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The space charge that surrounds the cathode tends to prevent further electrons from being emitted and forms an equillibrium. The positive electric field provided by the anode neutralises the space charge and starts a stream of electrons to flow from cathode to anode. This stream is called the space current, or more commonly the anode current.

The resistance offered to the flow of anode current is known as the anode impedance or internal resistance of the valve. It depends on both the anode voltage and th physical construction of the valve.

The grid.

The introduction of a control grid turns a 2-electrode diode into a 3-electrode triode. It normally consists of a mesh or spiral of wire in close proximity to the cathode. A positive potential on the grid will accelerate electrons from the cathode, and a negative potential will retard them. Thus the grid controls the anode current.

When the grid is positive it acts as an anode and establishes a grid-cathode current known as positive grid current.

Secondary emission.

Secondary emission is the result of bombardment by high velocity ions or electrons. If a surface releases electrons easily when bombarded this can generate a considerable secondary electron current and affect the functioning of a valve. Secondary emission from the glass walls of a valve reduce the internal resistance at high frequencies, and often valves have an internal metallic or carbon coating joined to the cathode to reduce this effect. When secondary emission occurs from the grid this produces a negative grid current. Often to prevent this the grid is fitted with heat radiation fins to attempt to keep it cool. In addition the anode may be made of a mesh rather than a solid plate in order to reduce the effects of reflected radiation heating the grid.

When the grid modifies the electron stream the internal conductance of the valve is altered. The factor which defines this is given the abbreviation µ, and is commonly known as the amplification factor, or more correctly the voltage amplification factor. When this factor is applied the internal conductance of the valve becomes the mutual conductance, or transconductance..

 

This file was last modified 15:13:37, Tuesday September 02, 2014