The class AB type of audio amplifiers has been around for a long time, and most high quality ones are still typically class AB. Two things plague this excellently performing device – a quiescent bias current to keep the output semiconductors in their active region of operation and an operating efficiency that refuses to rise above 75%. In reality, this type of audio amplifiers operates with a compromise between frequency response, power output, low distortion and efficiency. Although special efforts and additional circuitry can improve the performance and efficiency of class AB amplifiers, it also makes the resulting audio amplifier prohibitively expensive.
To rid the audio amplifiers of their dependence on quiescent currents for biasing, the output semiconductors in a class D amplifier are switched on and off at a high rate. That allows the signal to appear at the output as chopped with respect to time. An averaging circuit transforms this chopped output back into analog form for feeding to the speakers. The result is an amplifier that has zero quiescent current requirement and efficiencies above 90%.
Class D amplifiers are also called digital amplifiers, of which there are two types. The first type is entirely digital while the second is a switching amplifier with analog control. Although both use switching power stages and exhibit high power efficiencies, the fully digital version has no feedback. The analog signal input is converted into digital using Analog to Digital converters and fed to the PWM switching output stage. However, achieving low distortion and good performance in all-digital class D amplifiers demands an extremely complex and expensive design.
In contrast, class D amplifiers with switching outputs but with analog controls are simpler to design and achieve very good performance with surprisingly simple circuitry. They can easily achieve extremely low output impedance over and beyond the audio range with equally low frequency-independent distortion levels.
Listening tests bear out the good performance shown in measurements. At low frequencies, because of low filter impedance, the bass is commanding and dynamic. Low THD and level response at 20 KHz produce transparent and neutral sound that real audiophiles admire.
With the music industry moving towards mobile gadgets as their main sound producing devices, interest in class D amplifiers is gaining momentum. With conservation of battery power at top priority, designers find the high efficiency of a class D amplifier very attractive. Fully integrated class D amplifiers that require the bare minimum components are now the norm in tablets and computers.
For example, the CX20952 is a high-definition audio codec from Conexant. It offers high-quality audio, low power consumption, two capacitor-free headphone amplifiers and a fully integrated class D amplifier. The intelligent single chip device keeps a check on the amount of power it delivers to the speakers, ensures maximum sound pressure levels and an optimal performance without allowing damage to system components.
With two headphone/line outputs, the CX20952 accepts a universal jack, while automatically detecting and configuring itself to different type of headsets. The single 3.5mm audio jack accepts various peripheral devices such as line-in, microphones, powered speakers, headsets and headphones.
