amplifier classes Audio power amplifiers are classified according to the
relationship between the output voltage swing and the input voltage swing, thus
it is primarily the design of the output stage that defines each class.
Classification is based on the amount of time the output devices operate during
one complete cycle of signal swing. This is also defined in terms of output bias
current [the amount of current flowing in the output devices with no applied
signal]. For discussion purposes, assume a simple output stage consisting of two
complementary devices (one positive polarity and one negative polarity). These
could be tubes (valves) or any type of transistor (bipolar, MOSFET, JFET, IGFET,
etc.).
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. Consequently
class A is the most inefficient of all power amplifier designs, averaging only
around 20% (meaning you draw about 5 times as much power from the source as you
deliver to the load!) Thus 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. [Much mystique and confusion
surrounds the term class A. Many mistakenly think it means circuitry comprised
of discrete components (as opposed to integrated circuits). Such is not the
case. A great many integrated circuits incorporate class A designs, while just
as many discrete component circuits do not use class A designs.]
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 is the intermediate case. Here both devices are allowed 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.
[and while you didn't ask, heres a few more classes]
class C use is restricted to the broadcast industry for radio frequency (RF)
transmission. Its operation is characterized by turning on one device at a time
for less than one half cycle. In essence, each output device is pulsed-on for
some percentage of the half cycle, instead of operating continuously for the
entire half cycle. This makes for an extremely efficient design capable of
enormous output power. It is the magic of RF tuned circuits (flywheel effect)
that overcomes the distortion created by class C pulsed operation.
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
(Sampling Theorem). Theoretically since the output devices are either completely
on or completely off they do not dissipate any power. If a device is on there is
a large amount of current flowing through it, but all the voltage is across the
load, so the power dissipated by the device is zero (found by multiplying the
voltage across the device [zero] times the current flowing through the device
[big], so 0 x big = 0); and when the device is off, the voltage is large, but
the current is zero so you get the same answer. 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 E operation involves amplifiers designed for rectangular input pulses, not
sinusoidal audio waveforms. The output load is a tuned circuit, with the output
voltage resembling a damped single pulse.
The following terms, while generally agreed upon, are not considered "official"
classifications
class F no info available
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 [thus the nickname rail-switcher]. 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. [Historical note: Hitachi is credited with pioneering class G designs
with their 1977 Dynaharmony HMA 8300 power amplifier.]
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 [thus the nickname rail-tracker]. The
efficiency of class H is comparable to class G designs. [Historical note:
Soundcraftsmen is credited with pioneering class H designs with their 1977
Vari-proportional MA5002 power amplifier.]