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Below is a numerical simulation of voltages and fields on the cathode
portion of the electron gun. The accelerator plate is held at 100
V, which pulls electrons off of the grounded, super-heated cathode
plate. The choke voltage is varied from 0 to -100 V and diminishes
the strength of the fields so that beam intensity can be modulated.
This is how the brightness of a video signal is changed from one point
on the screen to another. |
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Cathode Fields (Choke Varied
from 0 to -100 V) |
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Click on the thumbnails to see the color voltage plots
for this simulation. |
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Top View |
Perspective View |
Side View |
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Notes: |
- For the animated contour plot, the green lines are positive
voltage contours, the red lines are negative voltage contours, and the
white line is the zero-voltage contour. Contours are spaced
every 10 V.
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- Remember that electrons are negative and accelerate opposite
the direction of electric fields. Thus, the green fields
pointing towards the cathode will pull electrons off and the red
fields pointing away from the cathode will hold electrons on the
surface. In terms of the 3D voltage plots, we say that positive
charges roll "downhill" and negative charges roll "uphill". The
side view of the voltage plot shows this most clearly with a
longitudinal cross-section of the electron gun (horizontal axis is
distance and vertical axis is voltage). Find the choke voltage
threshold that causes voltages to slope "downhill" just in front of
the cathode.
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- A similar cathode/choke configuration is used in some vacuum tubes
that pre-date the transistor age. The choke voltage was used to
modulate currents and provide signal amplification. A relatively
weak signal on the choke placed near the cathode could force large
current drops from cathode to anode (the positive terminal on the
other end of the vacuum tube). Early radios, televisions, and
computers used many vacuum tubes to perform switching and signal
processing.
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- At about -50 V, the choke has completely reversed the polarity of
electric fields above the cathode. Below -50 V, there is a
fairly linearly relationship between field strength and choke voltage.
Remember, the stronger the fields, the more electrons will be pulled
off the cathode and hurled into the picture screen at the other end of
the tube.
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