Headphone Amplifiers in the Audio Performance
Recently, the Smartphone and tablet usage in consumer electronics has rapid growth. At the same time, it has driven a requirement for a much higher standard of audio performance than that of previous generations. It presents a number of technical and system level challenges which the audio architect has to overcome when ever increasing consumer demand for an exceptional listening experience every time a user picks up their mobile phone or plugs in their headphones. It begins by examining a range of problems which can degrade the systems audio performance, presenting the designer with a number of challenges.
The two common use cases of the end user’s listening experience are listening to audio through headphones and listening to audio via the loudspeaker. Consumers now demand crystal clear voice calls in noisy environments; hi-fi quality audio playback for watching mobile video content; the ability to use their device as a conference phone in hands-free mode; and, of course, exceptional playback time for digital audio playback.
Despite these demands, when using headphones and speakers for all content types the playback time is still often too short, and the speaker response is sub-standard when using loudspeakers in hands-free mode. There is also the relentless quest to minimise PCB height and area, reduce the hardware bill of materials, and reduce software development time to meet demands for sleek, low-profile gadgets which can hit the market as fast as possible and also make the manufacturer a profit.
Headphone amplifiers are not always ground-referenced, meaning that decoupling capacitors need to be charged to a reference level (VMID) on power up, and discharge to ground on power down. This can result in audible pops and also reduces bass response due to the capacitor forming a high pass filter with the headphone load impedance (50Hz in the case below). The additional expense of AC coupling capacitors is to be avoided, if possible.
Listening to headphones over a prolonged period of time will undoubtedly drain the battery; the challenge is to reduce the headphone amplifier’s active power consumption as much as possible to extend the battery life. Using a highly-efficient class G headphone amplifier with ground-referenced outputs will not only extend battery life, but also remove the need for AC coupling capacitors, improve headphone bass response and minimise troublesome power-on/off pops and clicks. DC-offset removal at the headphone outputs can be achieved by using a circuit such as a DC servo, which measures output DC offset and automatically zeroes DC voltage (achieving sub 1mV) during device initialisation. Class G headphone amplifiers will also extend battery life for headphone playback over traditional single supply class AB headphone amps, as often found on highly integrated SoC, due to adaptive voltage-rail switching and charge pump switching frequencies. A charge pump-based approach to power headphone amplifiers is favoured over a buck converter-based design in order to keep the size of the external components down. The buck converter requires an inductor in the region of 4.7µH, which often has a larger PCB footprint than three 2.2µF capacitors required for a charge pump-based scheme.The impact of Class G and switching-frequency thresholds can be illustrated by looking at quiescent current consumption of WM9090.