The power amplifier forms the output stage of the audio amplifier system.
As such a low output resistance is required so that it can deliver the
output signal without a loss of gain. The most challenging requirement
in the design of a power amplifier is that it must deliver the required
amount of power to the load in an efficient manner. Output stages
are classified according to the collector current waveform that results
when an input signal is applied. MOSFETs are more commonly use nowadays
due to the fact that they are more efficient than BJTs.
