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Design 101: Class D amplifiers

Posted: 01 Apr 2009 ?? ?Print Version ?Bookmark and Share

Keywords:Class D amplifier? linear amplifier? audio application? design Class D?

By Eric Haber
Wolfson Microelectronics

Class D audio amplification is fast becoming the solution of choice in consumer audio because of its ability to extend long battery life and compact dimensions. However, designers must take care to avoid complexity elsewhere in the system, including the power supply, output filtering and input signal conditioning.

The switching amplifier, or Class D amplifier, has risen quickly to prominence in consumer audio applications, from MP3 devices including mobile phone handsets to games consoles, LCD TVs and home cinema. The ace in the pack for Class D is its vastly superior efficiency, which can be as high as 85 to 90 percent in practice. A linear Class AB implementation will normally achieve around 25 percent at typical listening levels.

In handheld applications the low power dissipation of Class D allows designers to combine high audio performance with a long battery recharge interval. Battery life is a key figure of merit for all personal communication and audio devices. For mains powered equipment, such as audio-visual (AV) products and games consoles, the high power efficiency of Class D brings the advantage of reduced heat dissipation.

Hence, designers can specify smaller heat sinks, to achieve lower profile styles as well as lower BOM and assembly costs. In fact, careful design of the power supply can allow heat sink-less operation up to several watts per channel of output power.

Single-chip solution
The basic topology of a Class D amplifier comprises a pulse width modulator, a power bridge output circuit and a low pass filter. Class D amplifier ICs currently available take away much of the design effort, such as managing the EMI produced by the amplifier's switching operation, and selecting the optimal switching frequency. Increasing the switching frequency reduces output filtering requirements, but results in greater losses due to MOSFET gate capacitance. Hence, switching frequency selection requires a balance between external components and power efficiency.

The design of the power bridge depends upon the desired output power of the amplifier. For instance, Class D ICs are available with headphone drivers or with drivers for loudspeakers, with the design of the output stage being one of the key differences between these various configurations.

Amplifiers designed for use with loudspeakers are capable of producing from less than 1W up to several watts of output power, without requiring a heat sink. These ICs enable a single-chip solution in many consumer applications, from portable media players to games consoles and some LCD TVs. In the majority of these applications, particularly handheld products, a single-chip solution is essential.

However, for very high output power, a Class D amplifier IC can be combined with an external output stage built using audio power MOSFETs. The IC must provide a suitable pre-driver, and the chosen discrete MOSFETs must be optimized for digital audio operation.

The output from the Class D MOSFET H-bridge is a square wave representation of the audio signal. The switching frequency components must be attenuated, to prevent interference and to ensure the end product will pass EMC certification. Low pass filtering, with a cut-off frequency just above the audible band, is required. Hence, attenuation of these components is greater with higher switching frequencies. This allows smaller external filtering components.

On the other hand, MOSFET losses tend to increase with switching frequency, thereby driving down efficiency and leading to increased power dissipation and associated thermal management issues. In particular, switching losses are incurred in the MOSFET gate capacitance, increasing linearly with the operating frequency. Thus, the design of a Class D amplifier IC output stage is predicated on fabricating low loss MOSFETs and setting the switching frequency low enough to correctly meet the specified target for EMI.

Filterless connection to a loudspeaker, such as a cellular phone speaker, is a distinct advantage in size-and-cost sensitive applications. When the Class D output is physically close to the speaker, the parasitic resistance and inductance of the speaker coil may be used as a suitable R-L low pass filter, enabling one inductor and one capacitor to be eliminated from each output connection. An example of a Class D amplifier chip that may be used in a filterless configuration is the Wolfson WM8960. If the distance from the amplifier output to the speaker is longer, a small amount of additional inductance, in the form of a ferrite bead, will be required to improve EMC performance. Figure 1 compares headphone driver outputs with and without a ferrite bead.

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