A Class D amplifier or switching amplifier is an electronic amplifier where all power devices (usually MOSFETs) are operated as binary switches. They are either fully on or fully off. Ideally, zero time is spent transitioning between those two states. 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 a 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%.

The output of a class D amplifier is a square wave. Low pass LC-filtering smoothes the pulses out and restores the signal shape on the load.

The term “class-D” is sometimes misunderstood as meaning a “digital” amplifier. While many class-D amps are indeed digital, the quantization of the output signal at the power stage can be controlled by either an analog signal or a digital signal. Only in the latter case would an amplifier be using fully digital amplification.

Signal modulation

Output stages such as those used in pulse generators are examples of class D amplifiers. However, the term mostly applies to devices intended to reproduce signals with a bandwidth well below the switching frequency. These amplifiers use pulse-width modulation (PWM), pulse density modulation (sometimes referred to as pulse frequency modulation), or more advanced forms of modulation such as delta-sigma modulation[1] or sliding mode control.

The input signal is converted to a sequence of pulses whose average value is directly proportional to the instantaneous value of the signal at that time. The frequency of the pulses is typically ten or more times the highest frequency of interest in the input signal. The final switching output consists of a train of pulses whose width is a function of the amplitude & frequency of the signal being amplified, and hence these amplifiers are also called PWM amplifiers. The output contains, in addition to the required amplified signal, unwanted spectral components (i.e. the pulse frequency and its harmonics) that must be removed by a passive filter. The filter is usually made with (theoretically) lossless components like inductors and capacitors in order to maintain efficiency.

A PWM amplifier operates similarly to a switched-mode power supply (SMPS), except that a PWM amplifier is feeding a varying audio signal voltage into a relatively fixed load, while an SMPS feeds a fixed voltage into a varying load. A switching amplifier must not be confused with an amplifier that uses an SMPS. A switching amplifier may use any type of power supply but the amplifier itself uses switching of output devices though to achieve amplification.

One way to create the PWM signal is to use a high speed comparator (“C” in the block-diagram above) that compares a high frequency triangular wave and the audio input and generates a series of pulses such that the width of the pulses corresponds to the amplitude and frequency of the audio signal. The comparator then drives a switching controller which in turn drives a high-power switch (usually made of MOSFETs) which generates a high-power replica of the comparator’s PWM signal.

This PWM output is fed to a low-pass filter which removes the high-frequency switching components of the PWM signal to recover the audio information and feeds it to a loudspeaker. A suitably high switching frequency (or triangular waveform) is mandatory in order to obtain reasonably good frequency response and low distortion. Most class-D amplifiers use switching frequencies greater than 100 kHz. These high frequencies require most of the components in the amplifier to be capable of high speed operation.

Another way to create the PWM signal is adopted when a SPDIF signal or other form of digital feed is available. The digital signal is fed to a DSP that uses software to create the PWM signal. This drives the MOSFETs through a suitable gate driver chip.

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 time” 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 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 an 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) heatsinks,
* Reduction in cost due to smaller heat sink and compact circuitry,
* Very high power conversion efficiency, usually ≥ 90%.

This still leaves the signal with significant out-of-band content, which may be filtered out. To maintain a high efficiency, the filtering is done with purely reactive components (inductors and capacitors), which store the excess energy until it is needed instead of converting it into heat.

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