The Class-I Amplifier
The Class-I Amplifier
Class-I, also known as BCA (Balanced Current Ampli-
fier) is Crown’s patented, cutting-edge technology that
gets more power out of an amplifier with less waste than
was ever before possible.
Class-I technology offers several key advantages. It
provides unprecedented efficiency, requiring less power
from the AC supply than other designs and that can add
up to significant cost savings over the life of the ampli-
fier. Class-I handles reactive loudspeaker loads easily
and gracefully, by reusing energy returned from the
loudspeaker rather than dissipating it as heat or forcing
the amp into premature current limiting. This character-
istic means class-I amplifiers run better and longer—
especially at lower impedances. It also makes them
more reliable, since they are not constantly stressed to
their limits or subjected to excessive heat. Best of all, as
proud owners can attest, amplifiers with class-I technol-
ogy sound great, with a powerful, accurate sound that
stands out from the competition.
Crown’s class-I “switching” technology is a completely
new adaptation of switching (PWM) amplifier design.
This paper provides a simplified overview of Class-I, but
before we explore its inner-workings, we need to look at
the foundation of all previous high-power amplifier de-
signs in order to fully appreciate how class-I stands
apart.
TRANSISTOR OPERATION
To understand the different amplifier classes, it helps to
understand a little about how transistors operate. Bias is
a technical term referring to the static operating condi-
tion of an electronic device, such as a transistor. In other
words, bias defines how much conduction takes place
in the transistor with no dynamic signal input. Transistors
may operate in three possible states: cutoff, saturation,
or somewhere in between. The cutoff state is when there
is not enough signal present to cause the device to
conduct. Saturation refers to when the device has reached
maximum conductivity. Amplifiers referred to as “dissi-
pative” control their output by operating in the region
between cutoff and saturation.
CLASS-A
Class-A amplifiers are the simplest in design, and can
be the most distortion-free of all amplifier classes. In
class-A, the output devices are biased on all the time
with a current large enough to produce the largest
output signal.
Some class-A amplifiers may employ both a positive
and negative device in a push-pull arrangement to
increase output power, but both devices still are biased
on and conduct all the time. Class-A amplifiers are
generally considered to be the most accurate of all
classes in low to moderate power ranges and are useful
for applications such as preamp stages; however, they
create tremendous amounts of heat due to their very low
efficiency, making them impractical for high-power am-
plification. Other amplifier classes have been devel-
oped over time to overcome the class-A efficiency
problem.
CLASS-B AND TIME ALTERNATION
Class-B was invented as a solution to the efficiency
problem with class-A. The invention of class-B is signifi-
cant in that with it came the concept of time alternation,
which has been the foundation of virtually all power
amplifier designs used for audio reproduction and in-
dustrial power since about 1931 to the present. While
many incremental improvements have been brought to
market since that time, none have varied from the basic
time-alternation paradigm.
The basic class-B amplifier implements two devices in
the output stage in a “push-pull” arrangement, with each
amplifying half of the waveform, and the devices oper-
ate in strict time alternation. When the signal goes
positive, the positive device conducts while the negative
Class-A Operation
Class-B Operation
