Class-D amplifiers swing with a digital audio beat

Class-D amplifiers swing with a digital audio beatSteve Bush  
 
Digital, or class-D, audio amplifiers have been around for at least 20 years.
In class-D amplifiers, the loudspeaker load is driven from a pair of transistors which are either hard on, or off. This digital nature leads to their primary advantage which is power efficiency. Theoretically this could be over 90 per cent, compared with around 30 per cent for conventional linear output amplifiers.
The disadvantage is that it is difficult to get good audio quality from a digital amplifier.
Whereas 1 per cent audio distortion over the audio range at 10W is almost trivial to produce with a few cheap transistors in a linear amplifier, it is impossible with a digital amplifier, without a lot of complexity.  
 
  AT A GLANCE l Digital in-digital out audio amplifier. l 1W into 8ž l 70% efficient at 1W l 0.1% distortion at 1W, 1kHz l 84dB S/N over 20KHz bandwidth.
This has relegated class-D amplifiers to applications like mobile public address systems where power consumption is more important than ultimate sound quality.
That is until recently, when monolithic class-D amplifiers began to appear for use in TV sets as part of the overall drive for power efficiency in consumer goods and the gradual creep of digital circuitry into all applications.
At ISSCC, Philips introduced a novel fully digital class-D audio amplifier chip with real Hi-Fi performance. It has been designed from the start to accept digital audio signals.
The conventional approach to producing a digital-in, digital-out amplifier, says Philips, uses a D to A converter followed by a pulse width modulator (PWM) with an anti-aliasing filter in between (top diagram).
According to the company, this is difficult to integrate on a single chip because of the different processes desirable for the two functional blocks.
If the original digital information is available in a noise-shaped one-bit digital code (pulse density modulation) there is no need for a PWM, leading to the approach in the middle diagram which can be cost-effectively integrated in a single chip.
The disadvantage here is that the process is essentially open-loop and any ripples or other variations present on the power rails go straight on to the output.
More subtly, output power is limited as 100 per cent modulation is impossible for stability reasons.
Philips is calling its topology PowerDAC, and claims it can be integrated on a single chip because it uses a noise-shaping approach, but it also takes into account output errors due to power supply ripple and stability.
The actual D to A conversion takes place between the noise shaper and the analogue loop in a latch with accurate time and amplitude references. It generates reference pulses with constant integrated area. This means that the feedback loop forces the output pulse width to increase if the output amplitude is reduced instantaneously by the supply voltage changing.
The frequency response of the feedback loop can be set to assure stability at full power. Hi-fi audio A/D gets CD sound on little power
CD quality analogue to digital conversion for 1.0mW.
That was the claim at ISSCC by a group from Rockwell Semiconductor Systems and the Georgia Institute of Technology (Georgia Tech).
The key to this low power consumption, claims Rockwell, is choosing an architecture that keeps clock speed down and minimises the number of high gain op amps.
The result is a 64-times oversampled ? converter with a single-loop modulator.
Single loop was chosen over a cascaded design because the single-loop type only places stringent requirements on the first op amp. The others are inside the feedback loop and specifications can be relaxed, leading to lower overall power dissipation.
By carefully tailoring the first amplifier gain, fine-tuning the architecture to minimise its output swing and using single stage amplifiers for the other op amps, overall power lost in the analogue section of the chip is 900?W. Of this, 80 per cent is due to the first amplifier. Measured performance Fully differential switched capacitor ? modulator 1.5V operationl 670?A supply currentl 98.2dB dynamic range 90dB signal to noise ratiol 2.82MHz clock 0.5?m CMOSl 1.02 x 0.52mm die


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