Чтение общенаучной литературы. Кытманова О.А. - 10 стр.

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11
General Classification of Amplifiers
It is common knowledge that amplifiers are divided into three general classes: А,
В and C, depending on the type of service in which they are to be used.
A class A amplifier is one which operates so that the plate output wave shapes of
current are practically the same as those of the exciting grid voltage. This is
accomplished by operating the tube with sufficient negative grid bias so that some
plate current flows at all times and by applying an alternating excitation voltage to
the grid of such value that the dynamic operating characteristic is essentially linear.
The grid must not go positive on excitation peaks, and the plate current must not
fall low enough at its minimum to cause distortion due to curvature of the
characteristics. We know the characteristics of class A operation to be free from
distortion and relatively low power output, practically all a-f amplifiers being
operated in this manner.
Radio-frequency amplifiers of the type used in receiving sets to amplify the signal
voltage prior to detection are also considered to be of this class. Class В amplifiers
are operated with a negative bias approximately equal to cutoff so that the plate
current is almost zero when the alternating grid excitation is removed. With a
sinusoidal voltage applied to the grid, the plate current consists of a series of half-
sine waves, similar to the output of a half-wave rectifier. The load impedance is
adjusted so as to obtain an approximately linear dynamic characteristic. The grid
swings positive on excitation peaks, causing grid current to flow. We know of class
В amplifiers being used in radio-telephone transmitters following the modulated
stage.
The power output obtainable from a given tube is much greater than with class A
operation, the plate efficiency being much higher. As with a-f power amplifiers,
tubes operating as class В r-f amplifiers may also be operated in push-pull.
A class С amplifier is one in which high output and plate efficiency are the
primary considerations. The grid is negatively biased to a point considerably
beyond cutoff, so that the plate current is zero with no grid excitation. The latter is
quite large and is often sufficient to cause the plate current to reach saturation on
positive swings. Plate efficiencies in the vicinity of 90 per cent may be obtained
with the larger tubes, these high efficiencies being made possible by allowing the
plate current to flow during less than 180 deg. of the cycle and only at a time when
the plate potential is comparatively low. In radiotelegraph transmitters all stages
are operated in class C, while with radio-telephony it is only the modulated
amplifier and the stages preceding it that are so operated.
Either triodes or screen-grid tetrodes may be used as power amplifiers. The latter
have the advantage of. not requiring neutralization. The screen-grid voltage in
transmitting tubes is usually about 15 per cent of the plate supply voltage, which is
proportionally much lower than in receiving tubes. Тhese tubes are difficult to
construct for power outputs much greater than 500 watts, and, where larger outputs
are required, triodes must be used.

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