From
http://www.faqs.org/faqs/AudioFAQ/part4/11.18 What is Amplifier Class A? What is Class B? What is Class AB?
What is Class C? What is Class D?
All of these terms refer to the operating characteristics
of the output stages of amplifiers.
Briefly, Class A amps sound the best, cost the most, and are the
least practical. They waste power and return very clean signals.
Class AB amps dominate the market and rival the best Class A
amps in sound quality. They use less power than Class A,
and can be cheaper, smaller, cooler, and lighter. Class D amps
are only used for special applications like bass-guitar amps and
subwoofer amps. They are even smaller than Class AB amps and
more efficient, yet are often limited to under 10kHz (less than
full-range audio). Class B & Class C amps aren't used in audio.
In the following discussion, we will assume transistor output
stages, with one transistor per function. In some amplifiers,
the output devices are tubes. Most amps use more than one
transistor or tube per function in the output stage to increase
the power.
Class A refers to an output stage with bias current greater than
the maximum output current, so that all output transistors are
always conducting current. The biggest advantage of Class A
is that it is most linear, ie: has the lowest distortion.
The biggest disadvantage of Class A is that it is inefficient,
ie: it takes a very large Class A amplifier to deliver 50 watts,
and that amplifier uses lots of electricity and gets very hot.
Some high-end amplifiers are Class A, but true Class A only
accounts for perhaps 10% of the small high-end market and none
of the middle or lower-end market.
Class B amps have output stages which have zero idle bias
current. Typically, a Class B audio amplifier has zero bias
current in a very small part of the power cycle, to avoid
nonlinearities. Class B amplifiers have a significant advantage
over Class A in efficiency because they use almost no
electricity with small signals.
Class B amplifiers have a major disadvantage: very audible
distortion with small signals. This distortion can be so bad
that it is objectionable even with large signals. This
distortion is called crossover distortion, because it occurs at
the point when the output stage crosses between sourcing and
sinking current. There are almost no Class B amplifiers on the
market today.
Class C amplifiers are similar to Class B in that the output
stage has zero idle bias current. However, Class C amplifiers
have a region of zero idle current which is more than 50% of
the total supply voltage. The disadvantages of Class B
amplifiers are even more evident in Class C amplifiers, so
Class C is likewise not practical for audio amps.
Class A amplifiers often consist of a driven transistor
connected from output to positive power supply and a constant
current transistor connected from output to negative power
supply. The signal to the driven transistor modulates the
output voltage and the output current. With no input signal,
the constant bias current flows directly from the positive
supply to the negative supply, resulting in no output current,
yet lots of power consumed. More sophisticated Class A amps
have both transistors driven (in a push-pull fashion).
Class B amplifiers consist of a driven transistor connected
from output to positive power supply and another driven
transistor connected from output to negative power supply.
The signal drives one transistor on while the other is off,
so in a Class B amp, no power is wasted going from the
positive supply straight to the negative supply.
Class AB amplifiers are almost the same as Class B amplifiers
in that they have two driven transistors. However, Class
AB amplifiers differ from Class B amplifiers in that they
have a small idle current flowing from positive supply to
negative supply even when there is no input signal. This idle
current slightly increases power consumption, but does not
increase it anywhere near as much as Class A. This idle current
also corrects almost all of the nonlinearity associated with
crossover distortion. These amplifiers are called Class AB
rather than Class A because with large signals, they behave like
Class B amplifiers, but with small signals, they behave like
Class A amplifiers. Most amplifiers on the market are Class AB.
Some good amplifiers today use variations on the above themes.
For example, some "Class A" amplifiers have both transistors
driven, yet also have both transistors always on. A specific
example of this kind of amplifier is the "Stasis" (TM) amplifier
topology promoted by Threshold, and used in a few different
high-end amplifiers. Stasis (TM) amplifiers are indeed
Class A, but are not the same as a classic Class A amplifier.
Class D amplifiers use pulse modulation techniques to achieve
even higher efficiency than Class B amplifiers. As Class B
amplifiers used linear regulating transistors to modulate output
current and voltage, they could never be more efficient than
71%. Class D amplifiers use transistors that are either on or
off, and almost never in-between, so they waste the least amount
of power.
Obviously, then, Class D amplifiers are more efficient than
Class A, Class AB, or Class B. Some Class D amplifiers have
>80% efficiency at full power. Class D amplifiers can also have
low distortion, although not as good as Class AB or Class A.
Class D amplifiers are great for efficiency. However they are
awful for other reasons. It is essential that any Class D amp
be followed by a passive low-pass filter to remove switching
noise. This filter adds phase shift and distortion. It also
limits the high frequency performance of the amplifier, such
that Class D amplifiers rarely have good treble. The best
application today for Class D amplifiers is subwoofers.
To make a very good full range Class D amplifier, the switching
frequency must be well above 40kHz. Also, the amplifier must be
followed by a very good low-pass filter that will remove all of
the switching noise without causing power loss, phase-shift, or
distortion. Unfortunately, high switching frequency also means
significant switching power dissipation. It also means that the
chances of radiated noise (which might get into a tuner or phono
cartridge) is much higher.
Some people refer to Class E, G, and H. These are not as well
standardized as class A and B. However, Class E refers to an
amplifier with pulsed inputs and a tuned circuit output. This
is commonly used in radio transmitters where the output is at
a single or narrow band of frequencies. Class E is not used
for audio.
Class G refers to "rail switched" amplifiers which have two
different power supply voltages. The supply to the amplifier
is connected to the lower voltage for soft signals and the
higher voltage for loud signals. This gives more efficiency
without requiring switching output stages, so can sound better
than Class D amplifiers.
Class H refers to using a Class D or switching power supply
to drive the rails of a class AB or class A amplifier, so that
the amplifier has excellent efficiency yet has the sound of a
good class AB amplifier. Class H is very common in professional
audio power amplifiers.
