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Mixed-Signal and Analog Products Amplifier and Comparator Products TI's Class-D Audio Power Amplifiers Take the Spotlight Architectural and Process Advances Make Class-D Amplifiers the Solution for Audio Systems in Portable Applications Profile of TI's Audio Power Amplifier Products
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Architectural and Process Advances Make Class-D Amplifiers the Solution for Audio Systems in Portable ApplicationsThanks to architectural and process advances in Class-D audio power amplifiers, the day is fast approaching when the sound systems of handheld devices will rival those of today's over-sized, cumbersome boom boxes. Until recently, popular audio power amplifiers (APAs) were grouped into three classes: Class-A, -B and -AB amplifiers. Class-D amplifiers, which are based on a completely different architecture, have been used for several years in public address systems, which were designed to mainly serve the voice portion of the audio spectrum. By applying its expertise in linear audio amplifiers, as well as its leadership position in switching power supply technology, Texas Instruments (TI) has broken new ground with highly integrated Class-D amplifiers that are three times more power-efficient and offer sound quality equal to or better than previous generations of linear amplifiers. This new family of devices will not only alter the face of next generation low-powered portable systems, but it will also give designers the ability to design quality sound systems in new types of applications like handheld computers and personal digital assistants (PDAs), wireless telephones and others.
Moving to the Head of the Class The first type of APA, Class-A, was originally based on one output element, a vacuum tube, which was soon replaced by a transistor. Class-A amplifiers add little distortion to the sound they amplify, but, since they are biased at the center of the active region, they consume a great deal of power. In many applications, this would require systems with very large power supplies. As a result, the effective use of Class-A amplifiers in portable applications was severely limited. Class-B addressed the problem of power consumption. This type of APA features two elements or transistors in the output stage, both of which are shut off when no signal is present. Unfortunately, this arrangement introduces significant distortion into the signal as it moves through the zero crossover point. Class-AB amplifiers removed the distortion by keeping each of the two transistors slightly on at all times, but this re-introduces the problem of high power consumption. Not quite as much as Class-A, but as much as 85 percent of the input power was still wasted in a Class-AB amplifier. This wasted power is dissipated as heat. In contrast to Class-A, -B, and -AB amplifiers’ use of linearly-biased output stages, TI’s Class-D APA devices have switching output stages like those used in switching power supplies, and variable speed motor drives. Class-D amplifiers process analog signals using pulse-width modulation (PWM) techniques, which is the key behind Class-D amplifiers’ increased efficiency. The PWM signals are applied to power DMOS H-bridges, which provide high output current capability. High-frequency square waves of constant amplitude, but varying width, are output from the IC. These pulses of varying widths contain the audio information. The output signal must be low-pass filtered to isolate the audio information from the high-frequency signal. Proper filtering assures the quality of the sound produced by the system. The efficiency of higher performance Class-A and -AB amplifiers is constrained by the bias current needed in the output stage. Bias current and audio performance are directly proportionate in Class-A and -AB amplifiers. This wasted current is dissipated as heat in the amplifier and must be extracted from the IC with the use of a heat sink. Since Class-D amplifiers do not have a linearly biased output stage, their efficiencies are much greater, making them ideal for use in battery operated systems. The use of a Class-D amplifier can extend the life of a battery by three times over the older Class-AB linear amplifiers. Extensive Expertise Over the last several years, TI has developed an expertise in linear audio power amplifiers. Following the company's introduction of the TPA486x family, came the TPA0102, which has garnered significant market share in notebook computers. Recent introductions include the TPA0202, TPA301 and TPA311 and TPA152. The TPA0202 is a 2-Watt, low-power stereo speaker amplifier that includes a unique "depop" filtering technique, which removes the popping distortion found in many analog amplifiers when the system is turned on. Moreover, TI has an established leadership position in switching power supply technology. This is a critical competency needed in the development of Class-D amplifiers, which some in the industry refer to as switching audio amplifiers. The first device in TI's family of Class-D APAs is the TPA005D02, a fully integrated 2-Watt stereo amplifier. Additional developments will lead to completely digital APAs that will accept digital information as input from devices such as compact disk (CD) or digital video disk (DVD) players. Further integration will lead to a digital APA with a digital-to-analog converter on-chip. This level of integration will reduce system cost, size and weight factors, and provide solutions for next generation portable, battery operated electronic products with quality sound systems. The Digital Future of Sound Because of their compact size, low heat dissipation and paltry power consumption, future generations of Class-D APAs will play an important role in digital sound systems. One such application is already taking shape. It uses a personal computer (PC) as the centerpiece of a home entertainment system. Class-D APAs are critical to the development of digital speakers capable of Dolby® Digital or Dolby Surround® playback. Because of the complexity of configuring advanced speaker systems like Dolby Surround, much of the sound processing tasks would be moved from the PC's sound card to smart digital speakers. The digital speakers could be powered from one of the new serial peripheral buses, like USB or the high-speed 1394 bus. Each speaker would include a digital signal processor (DSP), digital-to-analog converter capabilities and a Class-D audio power amplifier because of its low power consumption and small size. Best in Class Linear Class-A, -B and especially Class-AB audio power amplifiers will continue to serve the industry in many applications, but as more aspects of electronic systems become digital, the many advantages of Class-D APAs will emerge as critical for next generation sound systems. TI's long held expertise in audio amplifiers, digital signal processing and power technology assures the company's Class-D audio power amplifiers will be in a class by themselves.
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