04 Tube Operation.pdf - Kambing UI

these voltage variations are smooth sine-wave variations, 180° out
of phase (as the grid voltage rises and becomes more positive,
the anode voltage falls and becomes less positive), as indicated in
Figure 24. Note how these variations center about the d-c anode
voltage and the d-c control grid bias.
Figure 24. Variation of anode and grid voltage
Now let us see how such variations of the anode and grid voltages
of a tube appear on the constant-current curves of a tube. In Figure
25 these variations have been indicated next to the anode voltage
and grid voltage scales of a typical constant current curve. At some
instant of time, shown as “t” on the time scales, the grid voltage
has a value which is the point marked “eg” on the grid-voltage
sine wave. If one finds the point on the tube curves corresponding
to these values (where a line drawn from “eg” and a line drawn
from “ep” cross) he will be at Point A in Figure 25. As the values
of the grid voltage “eg” and anode voltage “ep” vary over the r-f
cycle, Point A moves up and down a line, which in the case of the
normal r-f power amplifier is a straight line. This line is called the
“Operating Line.”
Any point on the operating line (when drawn on constant current
curves as illustrated in Figures 25 or 27) tells the instantaneous
values of anode current, screen current, and grid current which
must flow when these particular values of grid and anode voltage
are applied to the tube. Thus, by reading off the values of the
currents and plotting them against time “t”, one can obtain a curve
of instantaneous values of anode and grid current (Figure 26).
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