Vacuum tube

The introduction of vacuum tube at the beginning of the 20th century was the starting point of the rapid growth of modern electronics. With vacuum tubes the manipulation of signals became possible, which could not be done with the early telegraph and telephone circuit or with the early transmitters using high-voltage sparks to generate radio waves.

The simplest vacuum tube, the diode, contains two electrodes: the cathode, a heated filament or a small, heated, metal tube that emits electrons through thermionic emission; and the anode, or plate, which is the electron-collecting element. In diodes, the electrons emitted by the cathode are attracted to the plate only when the latter is positive with respect to the cathode. When the plate is negatively charged, no current flows through the tube. If an alternating potential is applied to the plate, the tube passes current only during the positive halves of the cycle and thus acts as a detector.

The introduction of a third electrode, called a grid, interposed between the cathode and the anode, forms the triode, which for many years was the basic tube used for amplifying current. The function of the grid is to control the current flow. At a certain negative potential, the grid, because it repels electrons, can impede the flow of electrons between the cathode and the anode. At lower negative potentials, the electron flow depends on the grid potential. The grid usually consists of a network of fine wires surrounding the cathode. The capacity of the triode to amplify depends on the small changes in the potential between the grid and the cathode causing large changes in the number of electrons reaching the anode. Thus, the basic components of a triode vacuum tube are comparable to those of the transistor which include the emitter corresponding to the heated cathode of the triode tube as the source of electrons.

Vacuum tubes have now been almost entirely replaced by transistors, which are cheaper, smaller, and more reliable. Tubes still play an important role in certain applications, however, such as in power stages in radio and television transmitters, and in scientific and military instruments that must resist strong voltage pulses which destroy transistors. The cathode raytube is still a base of some kind of displays, e.g. in radar equipment.

Grid Anode

Cathode Anode

Grid

Heated Cathode

Appearance Electrodes (the glass tube Circuit symbol

Is broken)