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AMPLIFIER
CLASSIFICATION
All
radio receiving tubes except the rectifiers may be conveniently
considered as amplifiers. Oscillators and detectors or frequency
converters may be thought of as special cases of amplifiers
in which use is made of the non-linear relations between
the input voltages and output currents of the tube under
consideration.
There are three major classes of amplifier service. Definitions
describing these have been standardized by the Institute
of Radio Engineers.
Class A Amplifier
A Class A, or Class A1, amplifier is one in which the grid
bias and signal voltages are such that plate current in
the tube, or in each tube of a push-pull stage flows at
all times.
This is accomplished by operating at the center point of
the plate current vs. grid voltage curve and using signal
voltages which do not drive the grid into either the positive
region or into the sharp bend near cut-off voltage.
Class
A2 Amplifier
A Class A2 amplifier is the same as a Class Al amplifier
except that the signal may drive the grid into the positive
region. This is accomplished by operating at a lower bias
than the center point which would have been selected for
class A operation.
Class
B Amplifier
A Class B amplifier is an amplifier in which the grid bias
is approximately equal to the cut-off value, so that the
plate current is approximately zero when no signal voltage
is applied and so that plate current in the tube or in each
tube of a push-pull stage, flows for approximately one-half
of each cycle when an alternating grid voltage is applied.
An important characteristic is that the grid circuit draws
appreciable power which prevents it from being used with
ordinary resistance coupled driver tubes.
Class
AB1 Amplifier
A Class AB1 amplifier is one in which the grid bias and
peak signal voltage are in such proportion that it operates
as a Class A amplifier for small signals and as a Class
B Amplifier for large signals.
Clan
AB2 Amplifier
A Class AB2 amplifier is one in which the signal is allowed
to drive the grid slightly into the positive region but
not enough to require appreciable power from the driver.
This is accomplished by operating two tubes in push-pull
at very nearly the cut-off bias and applying a peak signal
equal to the bias. Resistance coupling may be used making
this is the best way of obtaining large power output with
low distortion. A good example of this rating may be found
under Type 6L6G.
Class
C Amplifier
A Class C amplifier is one in which the tubes operate at
a bias much greater than cut-off voltage so that plate power
is drawn only on the peaks of the signal voltage. It is
not used in audio amplifiers because the distortion is too
high but is the most efficient circuit for R. F. power amplifiers
where the harmonics can be reduced by use of resonant circuits.
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