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Radio Broadcast
a radio-frequency amplifier, before it reaches ihe detector.
Contrary to the general belief, the original signal potential is not passed along and amplified in these successive stages of radiofrequency amplification. The action in these units is more on the order of a trigger releasing device. To understand this, the action of an amplifier must be observed.
THE TRIGGER ACTION OF THE TUBE
WHEN a signal is applied to the grid of an amplifying tube, the electronic emission from the filament is interrupted in its path to the plate because the grid acts as a shutter or trigger device. By means of a local source of potential (the B battery), the variation in signal frequency is faithfully reproduced in the plate circuit of the tube in the form of a varying, direct current potential many times greater in strength than the original signal applied to the grid of the tube.
In other words, the vacuum tube has repeated and amplified the incoming signal without changing any of its characteristics.
We have so far traced the action in this circuit to the plate circuit of the first tube.
A typical two-stage radio-frequency amplifier with detector and one-stage audio amplifier is shown in Fig. i.
Now in this plate circuit is contained the primary coil P2 of the radio-frequency coupling unit. This unit, consisting of the primary and a secondary which is connected to the input of the next tube, performs the function of inductively coupling one tube circuit to the next so that the signal received by the antenna may be repeated at a greater ampli
Mu, OR AMPLIFICATION FACTOR OF TUBE
I Ph-lncoming Signal I (continuous wave) t— J which is impressed [J on Grid of Tube
FIG. 2
How a tube amplifies, graphically shown. This only takes into consideration the amplification factor of the tube. Actually, the amplification is greater, due to the step-up. value of the r. f. coupler unit
tude in each successive stage. In this instance, the unit is a radio-frequency transformer.
When the variation of current takes place in the first plate circuit, coinciding with the variation of frequency of the received signal, an electro-magnetic field is set up in and about the plate coil, the intensity of which varies with the variation of the plate energy. This varying magnetic field induces in the secondary of* the transformer, which is the grid coil of the next tube, a magnified voltage corresponding exactly to that to be found in the preceeding plate circuit. (The direction in which the current flows in the two coils is
A typical radio-frequency amplifier circuit.
tig. 1
Oscillation control is obtained by the potentiometer