Audio power
amplifiers are classified primarily by the design of the output stage.
Classification is based on the amount of time the output devices
operate during each cycle of signal swing. Also defined in terms of
output bias current, (the amount of current flowing in the output
devices with no signal).
Class A operation is where both devices conduct continuously for
the entire cycle of signal swing, or the bias current flows in the
output devices at all times. The key ingredient of class A operation is
that both devices are always on. There is no condition where one or the
other is turned off. Because of this, class A amplifiers are
single-ended designs with only one type polarity output devices. Class
A is the most inefficient of all power amplifier designs, averaging
only around 20%. Because of this, class A amplifiers are large, heavy
and run very hot. All this is due to the amplifier constantly operating
at full power.The positive effect of all this is that class A designs
are inherently the most linear, with the least amount of distortion.
Class B operation is the opposite of class A. Both output
devices are never allowed to be on at the same time, or the bias is set
so that current flow in a specific output device is zero when not
stimulated with an input signal, i.e., the current in a specific output
flows for one half cycle. Thus each output device is on for exactly one
half of a complete sinusoidal signal cycle. Due to this operation,
class B designs show high efficiency but poor linearity around the
crossover region. This is due to the time it takes to turn one device
off and the other device on, which translates into extreme crossover
distortion. Thus restricting class B designs to power consumption
critical applications, e.g., battery operated equipment, such as 2-way
radio and other communications audio.
Class AB operation allows both devices to be on at the same time
(like in class A), but just barely. The output bias is set so that
current flows in a specific output device appreciably more than a half
cycle but less than the entire cycle. That is, only a small amount of
current is allowed to flow through both devices, unlike the complete
load current of class A designs, but enough to keep each device
operating so they respond instantly to input voltage demands. Thus the
inherent non-linearity of class B designs is eliminated, without the
gross inefficiencies of the class A design. It is this combination of
good efficiency (around 50%) with excellent linearity that makes class
AB the most popular audio amplifier design.
Class AB plus B design involves two pairs of output devices: one pair operates class AB while the other (slave) pair operates class B.
Class D operation is switching, hence the term switching power
amplifier. Here the output devices are rapidly switched on and off at
least twice for each cycle. Since the output devices are either
completely on or completely off they do not theoretically dissipate any
power. Consequently class D operation is theoretically 100% efficient,
but this requires zero on-impedance switches with infinitely fast
switching times -- a product we're still waiting for; meanwhile designs
do exist with true efficiencies approaching 90%.
Class G operation involves changing the power supply voltage
from a lower level to a higher level when larger output swings are
required. There have been several ways to do this. The simplest
involves a single class AB output stage that is connected to two power
supply rails by a diode, or a transistor switch. The design is such
that for most musical program material, the output stage is connected
to the lower supply voltage, and automatically switches to the higher
rails for large signal peaks. Another approach uses two class AB output
stages, each connected to a different power supply voltage, with the
magnitude of the input signal determining the signal path. Using two
power supplies improves efficiency enough to allow significantly more
power for a given size and weight. Class G is becoming common for pro
audio designs.
Class H operation takes the class G design one step further and
actually modulates the higher power supply voltage by the input signal.
This allows the power supply to track the audio input and provide just
enough voltage for optimum operation of the output devices. The
efficiency of class H is comparable to class G designs.
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