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New analogue front end integrates quality audio and Class D amplifiers for portable applications.

High levels of integration and intelligent circuitry have brought quality audio to such devices as handsets, MP3 players, hands free car kits and digital tvs.
Apart from the need to provide quality audio in ever smaller board spaces, audio engineers also face the challenge of low supply voltages and low power consumption. Low heat dissipation, a reduction in ‘pop’, signal to noise ratios (SNR) of more than 90dB and excellent common mode rejection ratio (CMRR) are further requirements. Meanwhile, headphones, speakers, microphones and other audio sources need to be handled by the same analogue front end. Since the number of functions – such as video, camera and digital radio – is always increasing, new ways of integration are being developed.
Every device transferring audio to the outside world needs to convert signals from digital to analogue and vice versa. Alongside complex audio conversion, additional functions – such as equalisation, mute, treble and bass – are needed. Finally, loudspeakers, headphones and microphones need to be driven.
Battery lifetime is a key concern and the solution, when driving audio speakers, has been the Class D amplifier. The speaker is driven by a PWM signal in the range of 250kHz – achieving efficiencies of more than 90% and reducing heat dissipation by 80% – but external LC filtering is needed to reduce undesired EMI noise at the output.

Peter Peisker

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