Another advantage of sigma-delta is that the minimum pulse width is one sampling-clock period, even for signal conditions approaching full modulation. This eases gate-driver design and allows safe operation to theoretical full power. Nonetheless 1-bit sigma-delta modulation is not often used in class D amplifiers (Reference 4) because conventional 1-bit modulators are only stable to 50% modulation. Also, at least 64- oversampling is needed to achieve sufficient audio-band SNR, so typical output data rates are at least 1 MHz and power efficiency is limited.
Recently, self-oscillating amplifiers have been developed, such as the one in Reference 5. This type of amplifier always includes a feedback loop, with properties of the loop determining the switching frequency of the modulator, instead of an externally provided clock. High-frequency energy is often more evenly distributed than in PWM. Excellent audio quality is possible, thanks to the feedback, but the loop is self-oscillating, so it's difficult to synchronize with any other switching circuits, or to connect to digital audio sources without first converting the digital to analog.
Figure 3
The full-bridge circuit (Figure 3) can use "3 state" modulation to reduce differential EMI. With conventional differential operation, the output polarity of Half-bridge A must be opposite to that of Half-bridge B. Only two differential operating states exist: Output A high with Output B low; and A low with B high. Two additional common-mode states exist, however, in which both half-bridge outputs are the same polarity (both high or both low). One of these common-mode states can be used in conjunction with the differential states to produce 3-state modulation where the differential input to the LC filter can be positive, 0, or negative. The 0 state can be used to represent low power levels, instead of switching between the positive and negative state as in a 2-state scheme. Very little differential activity occurs in the LC filter during the 0 state, reducing differential EMI, although actually increasing common-mode EMI. The differential benefit only applies at low power levels, because the positive and negative states must still be used to deliver significant power to the speaker. The varying common-mode voltage level in 3-state modulation schemes presents a design challenge for closed-loop amplifiers.
Coming in the next installment: Taming EMI
Other available parts:
Part 1 Audio Amplifier Background
part 2 Class D Alternatives
Part 3 Class D AmplifierTerminology. Differential vs. Single-Ended Versions
Part 4 Sound Quality
