1-19
applications in other NEETS modules and later will see the diode in several pieces of electronic
equipment.
As with all inventions, Fleming’s diode was immediately the subject of much experimentation and
many attempts at improvement. An American experimenter, Dr. Lee De Forest, added another active
element to the diode in 1906. He was trying to improve the radio application of Fleming’s diode. His new
tube was eventually called a triode.
DeForest’s triode was not very successful as a radio "detector." (Detectors will be studied in a later
NEETS module.) However, in 1912, De Forest discovered that his original triode could AMPLIFY or
magnify very weak electrical impulses. It is because of the triode’s ability to amplify that De Forest is
honored as one of the great radio pioneers.
The immediate application of the triode amplifier was in telephone and radio. Both fields were
limited because electrical impulses (signals) became weaker and weaker as the distance from the signal
source increased. The triode, along with other developments of the time, made long-distance
communications possible. Looking back, we can now see that the amplifying tube was the real beginning
of modern electronics and influenced everything that followed. Let’s find out more about the idea of
amplification and how it is done in the triode.
You are already familiar with a type of amplification. In a previous NEETS module, step-up
transformers were discussed. You should remember that an input voltage applied to the primary of a step-
up transformer is increased in amplitude at the secondary by a factor determined by the step-up turns
ratio.
For example, if 5 volts were applied to the primary of a 1:3 step-up transformer, the secondary would
produce 15 volts. In other words, the input voltage was amplified by a factor of 3. When applied to
electronic circuits, these primary and secondary voltages are more often called signals, or input and
output signal, respectively. In electronics, the amplitude of an input signal must sometimes be increased
many times-often, hundreds or thousands of times!
Because of size and design limitations, transformers are usually not practical for use in electronics as
amplifiers.
DeForest’s first experiment with the diode was to place an additional metal plate between the cathode
and plate. He then placed an ac signal on the metal plate. When the circuit was energized, De Forest
found that the ammeter stayed on zero regardless of the polarity of the input signal.
What was happening was that the new element was blocking (or shadowing) the plate. Any electrons
attempting to reach the plate from the cathode would hit the new element instead. As the circuit didn’t
work, it was back to the drawing board.
In his next attempt, De Forest decided to change the element between the cathode and the plate.
Instead of a solid metal plate, he used a wire mesh. This would allow electrons to flow from the cathode,
THROUGH THE WIRE MESH, to the plate. This tube circuit is shown in figure 1-15. In view (A) you
see De Forest’s circuit with 0 volts applied to the third element, (today called a control grid or
occasionally just the grid). Under these conditions, assume that the ammeter reads 5 milliamperes. With
no voltage applied to the grid, the grid has little effect on the electron stream. For all practical purposes,
the control grid is not there. Most electrons flow through the open mesh. The tube functions as a diode.
