7 Modulation

Modulation is at the heart of Class-D amplifiers as opposed to Class-A, AB, and B amplifiers, which are linear and do not use modulation. Instead of amplifying the audio signal directly, a Class-D amplifier first uses the continuous analog audio input and creates a PWM signal. The PWM signal is then amplified by the output stage and an external LC filter is used to remove the high frequency modulation and leave behind the amplified audio signal. Class-D amplifiers are generally far more efficient than their counterparts, but amongst them there are several modulation schemes to consider. A modulation scheme refers to the method at which the continuous analog audio signal is converted into a PWM signal. A few modulation schemes include AD, BD, 1SPW, HEAD, and hybrid. The different schemes offer different benefits, such as higher performance or greater efficiency. While AD and BD use a 50% common duty cycle between the bridge-tied outputs, 1SPW uses a lower common duty cycle closer to 15-20%. This reduces the DC offset from half the supply voltage to about one-fifth of the supply voltage. Lower DC offset reduces the ripple current though the LC filter and the amplifier's supply (PVDD) current, which mitigates “pop” noise at startup, reduces EMI, and most importantly minimizespower losses especially at idle. The downside to 1SPW is that at high output, one output channel is effectively acting as a ground, so the BTL system resorts to single sided switching, making it almost like a single-ended (SE) system. SE audio performance is inferior to BTL, so there is a trade-off for the added efficiency. In the hybrid and HEAD modulation schemes, some processing is done to dynamically change the common duty cycle based on the audio output. In this way, a low DC offset is maintained for the idle state, but increased for higher output levels to avoid single-sided switching. These proprietary TI modulation schemes offer the best of both worlds, improving efficiency without sacrificing audio quality. Refer to section 2 of the LC Filter Design application note to understand more about AD and BD modulation. Refer to section 5 of the TAS5805 Hybrid Mode application note or the HEAD Modulation application brief to learn how Hybrid or HEAD modulation schemes can improve efficiency, and even EMI or click/pop-noises.

Regarding the modulation of a Class-D audio amplifier, there are other things to consider aside from just the scheme. The same modulation scheme can be implemented at different modulation or switching frequencies. As previously mentioned, the switching frequency is usually between 200 kHz to 1.5 MHz, or even as high as 2.1 MHz as seen in some of the industry leading automotive Class-D amplifiers such as the TAS6424-Q1. The modulation frequency impacts distortion, the EMI of a device, the demand on the LC filter and more. The modulation of a Class-D amplifier leaves artifacts in the audio signal at harmonics of the modulation frequency. Having a higher frequency eliminates some of the potential distortion from harmonics. The high frequency waves also propagate EMF, resulting in interference depending on the application. For example, some Class-D amplifiers pose a problem for radios in Europe, where digital audio broadcasting (DAB) is used broadly. The speaker amplifier in an application with DAB radios must pass more stringent EMI tests than otherwise, and choosing a modulation scheme and frequency accordingly is important. A similar problem exists with the AM frequency band, which is why some devices have an AM avoidance feature to manipulate the switching frequency in order to dodge the AM bands. However, using 2.1 MHz switching completely avoids the AM frequency band (540–1,600 kHz), which is especially useful in automotive head units, where traditionally a heavy-duty metal casing would be necessary to prevent emissions from the amplifier causing interference with the radio signal. Some Class-D devices offer a range of modulation frequencies to implement the device with, so it can be selected on a system-by-system basis.

Regardless of the modulation scheme, after the signal is amplified it needs to be converted from PWM back to a continuous audio signal to play through the speakers. To do this, an LC filter is used at the output of a Class-D amplifier. Depending on the modulation scheme, frequency, and power level, sometimes full LC filters are unnecessary. For output power levels below 10 W, some of TI’s new amplifiers have advanced features such as spread spectrum and de-phase control, which reduces the demand for an LC filter to the point where tiny ferrite beads can be used to filter PWM signal. Such a capability is incredibly valuable for saving cost on the overall BOM since ferrite beads are cheaper than inductors.