![]() Its output stage switches between the positive and negative power supplies so as to produce a train of voltage pulses. Thanks to a different topology (Figure 2), the Class D amplifier dissipates much less power than any of the above. The large drain-source voltage drops thus produce significant I DS× V DS instantaneous power dissipation. Unfortunately, even a well-designed class AB amplifier has significant power dissipation, because its midrange output voltages are generally far from either the positive or negative supply rails. Some control, similar to that of the Class B circuit, is needed to allow the Class AB circuit to supply or sink large output currents. Power dissipation, although between Class A and Class B limits, is typically closer to Class B. The small dc bias current is sufficient to prevent crossover distortion, enabling good sound quality. The Class B circuit has inferior sound quality, however, due to nonlinear behavior ( crossover distortion) when the output current passes through zero and the transistors are changing between the on and off conditions.Ĭlass AB, a hybrid compromise of Classes A and B, uses some dc bias current, but much less than a pure Class A design. This reduces output stage power dissipation, with only signal current conducted through the transistors. Its output transistors are individually controlled in a push-pull manner, allowing the MH device to supply positive currents to the speaker, and ML to sink negative currents. The Class B topology eliminates the dc bias current and dissipates significantly less power. Good sound quality is possible with the Class A output stage, but power dissipation is excessive because a large dc bias current usually flows in the output-stage transistors (where we do not want it), without being delivered to the speaker (where we do want it). The Class A topology uses one of the transistors as a dc current source, capable of supplying the maximum audio current required by the speaker. The amount of power dissipation strongly depends on the method used to bias the output transistors. Power is dissipated in all linear output stages, because the process of generating V OUTunavoidably causes nonzero I DSand V DSin at least one output transistor. The output stage could also be implemented with MOS transistors, as shown in Figure 1. If bipolar junction transistors (BJTs) are used in the output stage, they generally operate in the linear mode, with large collector-emitter voltages. ![]() Linear-amplifier output stages are directly connected to the speaker (in some cases via capacitors). Linear Amplifiers, Class D Amplifiers, and Power Dissipation This difference gives Class D significant advantages in many applications because the lower power dissipation produces less heat, saves circuit board space and cost, and extends battery life in portable systems. Compared with Class D designs, the output-stage power dissipation is large in even the most efficient linear output stages. ![]() The many possible implementations for audio systems include Classes A, AB, and B. In a conventional transistor amplifier, the output stage contains transistors that supply the instantaneous continuous output current. 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 (PSR). If the forward gain is part of a feedback loop, the overall loop gain will also be high. The forward voltage gain is usually high (at least 40 dB). Power capabilities vary widely depending on the application, from milliwatts in headphones, to a few watts in TV or PC audio, to tens of watts for “mini” home stereos and automotive audio, to hundreds of watts and beyond for more powerful home and commercial sound systems-and to fill theaters or auditoriums with sound.Ī 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. Audio frequencies range from about 20 Hz to 20 kHz, so the amplifier must have good frequency response over this range (less when driving a band-limited speaker, such as a woofer or a tweeter). 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. 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? What are the features of ADI’s Class D amplifier products? Find the answers to all these questions in the following pages. Class D Audio Amplifiers: What, Why, and HowĬlass D amplifiers, first proposed in 1958, have become increasingly popular in recent years. ![]()
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