Focus on the class of audio amps

The sweet sound you hear in your home theater, your portable CD headset, and your multi-speaker car audio may be sheer magic to your ears but it required diverse design techniques that presented several design challenges before it could go live.

Audio amp manufacturers are faced with different challenges depending on the end market. “Consumer Audio makers have experienced a rapid shift from stereo to 5.1 channel audio requirements with 7.1 channels on the horizon,” said Gary Adrig, Director of Marketing for the Audio Division at National Semiconductor. “This demand for more channels came with an initial surge in demand for more power-per-channel. The higher power systems are now complemented by the release of more systems with lower price points and more modest power levels ranging from 15-30 Watts per Channel. Besides DVD receivers, many mini and micro component systems now have 5.1 channels,” said Adrig.

Class Acts
Let's take a moment to review the amplifier Classes, their desirable qualities as well as shortcomings.
Class A
Pro : Used for high-end, high-power, low efficiency, premium quality audio.
Con : Extremely high-power dissipation; the maximum power dissipation is at idle.

Class AB Output operates in Class AB mode where output stage is push/pull

Pro It's simple to use and delivers high quality audio. It is the most popular amplifier from the mass market to high-end products.
Con There is a significant challenge dealing with heat in high-power and multi-channel systems. Various configurations and heat sinking schemes must be used.

Class D It is a switching amplifier that converts audio to high voltage square waves for high efficiency, then reconstructs the audio with a low pass filter on the output.
Pro It is an emerging design topology and starting to provide high efficiency with average quality audio.
Con It generates EMI caused by the high voltage switching waveforms, and needs output LC filter components.

Class H this is used for high-power amplifiers and some applications with limited voltage, such as automotive.
Pro The power supply rails track the input and keep the voltage only as high as needed for proper operation.
Con Limited use and appeal

Class G Switches between two discrete power supply voltages levels.
Pro the normal operation is at low voltage but it switches to high-power supply as needed by the signal levels. The market use is similar to Class H.
Con Limited use and appeal

It's almost a given that most home stereo systems use Class AB amplifiers, however, there is some migration to Class D. Class G/H amps offer chameleon-like characteristics where the power supply changes to accommodate the power needed at any given time. Classes G/H attempt to save power like Class D but still deliver Class AB quality audio.

“The use of Class AB amps means that for higher power and more channels, manufacturers must keep power levels per channel low for smaller audio-video receiver boxes while for higher power larger boxes and heat sinks are required,” observed National Semi's Mr. Adrig. Just as you would expect, the amp makers that use Class D amplifiers can fit more power into a smaller form factor. However, even though Class D amplifiers help reduce heat issues, many manufacturers still insist on the richer sound quality of traditional Class AB amps.

Portable Audio
What about the audio in portable products like cell-phones and portable DVDs? The manufacturers of these products are faced with their own set of issues when it comes to audio. For example, cell phone makers are continually challenged to make the hand sets smaller, lighter and thinner while increasing feature sets and offering higher power audio. “The challenge for audio design companies is to stay ahead of the fast moving audio trends as cellular handsets continue to offer more functionality that now include 3-D stereo ring sounds, 256kB color screens, 4 mega pixel cameras, FM radio, streaming video with audio record, Internet access, gaming, walkie-talkie, TV and movies,” says Adrig.

National Semi says it addresses these needs with its Boomer audio amplifiers and integrated audio subsystems that are optimized for portable audio applications. It targets a range of portable applications and includes a very large selection of portable audio. These products include low power mono and stereo headphone amplifiers, mono and stereo speaker amplifiers, combinations of headphone and speaker amps, Class D amplifiers (LM4667) as well as sophisticated easy-to-use audio subsystems in space saving micro SMD packages.

National's LM4667 is a fully integrated single-supply, high efficiency switching audio amplifier. It features an innovative modulator that eliminates the output filter used with typical switching amplifiers. The LM4667 processes analog inputs with a delta-sigma modulation technique that lowers output noise and total harmonic distortion plus noise (THD+N) when compared to conventional pulse width modulators. The LM4667 is designed to meet the demands of mobile phones and other portable communications devices. Operating on a single 3V supply, it is capable of driving 8Ohm transducer loads at a continuous average output of 450mW with less than 1% THD+N. Its flexible power supply requirements allow operation from 2.7V to 5.5V.

To enable designs for audio on wireless and personal digital assistant products TI introduced one of the smallest and highest performance filter-free Class-D mono audio power amplifiers in the market. This 2.5-W device is available in TI's state-of-the-art Wafer Chip Scale Package (WCSP), which measures only 1.45mm by 1.45 mm and comes in lead and lead-free ball options. The TPA2010D1 is ideal for end equipments where high efficiency and board space are critical, such as cell phones, smart phones and PDAs.

Maxim offers the MAX9700B, a Class-D audio amplifier targeted at mobile phone and PDA designs. This amplifier operates over the 2.5- to 5.5-V supply range, delivers a 12-dB fixed gain, and produces up to 1.2-W power per channel into an 8-ohm load. It also sports a 72-dB PSRR at 217 Hz, thus allowing the device to operate directly from a single lithium-ion cell. The amplifier has four clocking options. In addition, multiples of this amplifier can be used together in a master/slave configuration. The amplifier is provided in either a 10-pin DFN or 10-pin uMAX package. Pricing: $0.45 in 100,000-unit quantities

STMicroelectronics introduced a pair of compact stereo headphone amplifiers suitable for use in portable audio players, high-end TVs, portable digital assistants and computer motherboards. The TS486 and TS487 are 100mW stereo headphone amplifiers with standby mode. The standby mode can be active low in the TS486 or active high in the TS487. The parts are offered in the space-saving DFN, SO-8 and MiniSO-8 packages. The drivers can run off 2V as a minimum voltage and consume a maximum of 2.5mA, falling to a typical 10nA in standby mode. The parts can be ordered with an external gain configuration or with fixed gain of 0, 8, 12 dB (reducing the need for external components). The parts also include On/Off click reduction circuitry.
With a Vcc of 5V the output power of each amplifier is 102mW into 16ohm, measured at a total harmonic distortion plus noise figure of a maximum of 0.1%. Power output is 64mW into a 32ohm load at the same supply voltage.

With a Vcc of 3.3V the output power of each amplifier is 38mW into 16ohm, measured at a total harmonic distortion plus noise figure of a maximum of 0.1%. Power output is 26mW into a 32ohm load at the same supply voltage. Signal to noise ratio is typically 103 dB with a 5V supply, and cross-talk immunity is typically 83dB at 1 kHz. The power supply rejection ratio for the parts is typically 58dB, measured at 1 kHz with the inputs grounded. The parts are stable at unity gain and feature short-circuit protection.

Challenges for audio amps manufacturers
One of the challenges facing designers includes higher levels of integration. That means companies must constantly develop and evolve their audio products by adding more features and functions. “The challenge of audio amps includes making the designs available in ever smaller packaging such as ultra chip scale package (UCSP) and quad flat no-lead (QFN) packages to meet the needs for space savings and improved thermal performance,” said Andrew Rhind, Business Manager, Audio Products at Maxim. “As you might expect, lower cost and simpler application circuits are also high on the priority list of manufactures that use audio amplifiers. Finally, these cutting-edge manufacturers want these new designs to enable them to get to market more quickly by eliminating many of the external components traditionally required in audio amplifier designs.”

Maxim offers the MAX9700B, a Class-D audio amplifier targeted at mobile phone and PDA designs. This amplifier operates over the 2.5- to 5.5-V supply range, delivers a 12-dB fixed gain, and produces up to 1.2-W power per channel into an 8-ohm load. It also sports a 72-dB PSRR at 217 Hz, thus allowing the device to operate directly from a single lithium-ion cell. The amplifier has four clocking options. In addition, multiples of this amplifier can be used together in a master/slave configuration. The amplifier is provided in either a 10-pin DFN or 10-pin ¼MAX package. Pricing: $0.45 in 100,000-unit quantities

The key challenge, according to TI, is to meet the customers' performance needs at the price point they require. In what seems to be a never ending downward spiral, customers continue to require smaller solution sizes at a lower cost. “One way TI meets these challenges is with Class-D audio power amplifiers, said Eric Droge, Product Marketing Manager, for Class-D audio power amplifiers at TI. The company believes that Class-D is the right design technology because it greatly reduces heat in the system and power consumption compared to traditional Class-AB or linear amplifiers. “Once considered more expensive than Class-AB amplifiers, a total Class-D audio power amplifier solution is now cost-competitive due to the high level of integration, which shrinks the component count, and coupled with the reduced heat generation it eliminates the need for large heat sinks, said Mr. Droge.

TI says there are different layout techniques used for linear amplifiers, analog input Class-D amplifiers, and 100 percent digital systems. “It is a paradigm shift for many audio amplifier design engineers to learn the new design requirements/constraints of Class-D and digital amplifiers,” observed Droge. TI has addressed this challenge by providing reference designs that are virtual turnkey solutions, leading to minor adjustments prior to manufacturing. Another challenge has been the transition from linear power supply to a switch power mode power supply.

What do designers want from audio amps?
In the world of electronics, smaller is better, and it's certainly true for designs using audio amps. However, designers want more than small chip size. TI's Droge says the number one request from designers is for reducing solution size and cost while meeting the performance needs of the application. To meet this requirement, TI offers a complete line of Class-D and digital audio power amplifiers across a wide range of power stages and price points for the home theater and automotive infotainment markets as well as for wireless and consumer applications such as cell phones, PDAs, flat panel TVs, notebook computers and some portable audio devices. By offering specific amplifiers containing features for certain markets and groups of customers, TI says it is able to meet the performance requirements at the right solution size and cost.

Maxim says it can help you reduce component count to meet your design needs. The company offers a patented DirectDrive headphone amplifier architecture that eliminates the bulky DC blocking capacitors required by conventional amps, low click/pop performance, integrated 8kV ESD protection, and short circuit protection. The MAX9720 includes its SmartSense feature that can automatically sense and disable either the left or right output under a fault condition and report the status back to a host.

STMicroelectronics released two very small chips. The company offers the TS4851, a low-power audio amp with speaker and headset drivers and digital volume control. The chip can deliver 400 mW to a speaker (8-ohm load), and 30 mW into each channel of a 32-ohm bridge-tied load (headset). The company's TS4855 delivers 1.1 watts into a speaker, and 85 mW into a headset. The TS4855 also includes an input channel for a hands-free microphone. Both chips come in a 18-bump flip-chip package.

Another request from designers is for help minimizing EMI. TI's newest Class-D amplifiers employ a patented switching modulation scheme to reduce EMI. In addition, the company provides online resources such as application notes on board layout to help customers reduce EMI in their designs.

Maxim also offers Class D amps that employ a patented spread spectrum modulation scheme that reduces EMI noise by up to 6dB over conventional modulation schemes.

STMicro offers a web seminar for designers who want to catch up on techniques for low power audio amps that deliver optimum sound quality even in noisy environments. The need for these audio amps is being driven by demanding applications such as cellular phones and PDAs. The net seminar discusses fundamental audio concepts and presents the STMicroelectronics low power audio portfolio.

“Philips Semiconductor developed its patented overload protection technology to bring to market a robust solution for high performance audio amplification,” said Jan-Paul Huyser, international product marketing, audio amplifiers at Philips Semiconductors. “In addition, Philips has developed a family of these devices to support the various power requirements of today's consumer electronic devices – ensuring our customers a flexible amplification solution to complement their audio and video products.”

The company introduced the TDA8947J amplifier that will provide manufacturers of audio applications a low-cost solution for high-performance audio. Featuring Philips' patented overload protection; the TDA8947J enables sufficient headroom to prevent audio holes under extreme conditions such as low load impedances, which in many cases can happen when overdriving an amplifier or using external speakers. This performance, combined with its high supply voltage range, makes the TDA8947J suitable for applications such as home theatre applications, PC speakers and LCD, Plasma and CRT TVs.

The TDA8947J includes a 4-channel amplifier that can be used to enhance the sound of a simple 2.1 systems amplifier or to build a 5.1 system with just two chips — reducing the bill of materials for a 5.1 system. A voltage range of 9-V to 28-V allows the TDA8947J to be designed in to a wide range of applications, including those with a simple power supply structure. To complete the family Philips also released a 2-channel amplifier (TFA9842J for 2x7W and TFA9843J for 2x20W) and a 1-channel amplifier (TFA9841J for 1x7W) into the market, having the same feature list as the TDA8947J.

National Semiconductor says its Overture amplifiers are known for their robustness in a variety of applications. The company says the process technology and its internal SPiKe protection topology, make it a very robust design. SPiKe protection is an active thermal management system that monitors heat in the outputs and protects them from damage. When these devices are overdriven they can protect themselves from damage. Once the overload condition is removed, they will recover and operate normally.

The latest additions of the LM4780, LM4781 and LM4782 round out National's Overture family with high-power range stereo and three-channel options. The LM4780/81/82 audio amplifiers are fully protected by National's SPiKe protection circuitry. It protects the LM4780/81/82 outputs against over-voltage, under-voltage, overloads, shorts to the supply or ground, thermal runaway and instantaneous temperature peaks.

The future of audio amps
Looking into the near future, Maxim says it sees a wide range of future audio product needs, including even higher levels of integration, amplifiers with on-board DSP, Digital Audio products with integrated, high performance audio input and output paths

Segmented by application, National Semi says that consumer amplifier makers will continue to demand amplifiers that can deliver a full range of power levels, premium quality sound, effective packaging technology, robust architectures and lower prices per watt. Some are demanding high efficiency amplifiers for smaller and lighter boxes.

Additionally, National's Adrig says “portable products of the future will continue to place increasing demands on audio amplifiers for more power, more features, more functions, smaller packages, less heat, and lower system level price points. National will continue to develop advanced audio solutions in both stand alone amplifiers and higher level audio subsystems that serve these increasingly complex needs.”

According to TI's Droge, “wireless and consumer applications will require amplifiers that enable them to integrate even more audio capabilities, such as FM Radio, MPEG videos and even CD quality sound.” Additionally, TI's high efficiency and PurePath technology are ideally suited for future home theater requirements, such as wireless rear channel speakers.

STMicroelectronics is shipping a new single-chip audio power amplifier, the STA5150 that delivers 200W, the highest power available from an integrated commercial device. This new amplifier combines the company's patented high-efficiency technology licensed from Indigo Manufacturing with ST's proprietary Bipolar-CMOS-DMOS (BCD) process technology.

Merging the benefits of class AB and class D, the high-efficiency technology uses DMOS power transistors operated in class AB mode, reducing the EMI and distortion typical of Class D switching amplifiers. To maximize efficiency, by continually adjusting the power supply voltage, depending on the amplitude of the amplified audio signal, a constant voltage drop is achieved across the output transistors, completely independent of the output amplitude. Implementing this technique allows the STA5150 to deliver 200W into a four-ohm load with less than 10% distortion, and with an efficiency three times better than a conventional Class AB.

The future for designers using audio amps sounds sophisticated, and will have designs that continue to shrink and yet remain cool, very cool.

Company Contact List

Maxim Integrated Products
Tel: 408-737-7600

National Semiconductor Corp.
Tel: 800-272-9959

Philips Semiconductor
Tel: 800-234-7381

Tel: 781-861-2678

Texas Instruments Inc.
Tel: 800-477-8924, ext. 4500

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