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look at the conflicting set of definitions fo r
classes A, AB, B, and C, as published in th e
widely circulated ha ndboo k for radio am a­
te urs published by the ARR L (q uo te s ar e
fro m the 19 62 ed iti on):
" A class A am plifier is one o perate d so
that th e wave shape of th e out pu t voltage
is the same as th at of the signal voltage
applied to th e grid . ..
"A class AB am plifier is a push-pull
am plifier with higher bias tha n wo uld be
normal for pure class A operation , but less
than the cutoff bias req uired for class
B . . . "
(Class B operation is defin ed by mea ns
of a schemati c diagram and four waveform
s.)
" A radio-freq uency po wer am plifier
... can be used with an o pera ting angle of
less than 180 degrees. This is called 'class C
o peration ."
It ma y appear t hat we're overemphasizing
the differen ces in th ese sets of definiti
on s, bu t in pea rly 20 years of listening
to o n-the-air bull sessio ns get dow nright
acrimo nious sim ply because two people
att em pt ing to discuss am plifier operatio
n d idn 't mea n the same thing at all
by "class A" or " class AB" o r "class C,"
we feel that the ex isten ce of mu ltiple
defin iti ons is an essential fac t to know.
Th e major significan ce of the differences
is th at the en gineers' version is based
entirely upon an op erating charac teristic
wh ich can be precisely measure d and specifie
d; th e flo w of plate cu rre nt in th e
amp lifie r th rougho ut o ne cy cle of an inpu t
signal de termines th at amplifier's o perati ng
class. If plate cur re nt alwa ys flo ws, it's
class A. If current is cut off for part of the
cycle, but less than half, it 's AB. If cutoff
is for approxima te ly half a cycle , it 's B.
And if cutoff is appreciably mo re than half
a cy cle, it's class C.
Th e "amateur" def inition, on the other
hand , define s each different class by means
of a different char acteristic. It says that a
class A amplifier is on e whi ch is free of
dist or tion (that may not be wha t was
intended, but it's prec isely t he meaning of
the words used in the de finition , and it's
the way most peo ple read th em if you
listen to th e resulting arguments), while a
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7 3 M A G A Z I NE