Triode
A triode is an electrical circuit component that uses a small voltage to control a relatively large flow of current. A triode contains three electrodes, a cathode, an anode, and a control grid. The name triode is derived from the term tri-electrode (i.e., three electrodes).
The triode is actually a variation on the diode, a device that has just two electrodes sealed in vacuum. One electrode, the cathode, is attached to a negative electric potential, the other electrode, the anode, is attached to a positive potential. This device allows current to pass through it in one direction only.
Under normal operating conditions, the cathode is heated to high temperature and the anode remain cooler. Current flow through the device can be increased by a further increase in the temperature of the cathode, by increasing the potential difference between the cathode and anode, or by shortening the physical distance between the cathode and anode.
With a triode, additional control over current flow is achieved by introducing a third electrode between the cathode and anode. Usually this electrode is placed much closer to the cathode than the anode and, instead of being a small metal plate like the anode and cathode, is a grid with holes. The additional control over current flow is caused by the fact that the electrons streaming from the hot cathode must pass through the grid (usually made of a wire mesh) on their journey to the positive anode.
The addition of the grid, hence the creation of a triode, completely revolutionizes the diode's characteristics. The grid is a powerful controller of the electron stream flowing from the cathode to the anode. If it is slightly negatively charged it entirely chokes off the flow of electrons since they are sensitive to the repulsive force from the grid. Also it only needs a small negative potential to cause this effect. More importantly, it is interesting to consider the effect of a small positive charge on the grid. The cloud of electrons gathered around the hot cathode are repelled by the negative charge of the cathode but they are also powerfully attracted to the positive charge of the grid which is very close to them. They speed towards this positive charge and their momentum carries them through the holes in the grid and towards the strong positive potential of the anode. A very small increase in the positive charge on the grid brings about a disproportionately large increase in the electron stream flowing from cathode to anode, and large current changes are accomplished with small voltage changes.
These properties of triodes allowed them to be used in radio amplification circuits. A small vibrating electrical signal applied to the grid of a triode will translate into a large signal output from the triode. Accordingly, a small input signal is magnified, or amplified.
A similar effect can be produced in a modern version of the triode, the n-p-n or p-n-p transistor. In either case, a small variation in voltage to the core of the triode causes a large response in potential across the outside electrodes. This mirrors the effect of a small variation on the triode grid producing a large response across cathode and anode. Because of their robustness and the fact they can be produced in extremely small sizes transistors have virtually supplanted triodes in amplifier circuit design.
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