The Diode

Author: E.E. Kimberly

An electron tube with two elements only, as described, is called a diode. Fig. 27-1 shows a schematic diagram of a simple diode circuit. The filament or cathode F is heated by the battery A. The battery B produces the negative charge on the filament and the positive charge on the plate. The ammeter I indicates the flow of electrons constituting the electric current, which may be put to useful service. The maximum rate at which electrons may be drawn from the cathode cloud at high plate voltages is dependent on the number of electrons available. The rate at which electrons are "boiled off" the hot cathode and made available is determined by the cathode temperature. A curve of plate current vs. plate voltage like each of those shown in Fig. 27-2 is called the saturation curve of the electron tube.

Fig. 27-2 Saturation Curves for Cathode Temperatures T1, T2 and T3

Electrons of the space charge, by their repelling action on electrons at the surface of the cathode and by their proximity thereto, tend to limit the number of electrons emitted. For this reason, at the rated plate voltage or any lower voltage, the plate current of any particular electron tube is limited chiefly by the space charge and is said to be "space-charge limited" The characteristic curve flattens off because, for any particular cathode temperature, only a limited number of electrons can be emitted. Tubes operated in the flattened portion of the curve are said to be " temperature limited." Radio tubes are normally operated in the space-charge-limited range or the range below the knee of the curve in Fig. 27-2. This becomes important where high peak currents are required from the tube.

For a flat anode and cathode the characteristic curves of Fig. 27-2 for any cathode temperature may be expressed by Child's Law as long as the cathode is emitting freely and the current is therefore not space-charge limited. Under such conditions,

where d is the spacing between anode and cathode, in centimeters.