Clicking and popping: the amplifier can be annoying when turned on or off. Unfortunately, however, unless special attention is paid to the state of the regulator, the timing of the output stage, and the state of the LC filter when the amplifier is muted or not muted, it is easy to introduce clicking and popping sounds into class D amplifiers.

    Signal to noise ratio (SNR): to avoid audible hiss from the amplifier noise base, the SNR of low-power amplifiers in portable applications should generally exceed 90dB, 100dB for medium power design and 110dB for high power design. This is possible for many different amplifier implementations, but each noise source must be tracked during amplifier design to ensure a satisfactory and comprehensive SNR.

     Distortion mechanisms: these mechanisms include the non-linear characteristics of the modulation technique or regulator execution - and the dead time used in the output stage to solve the through current problem.

     Information about the audio signal level is usually encoded in the width of the output pulse of the class D regulator. The dead time is added to prevent the non-linear timing error caused by the through current of the output stage. This error will distort the loudspeaker, which is larger than the timing error related to the ideal pulse width. The shortest dead time to avoid passthrough is often the best for minimizing distortion. See the detailed design method in reference 2, which is used to optimize the distortion performance at the switching output stage.

     Other distortion sources include: improper matching of rise and fall time in the process of output pulse; Improper matching of timing characteristics of output transistor gate drive circuit; Nonlinear characteristics in LC low pass filter components.

     Power supply rejection (PSR): in the circuit shown in the previous section, power supply noise is almost directly coupled to speakers with very small rejection. This problem occurs because the output stage transistor connects the power supply to the low-pass filter with a very low resistance. The filter suppresses high-frequency noise, but is designed to allow all audio frequencies, including noise, to pass through. Reference 3 shows a complete description of the effects of power supply noise in single ended and differential switch output stage circuits.

    If neither distortion nor power supply problems can be solved, it is difficult to achieve PSR better than 10dB or total harmonic distortion (THD) better than 0.1%. Even worse, thd may be a high-order interference component of poor sound.

    Fortunately, there are better solutions to these problems. It is helpful to use feedback with high loop gain, as used in many linear amplifier meters. Feedback from the LC filter input will greatly improve the PSR and weaken all LC filter distortion mechanisms. The nonlinear characteristic of LC filter can be weakened by including the loudspeaker in the feedback loop. In a fully designed closed-loop class D amplifier, high fidelity sound quality with PSR > 60dB and thd < 0.01% can be achieved.

    Feedback complicates amplifier design, however, because loop stability must be addressed (there is a perfect consideration for higher-order design). Continuous time analog feedback is also necessary to capture important information about pulse timing errors, so the control loop must include the analog circuit that processes the feedback signal. In the implementation of integrated circuit amplifier, this may increase the bare chip cost.

    To minimize IC costs, some suppliers prefer to minimize or eliminate analog circuit components. Some products use digital open-loop regulators, plus an analog-to-digital converter to sense power changes - and the compensation behavior of the regulator is adjusted by the scheme proposed in reference 3. This improves the PSR but does not solve any distortion problems. Other digital regulators try to pre compensate for possible output stage timing errors or correct the non ideal characteristics of the regulator. This can at least partially solve some distortion mechanisms, but not all distortion mechanisms. Applications that do not have strict requirements for sound quality can be handled by this open-loop class D amplifier, but some forms of feedback are indispensable for obtaining the best audio quality.

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