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Sound advice on Class D audio amps designs

Posted: 01 Jun 2007 ?? ?Print Version ?Bookmark and Share

Keywords:Class D amp? audio amps? amplifier designs?

The goal of audio amplifiers is to reproduce input audio signals at sound-producing output elements!with desired volume and power levels!faithfully, efficiently and at low distortion. Class D amplifiers offer many advantages in getting that job done.

A straightforward analog implementation of an audio amplifier uses transistors in linear mode to create an output voltage that is a scaled copy of the input voltage. The forward voltage gain is usually high (at least 40dB). If the forward gain is part of a feedback loop, the overall loop gain will also be high. Feedback is often used, because high loop gain improves performance!suppressing distortion caused by nonlinearities in the forward path and reducing power supply noise by increasing the power supply rejection.

Power is dissipated in all linear output stages, because the process of generating VOUT unavoidably causes non-zero IDS and VDS in at least one output transistor. The amount of power dissipation strongly depends on the method used to bias the output transistors.

The Class D amplifier dissipates much less power than Classes A, B and AB. 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.

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 EMI and avoid driving the speaker with too much high-frequency energy. The filter needs to be lossless (or nearly so) 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.

Minimizing system cost
The active components of the Class D amplifier are the switching output stage and modulator. This circuitry can be built for roughly the same cost as an analog linear amplifier. The real trade-offs occur when considering other components of the system.

The lower dissipation of Class D saves the cost and space of a cooling apparatus, like heat sinks or fans. A Class D IC amplifier may be able to use a smaller and cheaper package than is possible for the linear one. When driven from a digital audio source, analog linear amplifiers require DACs to convert the audio into analog form. This is also true for analog-input Class D amplifiers, but digital-input types effectively integrate the DAC function.

Filterless Class D amplifier block diagrams can be used for mobile applications both as open-loop and closed-loop versions.

On the other hand, the principal cost disadvantage of Class D is the LC filter. The components!especially the inductors!occupy board space and add expense. In high-power amplifiers, the overall system cost is still competitive, because LC filter cost is offset by large savings in cooling apparatus. But in cost-sensitive, low-power applications, the inductor expense becomes onerous. In extreme cases like cheap amplifiers for cellphones, an amplifier IC can be less expensive than the total LC filter cost. Also, even if the monetary cost is ignored, the board space occupied by the LC filter can be an issue in small-form-factor applications.

To address those concerns, the LC filter is sometimes eliminated entirely to create a filterless amplifier. This saves cost and space, although the benefit of low-pass filtering is lost. Without the filter, EMI and high-frequency power dissipation can increase unacceptably. Unless the speaker is inductive and kept very close to the amplifier, current-loop areas are minimal and power levels are kept low. Although often possible in portable applications like cellphones, this technique is not feasible for higher-power systems such as home stereos.

Another approach is to minimize the number of LC filter components required per audio channel. This can be accomplished by using single-ended half-bridge output stages, which require half the number of L's and C's needed for differential, full-bridge circuits. But if the half-bridge requires bipolar power supplies, the expense associated with generating the negative supply may be prohibitive, unless a negative supply is already present for some other purpose or the amplifier has enough audio channels to amortize the cost of the negative supply. Alternatively, the half-bridge could be powered from a single supply, but this reduces output power and often requires a large DC blocking capacitor.

The output stage of a Class D amplifier switches between positive and negative power supplies, producing a train of voltage pulses instead of the linear output produced by the Class A, B or AB amplifiers trADItionally used in audio applications. The output drives a speaker through a passive LC low-pass filter. The output transistors have zero current when turned off and a small voltage drop when on, causing the power dissipation of Class D amplifiers to be much lower than that of the alternatives. Thus, Class D amplifiers produce less heat, occupy less circuit board area and provide longer battery life in portable systems, making them ideal for audio applications.

Audio designers need to pay careful attention to many details when they are building a high-performance Class D audio amplifier, including the choice of output transistor size, output stage protection, modulation technique and filter topology. EMI reduction and system cost must also be considered along with the desired sound quality.

Fortunately, commercially available ICs implement complete Class D amplifiers that incorporate programmable-gain amplifiers, modulators and output stage to simplify the audio designer's job and speed up time-to-market. Evaluation boards, PCB layout and a reasonable BOM make it possible to quickly design cost-effective audio systems without having to re-invent the wheel to solve the major Class D design challenges.

- Eric Gaalaas
Senior Design Engineer
Analog Devices Inc.

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