The efficiency and size advantages of Class D audio amplification in battery- powered devices are well known. These advantages are now extended to amplifiers up to 500W, made possible by solidstate driver ICs designed specifically for Class D. Systems based on these new ICs outperform Class AB in THD+N measurements, and simplify the designer’s job by accepting ground-based analog audio inputs.

Features such as overcurrent protection for both rails and programmable dead time make these drivers additionally attractive. In this article, we examine the performance, size, and cost benefits of Class D versus Class AB topologies for medium power levels.

THE HISTORY

Audio amplification requires that a speaker (also called a driver) is driven back and forth in opposite directions, moving air to produce a sound wave that’s decipherable with the human ear. To accomplish this, a voltage of alternating polarity is impressed upon the speaker by means of either a halfbridge or full-bridge topology, as shown in Figure 1 for Class D topologies. The half-bridge amplifier requires a split-rail power supply, having positive and negative voltages of equal magnitude, and two power switches between them. When the load is tied between the common switch point and system ground, it’s referred to as a single-ended load (SEL).

The full-bridge amplifier, referred to as a bridge-tied load (BTL), is made up of two half bridges with the load tied between their center points. The switches are turned ON and OFF in such a way that the speaker moves to recreate the audio output, which must average to zero. BTL configurations produce higher power for a given switch rating, and a single power supply and output capacitor allows it to be ground-referenced, simplifying input controls at the expense of two more power switches and gate drivers. An SEL or BTL topology is used for either Class AB or Class D.

Class A was the earliest audio amplifier design, whereby both switches were ON simultaneously, although not fully, to produce the required voltage at the load (Fig. 2). This produced excellent audio performance, but very poor efficiencies of about 15%, resulting in large and expensive systems.

Class B followed, where only one switch at a time was turned ON. While efficiency improved to approximately 75%, it was hampered by significant problems at the zero crossing of the output waveform; instead of crossing smoothly through zero, Class B had a flat section, or zero voltage, between the positive and negative halves of the waveform, producing high distortion. Class AB compromised the two by turning on both switches simultaneously. Yet, the switch not carrying load current was only minimally ON so that the nonlinearity due to the loss of gain at the zero crossing was greatly reduced. This improved zero-crossing distortion to acceptable levels and boosted efficiency over Class A, but still an overall Class AB efficiency of 30% was typical.

These three topologies vary the bridge output voltage with the audio frequency, and are, therefore, relatively low-frequency designs. Class AB dominates the field of linear amplifiers, and bipolar transistors are typically used as the control devices.

CLASS D AMPLIFICATION

Today’s switching power supplies are far smaller and lighter than the linear, line-frequency supplies of the past due to the advent of high-frequency power conversion, made possible by improvements in power silicon, control ICs, magnetics, and capacitors. Likewise, thanks to the continuous improvements of key electrical components, Class D amplifiers decrease the size, weight, and system cost of audio amplifiers by switching at 200 to 800kHz instead of being linearly driven by 20 to 200kHz audio frequency signals. MOSFETs are commonly used as the switches due to their fast switching speeds.

Each power switch of opposite polarity is fully turned ON or OFF one at a time with dead time between the ON states, and the I² X R_DS-ON conduction and V_SI_St_Sf_S switching losses are far less than the (V_RAIL – V_OUT) X I loss of the linear Class AB. Even though switching losses increase with frequency, Class D efficiencies of 90 to 96% for medium power are now achievable.

A Class D amplifier half-bridge output produces a rail-to-rail switched digital power signal (see the waveform in Fig. 2); switching losses occur in the green areas and conduction losses in the blue areas. The analog output is reconstructed at the load by an output filter’s LC stages.

The duty-cycle D of the powered signal determines the filtered output voltage, as shown in the Fig. 1 half bridge. As D approaches unity, the output voltage approaches the positive rail or positive peak of the waveform; when D is 50%, the output voltage is zero; and when D approaches zero, the output voltage approaches the negative rail, or negative peak of the waveform. At switching frequencies of 400kHz and above, a single stage output filter can be used, comprised of one inductor and one capacitor.

Note that for the case shown, feedback is from the switch node only. To achieve the THD curves of Fig. 3, a Class D motherboard containing the output filter and a two-channel, power-stage daughtercard was plugged into a commercial Class AB stereo receiver. Having an identical power supply and input controls (Fig. 4), the power specs and noise floor are identical, permitting fair measured performance comparisons. The Class D metal mounting plate covers the large heat sink of the original Class AB amplifier.

Continue to next page