Digital amplifiers improve audio quality and system performance.
    Class D amplifiers have evolved tremendously over the past few generations, with system designers dramatically improving the durability of the system and enhancing its audio quality. In fact, for most applications, the benefits of using these amplifiers far outweigh their disadvantages.
    In a traditional Class D amplifier, a controller converts an analog or digital audio signal into a PWM signal before it is amplified by a power MOSFET tube integrated into a power back-end device. These amplifiers are highly efficient, use very small heat sinks or no heat sinks at all, and reduce the power requirements on the power supply output. However, compared with traditional Class A/B amplifiers, they also have inherent cost, performance and EMI problems, and solving these problems is the new trend of Class D amplifiers.

Reduce EMI
    Since the birth of Class D amplifiers, the high level of radiated EMI caused by their own rail-to-rail power supply switching characteristics has been a problem for system designers, which will prevent the equipment from being certified by the FCC and CISPR.
    In a Class D modulator, the digital audio signal is converted into a PWM signal by comparing the audio signal with a high frequency fixed frequency signal and modulating the result on a fixed frequency carrier. The formed signal is a fixed carrier frequency of variable pulse width (usually in a few hundred kHz), and then these PWM signals are amplified by a high-voltage power MOSFET, and the enlarged PWM signal is then removed by a low-pass filter to restore the original baseband audio signal.
    While this topology works well, it also leads to some undesirable consequences, such as large amounts of radiated EMI. Because the modulator uses a fixed frequency carrier, multiple harmonic radiation of the base carrier will be generated. Moreover, due to the switching characteristics of the PWM signal itself, the overshoot/undershoot and ringing will produce a fixed ratio of high frequency (10 to 100 MHZ range) radiated EMI. In order to suppress radiated EMI, the development trend of the latest generation PWM modulator is to use spread spectrum modulation technology.
    Spread spectrum modulation technique is used to extend the spectral energy of a switching PWM signal over a larger bandwidth without changing the content of the original audio. An effective way to improve the high radiation EMI of a conventional modulator is to change the two edges of the PWM switch signal, as shown in Figure 1. The signal is centered on the carrier frequency, but neither edge repeats periodically. This not only maintains the fixed carrier frequency, but because the edge does not jump at a fixed rate, the radiated energy at the carrier frequency is greatly reduced.

Improve audio quality
    Compared with class A/B amplifiers with good performance, the audio performance of Class D amplifiers is very poor, not only large distortion, but also narrow dynamic range. Therefore, the designers of current class D amplifiers must improve their performance. By integrating high-performance sample rate converters (SRC) and Delta-σ processing technology, the new generation of solutions enables greater improvement in distortion (THD+N) and a dynamic range of more than 100dB.
    Currently, a noise source for Class D amplifiers is the jitter of the audio sampling clock. While the clock is usually generated by SOCs (MPEG decoders and DSPS, etc.), even small jitter can quickly affect the performance of a conventional Class D amplifier, because the audio clock is associated with the output clock of the modulator.
    One way to solve this problem is to adopt SRC technology. Because SRC uses a locally stable clock source to synchronize the clock for digital audio, such as a quartz crystal oscillator, the output jitter of the modulator is virtually independent and unrelated to other audio clocks. Another advantage of SRC is that its output switching ratio is fixed regardless of how the sample rate of the input audio fluctuates, unlike PLL-based modulators. SRC also improves the durability of the system by eliminating audible noise when the audio input source changes or the input clock is missing.
    Similar to the technology used in today's high-end Dacs, the audio quality of Class D amplifiers is also improved by integrating higher-order Delta-σ processing technology. Modulators based on Delta-σ technology use internal feedback that can reduce modulation errors. By reducing the sampling error, the modulator can improve the output distortion, resulting in better sound quality.

Reduce system costs
    In order to pursue the lower cost of Class D amplifier, the designer adopts the half-bridge amplifier topology in the power amplifier stage to achieve the purpose of reducing complexity and material cost. Because the output of the half-bridge structure is usually half that of the full bridge, the number of power MOSFETs and external filter elements is also reduced by half. This also increases the number of power channels per unit of back-end devices. However, half-bridge amplifiers also require a straight capacitance at the output and are extremely sensitive to noise on the supply line.
    At start-up, the DC capacitor must be charged to the bias point (half the voltage of the high voltage supply main line). If the output signal does not rise from the ground potential to the bias point, it will produce a large "poof" sound (power-on shock sound) in the speaker. The new Class D amplifier uses a pre-charged capacitor to keep the speaker silent during startup.
    One of the ways to keep the speaker free of shock sound while the capacitor is charged is to use digital voltage boost technology, that is, to slowly increase the PWM duty cycle from the off-switch state to 50%. This will not produce a large "poof" sound in the speaker, but due to the large amount of transient current generated when the MOSFET is switched on and off, the speaker is not voiceless either.
    Another way to keep the speaker free of shock sound while the straight capacitor is charged is analog voltage boost technology. During this type of voltage boost, a current source charges the capacitor to the bias point. Once the voltage at both ends of the capacitor reaches the bias point, the current source is turned off.

Power feedback
    Since the half-bridge is a single-ended topology, there is no common mode suppression in the differential full-bridge topology. In a full-bridge amplifier, since the differential output of the amplifier is fed from the same voltage source, the noise on the common voltage source will be cancelled out at the output. In a half-bridge topology, any AC ripple noise on the amplifier's power supply will be coupled directly to the output. Because the half-bridge topology is sensitive to power supply noise, it is often necessary to provide power supply suppression feedback (PSR) circuit for noise reduction.
    Analog Class D amplifiers have many inherent PSR characteristics, while fully digital Class D amplifiers do not. In current digital PSR schemes, an external ADC is usually used to monitor the amplifier's power supply.
    Feedback and noise cancellation processing is performed in the digital domain of the modulator. Some manufacturers use this feedback method only to compensate for AC noise coupled from the supply trunk to the PWM output that degrades system performance. Other manufacturers also use it to compensate for changes in the DC supply voltage (voltage drop) due to load changes, for example, the fast inrush current required by the subwoofer (superheavy subwoofer), or voltage fluctuations in the supply line. The benefits of PSR feedback in AC and DC devices have been extended to full-bridge amplifiers and improved isolation between channels in current multichannel home theater amplifiers, effectively cancelling out crosstalk and line voltage changes before they reach the output.