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
providesunprecedented efficiency,requiring lesspower
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.
Class-A Operation
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.
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.
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.
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
inthetransistorwithnodynamic signalinput. 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.Saturationreferstowhenthedevicehasreached
maximum conductivity. Amplifiers referred to as “dissi-
pative” control their output by operating in the region
between cutoff and saturation.
The basic class-B amplifier implements two devices in
theoutputstageina“push-pull” arrangement, witheach
amplifying half of the waveform, and the devices oper-
ate in strict time alternation. When the signal goes
positive, thepositivedeviceconductswhilethenegative
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
Class-B Operation
Download from Www.Somanuals.com. All Manuals Search And Download.
The Class-I Amplifier
amount. Class-I uses symmetrical interleaved PWM,
meaning both the leading edges and trailing edges of
the pulse are varied according to the amplitude of the
signal, and the spacing between pulse centers remains
constant. Both the positive and negative switch pulses
remain aligned on-center, and the net output current is
positive.
tive energy returned from the loudspeaker to the ampli-
fier is reabsorbed and output again to the loudspeaker
with little loss. Non-switching, dissipative amplifiers are
forced to dissipate all of the returned energy and more
in the form of heat. This means class-I amplifiers are
louder than other amplifiers with comparible power
specs, yet they stay much cooler, and can be lighter
since they don’t need nearly as much mass for heat
dissipation.
Class-I Amplifier
Reactive Energy
Class-I Switches, Positive Signal
Dissipative Amplifier
Reactive Energy
Likewise, to produce a negative output signal, the
output of the negative switching device is increased in
duty while the positive switching device is decreased by
the same amount. Again, both switch pulses remain
aligned on-center, and the net output current is negative.
Reactive Energy Returned to Amplifier
From Loudspeaker
With it’s rock-solid reliability, high quality audio repro-
duction and unmatched efficiency, Crown’s patented
class-I technology creates a totally new paradigm for
amplifier design that represents the future of profes-
sional amplifiers.
Class-I Switches, Negative Signal
The result of using interleaved PWM is that by operating
theswitching devicesat250kHz, thesignaliseffectively
modulated at500kHzsinceboththeleading and trailing
edges of each pulse contribute to the output ripple
current. This arrangement further increases efficiency,
since switching losses are effectively halved by operat-
ing the switching devices at 250 kHz, rather than at 500
kHz as would be necessary with class-D designs to
achieve the same effective sampling of the waveform.
CrownAudio,Inc.
P.O. Box1000
Elkhart,IN46515-1000
TEL:219-294-8200
FAX:219-294-8FAX
Class-I amplifiers also have all of the nearly ideal power
converter attributes of class-D PWM amplifiers, in that
reactive loudspeaker loads are easily driven. The reac-
Trademark Notice:
Crown® and BCA® are registered trademarks of Crown International.
Printed in U.S.A.
137234-1
12/03
Download from Www.Somanuals.com. All Manuals Search And Download.
|