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Class D Audio Amplifiers: What, Why, and How - Part 2
Class D amplifiers, first proposed in 1958, have become increasingly popular in recent years. What are Class D amplifiers? How do they compare with other kinds of amplifiers? Why is Class D of interest for audio? What is needed to make a "good" audio Class D amplifier? Part 2 addresses different Class D alternatives.
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By
Eric Gaalaas, Analog Devices Inc
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Page 1 of 2
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Audio DesignLine
(01/23/2007 11:23 AM EST)
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Editor's note: This multi-part article is an expanded version of the article by the same title published in the EE Times print edition.
Thanks to a different topology (Figure 2), the Class D amplifier dissipates much less power than any of the above. Its output stage switches between the positive and negative power supplies so as to produce a train of voltage pulses. This waveform is benign for power dissipation, because the output transistors have zero current when not switching, and have low VDS when they are conducting current, thus giving smaller IDS - VDS.
Figure 2: Class D open-loop-amplifier block diagram.
Since most audio signals are not pulse trains, a modulator must be included to convert the audio input into pulses. The frequency content of the pulses includes both the desired audio signal and significant high-frequency energy related to the modulation process. A low-pass filter is often inserted between the output stage and the speaker to minimize electromagnetic interference (EMI) and avoid driving the speaker with too much high frequency energy. The filter (Figure 3) needs to be lossless (or nearly so) in order to retain the power-dissipation advantage of the switching output stage. The filter normally uses capacitors and inductors, with the only intentionally dissipative element being the speaker.
Figure 3. Differential switching output stage and LC low-pass filter.
Figure 4 compares ideal output-stage power dissipation (PDISS) for Class A and Class B amplifiers with measured dissipation for Analog Devices' AD1994 Class D amplifier, plotted against power delivered to the speaker (PLOAD), given an audio-frequency sine wave signal. The power numbers are normalized to the power level, PLOAD max, at which the sine is clipped enough to cause 10% total harmonic distortion (THD). The vertical line indicates the PLOAD at which clipping begins.
Figure 4. Power dissipation in Class A, Class B, and Class D output stages.
Significant differences in power dissipation are visible for a wide range of loads, especially at high and moderate values. At the onset of clipping, dissipation in the Class D output stage is about 2.5 times less than Class B, and 27 times less than Class A. Note that more power is consumed in the Class A output stage than is delivered to the speaker—a consequence of using the large dc bias current.
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