We learned in school that atoms comprise a nucleus of positively charged
protons and neutral neutrons with a cloud of electrons orbiting that nucleus. At
typical temperatures, some electrons will randomly gain enough energy to escape
the atom but will soon lose that energy--by colliding with other nearby
particles, etc.-- and return to the atom. Others may escape the atom to be
replaced by other nearby free electrons. Therefore, an atom will typically have
one electron in the cloud for each proton in the nucleus.[1]
If heat is applied to the material, more electrons will gain enough energy to
escape the atoms. For example, a heated, negatively charged object will
discharge and become neutral. If the hot material is inside a vacuum chamber,
many electrons will stream away from the material and impact the walls of the
vacuum vessel. This phenomenon was discovered by Edmond Becquerel in 1853 but
rediscovered by Thomas Edison in 1873. The cause was a mystery before Joseph
Thompson discovered the electron in 1897, after which the phenomenon was called
the Edison Effect.
The vacuum tube diode
An early incandescent lightbulb was a vacuum chamber with a hot carbon
filament at the center.[2]
Experimenting with this phenomenon, Edison constructed lightbulbs with a metal
cylinder surrounding the filament. Using a galvanometer, he measured no
detectable current unless he connected a battery between the filament and the
cylinder with the positive terminal at the cylinder. Then he measured a
significant current. Thus, Edison invented a one-way valve that passed current
in one direction but not the opposite. After the electron was discovered, Owen
Richardson measured the energy required for an electron to escape different
materials. He also gave the phenomenon the modern name of thermionic emission.
John Fleming developed the first practical vacuum tube diode to detect radio
waves. They were soon also used to rectify alternating current in DC power
supplies.
Practical vacuum tubes (aka valves) usually have a cylinder surrounding the filament. The
filament heats this cylinder which becomes the cathode that emits electrons.
The vacuum tube triode
Culminating in 1907, Lee de Forest developed the vacuum tube triode. This had
an electrode consisting of a spiral of wire or a grid between the cathode and
the positive anode (the plate).
Structure of a vacuum tube triode.
Schematic symbol for a vacuum tube triode.
Applying a small negative voltage to this grid(compared to the voltage
at the filament) controls the current flow from the cathode to the anode. A
small variation in voltage at the grid causes a large variation in current flow
to the anode, creating the first electronic amplifier circuit.
Later developments
Walter Schottky discovered that a second grid with a slight positive charge,
placed between the cathode and the control grid, partially neutralized
undesirable capacitance between the cathode and the anode. His two-grid tubes
with such a screen grid were called tetrodes (having four electrodes, the
cathode, the screen grid, the control grid, and the anode). However, the
presence of the screen grid placed a lower limit on the anode voltage. Electrons
striking the anode cause secondary emission of electrons, which will travel to
the screen of a tetrode. This causes instability and can cause the tube to
exceed its power capability. In 1926, Bernard Tellegen developed the pentode by
adding a suppressor grid between the anode and the screen grid. The suppressor
grid is usually connected to the cathode to receive the necessary negative
voltage.
Schematic symbol for a vacuum tube pentode.
Before the invention of the transistor, vacuum tubes did the jobs solid-state
devices do today. Some vacuum tubes are still in use. The most common is the
magnetron, which generates microwaves for ovens and radar.
A design for a triode audio amplifier from a discussion at
stackexchange.com. The filiment and its power supply are not shown
to simplify the diagram.
Vacuum tube circuits will not be discussed in detail in this class as they
are uncommon in modern electronics; they should be covered as a specialty.
Anayzing Vacuum Tube Circuits with Conventional Current
