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Pronunciation Drill 2

While reading these words pay attention to the stress markings;

‘Purpose con`sist `neutralize ` similarly

reason exist moderate consequently

value reduce capable accumulate

zero instead circular positively

TEXT 7

MODE OF OPERATION OF TRIODES

1. Now a voltage has been applied to the heater and the
cathode is emitting a normal supply of electrons. The plate
is at a high positive potential and would normally attract
a large number of electrons from the space charge, if it were
not for the large negative bias voltage applied to the grid
from the Egg battery.¹

2. Because of this large negative potential, the electrosta-­
tic field normally existing between plate and cathode cannot
penetrate to the cathode and actually terminates on the grid
wires. Under these conditions the grid entirely neutralizes
the electrostatic field and, hence, the attraction of the plate.

Since there is no electrostatic field near the cathode to draw away the electrons, the plate current through the tube is zero and a large space charge accumulates in the region be­tween cathode and grid. The smallest negative voltage be­tween grid and cathode that is just capable of cutting off the plate current is called the cut-off bias.²

3. Consider now that everything has been left unchanged,
except that the negative bias voltage has been reduced to a
value less than cut-off. The grid is now no longer capable of
neutralizing the field between plate and cathode completely
and some of the lines of force penetrate between the grid wires
to the cathode. Consequently, some electrons are attract­
ed away from the space charge and move between the grid
wires towards the positive plate.

4. This results in a moderate flow of plate current. As
the negative grid voltage is further reduced, that is, made
less negative, progressively more electrons are able to pass
between the grid wires to the plate and the plate current con­
tinues to increase.

5. When the bias voltage is removed and the grid voltage is zero, the positive voltage on the plate produces a substan­tial electric field at the cathode and large numbers of elec­trons are attracted through the grid wires to the plate, re­sulting in a fairly large plate current. The action is similar to that of a diode, except that the grid still has some retard­ing effect on the electrons because of its shielding action, and hence the plate current is somewhat less than it would be with the grid removed entirely.

6. When the bias voltage has been reversed in polarity, the grid is being made positive with respect to the cathode. The grid potential now adds the plate voltage and produces a very strong electrostatic field at the cathode, resulting in л large plate-current flow through the tube. If the grid is made sufficiently positive with respect to the cathode, a point will be reached when the electrons are attracted to the plate as fast as they can be emitted from the cathode. No space charge can accumulate under these conditions and the plate current reaches its saturation value. Still further increases in either the grid or the plate voltage cannot cause an increase in the plate current.

7. A part of the electrons is attracted to the positive grid and causes a grid current to flow between grid and cathode. Under these conditions power is dissipated in the grid circuit. To avoid this power consumption and also the large satura-tion plate current, which eventually can damage the tube, electron tubes are generally operated at negative grid poten­tials with respect to the cathode.

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