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Boost switched-mode audio power amps (Part 1)

21 Mar 2013  | Tom Lawson

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Even after taking careful measures to minimise the error sources cited above, negative feedback is required to obtain good performance. Negative feedback is problematic in a Class D amplifier because the output filter necessarily inserts a delay, which tends to destabilise any feedback loop.

The Class D amp is essentially a buck converter. Buck converters need compensation to remain stable for the same reasons. In order for feedback from the output to be applied without inducing a tendency towards oscillation, the feedback must be processed. That processing must be done with care to avoid causing frequency-dependent delays appearing as distortion. Otherwise, the feedback must be taken before the filtration, which means the non-linearity of the filter, and the interaction of filter and load reactance, is outside the loop.

The conflicting aims of faster correction of errors and limiting the tendency towards unwanted oscillation are all too familiar to those who work with this topology. Ultimately, it is this compromise which defines the limits of performance for Class D amplifiers.

Another limitation of the Class D amplifier of Figure 1 is easier to address, i.e., the limited output voltage swing. In a cell phone, the power input is apt to be 4V, and a 4V output swing is often not enough for audio. The solution is the Bridge Tied Load (BTL) topology. In figure 2, a Class D BTL amplifier is shown driving a piezo speaker, which is essentially capacitive. Piezo, or ceramic, speakers are desirable in cell phones because of their thinness and efficiency, but they require higher voltages to drive them properly.

Bridge tied load variation on Class D amplifiers
The differential voltage across the speaker can now be twice the Power Input voltage. In operation, both totem poles need one, and only one, switch to be ON at any time.

The efficiency of the BTL amp can be very good, but it is reduced by the need for a second switching element in series with the power path. The BTL form is more flexible, and it exhibits less bus pumping. In the piezo speaker case, a larger efficiency issue appears. Series resistance needs to be added to prevent oscillations caused by the capacitive loading. That resistance can easily become the largest single source of inefficiency in the system. On balance, the compromises work out to favour the BTL configuration for handheld devices.

Filterless variations
In some cases, the reactance of the speaker itself is relied on to filter out the chopping frequency. That can be satisfactory, but it requires the speaker to dissipate extra power due to the high-frequency ripple current. Not all dynamic speakers are sufficiently inductive at those frequencies to be efficient as a filter. Equivalent Series Resistance, ESR, causes analogous losses in piezo speakers.

In either case, efficiency suffers, the speaker heats, and the sound quality is compromised. Also, if the speaker is at any distance from the amplifier, the wiring is required to carry square waves. The edges are attenuated by stray inductance, degrading the sound, and those edges are a source of electromagnetic interference.

Care must be taken to avoid damage to dynamic speakers in the filterless case. Above the audible range, voice coil movement is proportional to 1 / frequency2. If the modulation frequency is high enough, the movement is small, so the voice coil won't hit the limit of travel and cause damage. As long as the speaker is sized to dissipate the extra energy, the filterless variation can be satisfactory. An additional efficiency implication is concealed here. You cannot simply reduce the modulation frequency to save power, without putting the speaker at risk.

Figure 2: Here, a Class D BTL amplifier drives a piezo speaker.

Filterless amplifiers for piezo speakers do not need to take voice coil damage into account. These amplifier/speaker combinations are the most efficient option now available. One manufacturer documents power consumption approaching half the power of Class D driving a dynamic speaker. That comparison includes a 10 ohm series resistor for stability in the piezo case. The largest single source of inefficiency is that resistor, so there is still room here for improvement.

The awkward overall conclusion is that the efficiency advantages of Class D amplifiers stem from the digital nature of their output, while the performance disadvantages of Class D amplifiers stem from that same digital nature. What is really called for is a digital amplifier with an analogue output. There is now an option in that category, enabled by improved FET switches and a control strategy called Predictive Energy Balancing.

About the author
Tom Lawson began working in instrumentation in 1968. He formed Lawson Labs in 1981, and CogniPower in 2009. His patents span 5 decades.

To download the PDF version of this article, click here.

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