch in its on state will conduct all current but has no voltage across it, hence no heat is dissipated. And when it is off, it will have the full supply voltage standing across it, but no current flows through it. Again, no heat is dissipated. Real-life power MOSFETs are not ideal switches, but practical efficiencies well over 90% are common. By contrast, linear AB-class amplifiers are always operated with both current flowing through and voltage standing across the power devices. An ideal class B amplifier has a theoretical maximum efficiency of 78%.
Terminology
The term "class D" is sometimes misunderstood as meaning a "digitalDigital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
" amplifier. While some class D amps may indeed be controlled by digital circuits, the power stage deals with voltage and current as a function of time. The smallest amount of noise, timing uncertainty, voltage ripple or any other non-ideality immediately results in an irreversible change of the output signal. A digital circuit also uses physics to operate, but those same errors will only lead to incorrect results when they become so large that a signal representing a digit is distorted beyond recognition. Up to that point, non-idealities have no impact on the transmitted signal. The difference between digital and analogue signals is that digital signals are subsequently interpreted as numbers whereas in analogue signals the exact waveform matters.
Nevertheless, the term "digital amplifier" has gained currency to denote class D amplifiers with significant amounts of digital processing in them.
Signal modulation
The binary waveform is derived using pulse-width modulationPulse-width modulation
Pulse-width modulation , or pulse-duration modulation , is a commonly used technique for controlling power to inertial electrical devices, made practical by modern electronic power switches....
(PWM), pulse density modulation (sometimes referred to as pulse frequency modulation), sliding mode control
Sliding mode control
In control theory, sliding mode control, or SMC, is a nonlinear control method that alters the dynamics of a nonlinear system by application of a discontinuous control signal that forces the system to "slide" along a cross-section of the system's normal behavior. The state-feedback control law is...
(more commonly called "self-oscillating modulation" in the trade.) or discrete-time forms of modulation such as delta-sigma modulation
Delta-sigma modulation
Delta-sigma modulation is a method for encoding high-resolution or analog signals into lower-resolution digital signals. The conversion is done using error feedback, where the difference between the two signals is measured and used to improve the conversion...
.
The most basic way of creating the PWM signal is to use a high speed comparator ("C" 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 duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
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 switches (usually MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
s). 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 recovers the audio information that the speaker can use.
DSP-based amplifiers which generate a PWM signal directly from a digital audio signal (e.g. SPDIF) either use a counter to time the pulse length e.g. or implement a digital equivalent of a 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 quantized
Quantization (sound processing)
In signal processing and digital audio, quantization is the process of approximating a continuous range of values by a relatively small set of discrete symbols or integer values...
, resulting in quantization distortion. In both cases, negative feedback is applied inside the digital domain, forming a noise shaper
Noise shaping
Noise shaping is a technique typically used in digital audio, image, and video processing, usually in combination with dithering, as part of the process of quantization or bit-depth reduction of a digital signal...
which has lower noise in the audible frequency range.
Design challenges
Two significant design challenges for MOSFET driver circuits in class D amplifiers are keeping dead times and linear mode operation as short as possible. "Dead timeDead time
For detection systems that record discrete events, such as particle and nuclear detectors, the dead time is the time after each event during which the system is not able to record another event....
" is the period during a switching transition when both output MOSFETs are driven into Cut-Off Mode and both are "off". 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. The MOSFETs effectively short the output power supply through themselves, a condition known as "shoot-through". Meanwhile, the MOSFET drivers also need to drive the MOSFETs between switching states as fast 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 a significant resistance, creating significant heat. Driver failures that allow shoot-through and/or too much linear mode operation result in excessive losses and sometimes catastrophic failure of the MOSFETs .
Error control
The actual output of the amplifier is not just dependent on the content of the modulated PWM signal. The power supply voltage directly amplitude-modulates the output voltage, dead time errors make the output impedance non-linear and the output filter has a strongly load-dependent frequency response.An effective way to combat errors, regardless of their source, is negative feedback. A feedback loop including the output stage can be made using a simple integrator. To include the output filter, a PID controller
PID controller
A proportional–integral–derivative controller is a generic control loop feedback mechanism widely used in industrial control systems – a PID is the most commonly used feedback controller. A PID controller calculates an "error" value as the difference between a measured process variable and a...
is used, sometimes with additional integrating terms. The need to feed the actual output signal back into the modulator makes the direct generation of PWM from a SPDIF source unattractive .
Advantages
Despite the complexity involved, a properly designed class D amplifier offers the following benefits:- Reduction in size and weight of the amplifier,
- Reduced power waste as heat dissipation and hence smaller (or no) heat sinks,
- Reduction in cost due to smaller heat sink and compact circuitry,
- Very high power conversion efficiency, usually above 90% above one quarter of the amplifier's maximum power, and around 50% at low power levels.
Uses
- Home Theatre systems. In particular the economical "home theatre in a box" 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 common.
- Mobile phones. The internal loudspeaker is driven by up to 1 W. Class D is used to preserve battery lifetime.
- Powered speakersPowered speakersPowered speakers, also known as self-powered speakers and active speakers, are loudspeakers that have built-in amplifiers. They can be connected directly to a mixing console or other low-level audio signal source without the need for an external amplifier...
- High-end audioHigh-end audioHigh-end audio is a term used to describe a class of consumer home audio equipment marketed to audio enthusiasts on the basis of high price or quality, and esoteric or novel sound reproduction technologies. High-end audio can refer simply to the price, to the build quality of the components, or to...
is generally conservative with regards to adopting new technologies but class D amplifiers have made an appearance - Active subwoofers
- Sound Reinforcement and Live SoundSound reinforcement systemA sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers that makes live or pre-recorded sounds louder and may also distribute those sounds to a larger or more distant audience...
. The Crest AudioCrest AudioCrest Audio, Inc is an American company that produces professional sound equipment, including audio mixers, power amplifiers, and loudspeakers. Crest Audio was founded in the late 1970s by Jean-Pierre Prideaux in California...
CD3000, for example, is a class-D power amplifier that is rated at 1500 W per channel, yet it weighs only 21 kg (46 lb). Similarly, the Powersoft K10K10K10 can mean:* K-10 , Eric Cartman's robot dog from South Park* K-10 , a state highway in Kansas* K10 * K-10 , a sounding rocket...
is a class-D power amplifier that is rated at 6000 W per 2-ohm channel, yet it weighs only 12 kg (26.5 lb). - Bass amplifiersBass instrument amplificationBass instrument amplification, used for the bass guitar, double bass and similar instruments, is distinct from other types of amplification systems due to the particular challenges associated with low-frequency sound reproduction. This distinction affects the design of the loudspeakers, the speaker...
Again, an area where portability is important. Example: Yamaha BBT500H bass amplifier which is rated at 500 W, and yet it weighs less than 5 kg (11 lb). The Promethean P500H by Ibanez is also capable of delivering 500 W into a 4 Ohm load, and weighs only 2.9 kg.
See also
- Class A amplifier (a linear, non-PWM, amplifier class)
- Class B amplifier (a linear, non-PWM, amplifier class)
- Class C amplifier (a non-PWM amplifier class)
- Class T amplifierClass T amplifierA Class T amplifier is an audio amplifier IC design. Rather than being a separate "class" of amplifier, Class T is a registered trademark for Tripath's amplifier technologies....
(a proprietary development of Class D) - Delta-sigma modulationDelta-sigma modulationDelta-sigma modulation is a method for encoding high-resolution or analog signals into lower-resolution digital signals. The conversion is done using error feedback, where the difference between the two signals is measured and used to improve the conversion...
- Sliding mode controlSliding mode controlIn control theory, sliding mode control, or SMC, is a nonlinear control method that alters the dynamics of a nonlinear system by application of a discontinuous control signal that forces the system to "slide" along a cross-section of the system's normal behavior. The state-feedback control law is...
External links
- List of PWM Amplifiers
- Sánchez Moreno, Sergio Class D Audio Amplifiers - Theory and Design http://www.coldamp.com/opencms/opencms/coldamp/files/Class_D_audio_amplifiers_White_Paper_en.pdf - Contains material on the theory and design of Class-D amplifiers.
- Burchers, Glen Digital Amplifiers Come of Age
- Hatfield, Robert Class D Amplifiers Empower Mobile Multimedia - Gives information on working with Class D amplifiers in mobile platforms
- Haber, Eric Designing With Class D Amplifier ICs - Some IC-oriented Class D design considerations
- Harden, Paul Introduction to Class C,D,E and F, The Handiman's Guide to MOSFET "Switched Mode" Amplifiers, Part 1 - An article on basic digital RF amplifier design intended for ham radio operators but applicable to audio Class D amplifiers.