Power-Supply Design
Unlike linear amplifiers, designers using class D amplifier ICs also must pay closer attention to the effect of power-supply behavior on audio-output quality. Because the class D output is a switching stage, effectively connecting the supply rail directly to the audio output, audio-band fluctuations in the supply will modulate the output signal directly. Therefore, designers must ensure high load-regulation in the audio band, or take steps to eliminate the effects of mains or audio-band ripple.
If load regulation needs to be improved, a number of manufacturers provide floating regulators that can be added to existing supplies. Using a separate regulator for each amplifier output has the extra benefit of reducing crosstalk between audio channels. However, an additional regulator, or pair of regulators, raises the overall cost of the implementation. Also, power dissipation in the voltage regulator offsets the efficiency gains that are the key justification for a class D implementation.
Alternatively, increasing the amplifier's power-supply rejection ratio (PSRR) reduces the effect of load regulation on the audio output signal. Adding feedback from the PWM output to the analog audio input raises the PSRR by compensating for supply voltage variations. This can achieve a PSRR of up to around 80 dB, which is very close to the PSRR of a differential class-AB amplifier for portable applications. If the class D input signal is digital, however, this technique can't be applied without first converting it to the analog domain. The PSRR of an all-digital class D amplifier is 0 dB, so the designer must ensure close regulation of the supply voltage.
The power supply's transient performance should also be considered. To reproduce the PWM waveform accurately, the power supply must be able to react quickly to sudden changes in current draw. A linear amplifier is much less demanding in this respect, since the bandwidth of the output stage is limited to the audio range. In a power supply for a class D amplifier, voltage fluctuations outside the audio band, resulting from poor transient response, will modulate the PWM signal and thus introduce harmonic distortion that can be heard in the audio output.
Of course, high-value capacitors can be used to deal with these fluctuations. But physically large capacitors aren't desirable in handheld products. On the other hand, high-value capacitors in small-outline packages are expensive.
A helpful technique is to have the MOSFETs in the different output stages switch at different times, thus reducing the peak supply current. For example, the Wolfson WM8608, a 5- to 7.1-channel digital power-amplifier controller, has a built-in "PWM output phase" function that introduces a short delay between the PWM signals for each output channel. This has the effect of spreading the switching transients around the PWM cycle (Fig. 4) , although the added delay is far too short to make an audible difference to the output. In a multichannel system with six channels, this technique significantly diminishes the maximum instantaneous load current and reduces crosstalk.
Switching Supply, Switching Amplifier
One potential concern with switched supplies is EMI, caused by the rapid switching of large currents. This problem is exacerbated when a switching supply and switching amplifier operate in the same system at different switching frequencies. Intermodulation produces tones that may be audible in the output. Synchronizing power-supply switching with that of the class D PWM modulator can eliminate this effect.
Alternatively, a class D amplifier may be powered from a regulated linear supply. This may be attractive where extremely low-cost targets dominate design, but switching power supplies are usually preferred for their high efficiency and small size.
Careful design of the power supply, with consideration for the amplifier PWM and output stage, can achieve SNR and THD performance comparable to most consumer analog amplifiers. With its inherent efficiency, size, and thermal-management advantages, as well as further developments that will enhance performance in the future, the class D amplifier will extend its domination in consumer audio products.