1. Field of the Invention
The present invention is generally in the field of electronic circuits and systems. More specifically, the present invention is in the field of amplifier circuits and systems.
2. Background Art
Audio power amplifiers form an important part of many mobile communications devices, such as cellular telephones and MP3 players. Because these mobile communications devices are typically battery-operated, it is highly desirable that the audio power amplifiers implemented in such devices consume low quiescent currents in order to extend the life of the battery. For example, a Class-A amplifier continuously consumes a large current even in the absence of an audio signal, which makes it unattractive for battery-operated hand-held devices. An alternative conventional amplifier, the Class-B amplifier, although designed so as not to consume any current when there is no signal, suffers from large cross-over distortion. This cross-over distortion may significantly degrade the output signal quality of a Class-B amplifier implemented to provide audio amplification.
A conventional approach to achieving a compromise between the relatively high fidelity of the Class-A design and the efficiency achieved by Class-B amplifiers is the Class AB amplifier, which consume a small current in the quiescent state to improve cross-over distortion. However, typical Class-AB amplifiers employ large output-stage transistors that are sized to handle peak load currents. This can create large variations in the output stage currents due to process mismatch, which may cause a stability problem if the output stage current is significantly reduced. In addition, the large output stage transistors and the small quiescent current reduce the headroom allocated to the first stage.
Thus, mobile communications devices may benefit from an audio amplifier that combines the good cross-over distortion performance of Class-AB amplifiers and the low quiescent current of Class-B amplifiers, without incurring stability problems due to current variations or headroom limitations. Accordingly, there is a need to overcome the drawbacks and deficiencies in the art by providing a Class-AB/B amplifier and a quiescent control circuit for implementation with the Class-AB/B amplifier.