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508
MOTION PICTURE SOUND ENGINEERING
D — £iL
• I\P T
if
where Rp = plate to filament resistance
Ep = plate voltage
Ip = plate current
Figure 357 shows a typical plate voltage-plate current curve for a triode and it will be noticed that the value obtained by dividing the plate voltage by the plate current will vary, depending upon what point on the curve is considered. Since, under operating conditions, we utilize a considerable portion of the curve, it is obvious that the direct-current plate to filament resistance would not necessarily give a true indication of the
situation under operating conditions. We, therefore, make use of what is known as the alternatingOutput //npe&vce current plate to filament resistance.
By reference to Figure 358 it may be seen that it is determined as follows:
P/ate Vo/taye Figure 3 57 — Plate current-plate voltage characteristic of threeelement vacuum tube.
of curve to t/?e curre/tf ar/s
P/ate l/o/taye
Figure 358 — The plate-filament resistance may be determined from the plate voltage-plate current curve.
RP =
P2
pl
IP2 — 1
Pi
This is known as the alternating-current plate-filament resistance and is also sometimes called the plate-filament impedance.
9, MUTUAL CONDUCTANCE
The conductance of an ordinary direct-current circuit is considered as the reciprocal of the resistance. If we designate the conductance by the lettd G, then
In The three-element vacuum tube, however, we are principally interested in The variation in plate current produced by a change of grid voltage rather than by a change of plate voltage. The mutual conductance therefore must equal the plate current divided by the grid voltage rather than the plate current divided by the plate voltage, and if we designate the mutual conductance by the symbol Gm, then