First, note that while class A, AB, B, C, and (I think) D are
recognized as defined, standard classifications and mean the same thing
no matter which manufacturer is using the term, other classes (H, G, K,
EIEIO) are not so universally defined, and someone's class "G" amp
might not be operating in the same "class" as someone else's class "G"
amp.
Having said that...
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 polarityoutput 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.
Music is art Audio is engineering