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MANAGERS AND PROJECTIONISTS 1215
Push-Pull Amplification
TO many the expression "push-pull" as applied to amplification is a dense mystery. In order to explain this process, let's think back for a moment to the fundamental idea of amplifier action. It will be recalled that in a vacuum tube amplifier the essential feature of its action is that if we change the voltage on the grid there will be a corresponding change in the current flowing through the plate circuit of the tube. Since the grid draws practically no current and the voltage change on it need only be of the order of a few volts, whereas the plate may be operated with 100 volts or more and may draw several milliamperes, obviously we have here a means of using an infinitesimal amount of power to control a much larger amount, in fact, practically as large as we please. In other words, whatever frequency change or wave-form we apply in the shape of grid voltage, we obtain in the plate circuit a greatly magnffled copy of this frequency change or wave-form.
However, this device, like everything else, must somewhere or somehow have a limit to its capabilities, and its particular limitation occurs in regard to the amount of electrical magnification it will afford, or putting it another way, if we need a certain amount of magnification, it may be possible to get it all with one tube, but the magnification may not be accurate, and the output from the plate may not be a true copy of the input to the grid.
Why does this happen? Simply because the change