Industrial Electronics Presentation on Vacuum Tubes----- Invention in 1904 A brief overview of Vacuum Tube

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 Vacuum tubes (or electron valves) were invented in 1904 by John
Ambrose Fleming and served as a basic component for electronics
throughout the first half of the 20th century
 Applications included radio, television, radar, sound recording, large
telephone networks, industrial process control and first generation
computers

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 The operating principle of a vacuum tube is based on the process of
thermionic emission
 Vacuum tubes involve the
transfer of electrons between
electrodes. The energy
required to overcome the
forces holding the electrons is
supplied by direct heating.
This provides additional
kinetic energy to the electrons
 At a sufficiently high temperature, some electrons will have enough
kinetic energy to escape from the surface of the material. These escaped
electrons then become the core reason of how a vacuum tube works

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 A vacuum tube is a sealed glass tube containing vacuum which
allows free passage of electric current
 The cathode is the electrode that
contains the heating source from which
electrons are released
 The anode is the electrode that receives
those electrons thus providing an electric
path in the tube
 The grid is an additional electrode by
which the current flowing in the anode
circuit can be controlled by another
potential

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 When there is no grid, vacuum tubes work like a diode
 This function is due to
the fact that electrons will
only move from the
cathode to the anode but
not in reverse as the anode
cannot release electrons
from it’s surface
 Therefore, if an AC
signal is applied, then it
will only allow half the
cycle through, thus
rectifying the signal

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 When a grid is placed between anode and cathode, vacuum tubes
work like a MOSFET
 By applying a potential to the control grid, it is possible to repel or
attract the electrons being emitted from the cathode and in this way
affect the current flow
 When there is no voltage on the grid,
the current between the cathode and
anode will be at its maximum. Placing a
negative voltage onto the grid will have
the effect of repelling some electrons
back to the cathode and thereby
reducing the number of electrons
travelling to the anode.

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 When 2 grids are used, vacuum tubes work as a tetrode. This type of
vacuum tube arose because the traditional triodes were very difficult to
use at frequencies above 50 or 100 kHz for radio applications
 The triode has drawbacks in terms
of gain and stability due to feedback
between the anode and control grid
circuits caused by internal
capacitance
The introduction of the screen grid
reduces the anode-control grid
capacitance and hence the
feedback. This improves the gain
and stability

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 Additional grids add more terminals to a vacuum tube. The following
table shows the naming scheme for such tubes:
Generic Name Total electrodes Number of grids
Diode 2 0
Triode 3 1
Tetrode 4 2
Pentode 5 3
Hexode 6 4
Heptode 7 5
Octode 8 6

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 Following the invention of semiconductor devices in the 1940s, the
use of vacuum tubes was gradually replaced by transistors
 When compared to transistors, vacuum tubes have several
disadvantages:
1. Large size
2. Higher operating voltages
3. Less efficient
4. Glass tubes are fragile
5. Higher cost
6. Needs regular replacement
 A few applications remained, such as the Cathode-ray tube (CRT) in
video monitors, but those too were eventually superseded in the 21st
century

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1. https://www.electronics-
notes.com/articles/electronic_components/valves-tubes/how-does-a-
vacuum-tube-work-theory.php
2. https://www.computerhope.com/jargon/v/vacuumtu.htm
3. https://electronics.stackexchange.com/questions/156301/how-does-
a-vacuum-tube-amplifier-work
4. http://www.effectrode.com/signal-tubes/vacuum-tubes-and-
transistors-compared/

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