The Class D amplifier has some added challenges. Real world transistor switching characteristics require that some degree of dead-time is present in each modulation cycle to control the cross-conduction (or shoot-through) current between the high and low-side FETs. The dead time is a source of error at the output as the pulse widths are necessarily modified. This aside, the output switching waveform varies somewhat from the idealized square-edged pulse stream. Also the requirement for an LC low-pass (or reconstruction filter) adds an additional source of distortion in series with the load. The frequency response of the system is a function of the LC values and the load impedance itself.
Figure 2 shows where in the amplifier these errors occur, and figure 3 shows the non-ideal switching waveform.
Figure 2: Error sources in a digital amplifier
Figure 3: Comparison of ideal and actual PWM switching edge
Applying the feedback loop in Class D
The methods applied to linear analog amplifiers can also be used with analog Class D amplifiers and a well engineered solution enables compensation for a series of potential error causes: power supply ripple, dead-time induced distortion.
Closing the loop for a digital amplifier is a different proposition. The most commonly thought of method is to convert the analog output of the amplifier back to the digital domain and compare with the original input. This approach is likely to fall foul of the relatively large group delay present in digital amplifiers, which would cause the system to be unstable.
Achieving feedback in a digital amplifier
Pulse width correction. The high voltage pulses produced by the output stage are normally very different to the finely tuned logic level input pulses. Dead time is one cause, as is ringing, RDS(on) modulation and mismatched components. A pulse width correction scheme seeks to compensate for this effect by pre-modifying the logic level pulses so the outputs become closer to the ideal. If successfully done, the effects are seen as reduced distortion. The effect of power supply noise has only a limited influence on edge positions, and hence the benefits in terms of Power Supply Rejection Ratio (PSRR) are only small.
Power supply feedback. An open-loop amplifier has no ability to compensate for errors on the power supplies. Schemes exist where the power supplies are monitored, through an analog-to-digital conversion, such that the digital gain is modified to compensate for any deviation from the nominal voltage. This approach is sometimes known as a feed-forward system and can achieve some power supply noise rejection as well as some reduction in inter-modulation distortion.