Page 138 - Electrical Engineering Dictionary

mation (FM) is preserved in a class B-D am- of the conduction angle, which is 180 de-
plifier, while all amplitude information (AM) grees. The unfiltered, broadband output cur-
is lost. Usually, class B-D power amplifiers rent waveform of a class D amplifier resem-
are designed in a push-pull configuration to bles a stepped squarewave. It is important
take advantage of both halves of a cycle. to note that only frequency related informa-
tion (FM) is preserved in a class D ampli-
class B-E amplifier transient switched fier, while all amplitude information (AM) is
mode amplifier where the device is biased lost. Usually, class D power amplifiers are
at cutoff, the input signal is large enough designed in a push-pull configuration to take
to drive the amplifier into heavy saturation advantage of both halves of a cycle.
such that only a small percentage of time is
spent in transition, and the design is such class E amplifier a transient switched
that during saturation the waveform is de- mode amplifier where the device is biased
termined by the switch circuit transient re- somewhere between class A and class B cut-
sponse, while the waveform during cutoff is off, the input signal is large enough to drive
determined by the transient response to the the amplifier into heavy saturation such that
entire circuit, including the load. The ampli- only a small percentage of time is spent in
fier is literally switched between cutoff and transition, and the design is such that dur-
saturation, the transient responses are well ing saturation the waveform is determined by
controlled, and thus the saturation angles ap- the switch circuit transient response, while
proach the conduction angle, which is 180 the waveform during cutoff is determined by
degrees. The final tuned output current wave the transient response to the entire circuit,
form of a class B-E amplifier resembles an including the load. The amplifier is literally
ideal squarewave. It is important to note that switched between cutoff and saturation, the
only frequency related information (FM) is transient responses are well controlled, and
preserved in a class B-E amplifier, while all thus the saturation angles approach the con-
amplitude information (AM) is lost. duction angle, which is 180 degrees. The fi-
nal tuned output current waveform of a class
class C amplifier a current source ampli- E amplifier resembles an ideal squarewave.
fier biased beyond the conduction cutoff such It is important to note that only frequency re-
that operation will not begin until the input lated information (FM) is preserved in a class
signal reaches a specific amplitude, and re- E amplifier, while all amplitude information
sults in less than half of an input sinusoid (AM) is lost.
being amplified. If the signal amplitude is
increased sufficiently, saturation and the as- class E-F amplifier a harmonic tuned or
sociated clipping will occur. Thus the con- harmonic reaction amplifier (HRA) in which
duction angle is between 0 and 180 degrees, devices, biased for class B operation, are
regardless of amplitude. Device saturation is arranged in a push/pull configuration, and
usually avoided. are utilized to inject each other with large
harmonic currents in order to modulate the
class D amplifier switched mode ampli- amplitude of the fundamental output current
fier where the device is biased somewhere through the device, resulting in improved
between class A and class B cutoff, and the switching efficiency. The even order har-
input signal is large enough to drive the am- monics must be shorted at the output, while
plifier from cut-off to heavy saturation such the odd order harmonics must be provided an
thatonlyasmallpercentageoftimeisspentin open at the output.
transition. The amplifier is literally switched
between cutoff and saturation, and thus the class F amplifier a high-efficiency opera-
saturation angle is a significant percentage tion in amplifiers. The class F amplifier has a

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