Grid current

As a grid current that is electric current referred to, which at a electron tube via the control grid or screen grid flows. Since electron tubes are ideally voltage-controlled, the grid current is a mostly undesirable side effect of real electron tubes. Electron tubes with a particularly low grid current are called electrometer tubes .

Since screen grids in electron tubes are generally operated with a positive bias voltage, but control grids with a negative bias voltage , the processes differ from the point of view of the grid currents.

Causes of the control grid current

When an electric voltage is applied between cathode and anode (anode voltage) to an electron tube forms a stream of electrons formed by the electric voltage can be influenced of the control grid. The magnitude of the grid current is essentially a function of the anode voltage and the grid voltage, based on the cathode potential.

The grid current is the sum of different current components that have different physical causes. Individual components of the grid current run in opposite directions and can compensate each other in certain operating points of the tube. Depending on the direction of the individual grid current components, a distinction is made between positive and negative grid current components.

If the grid is not connected, an equilibrium of the electric charge is created on the grid, which prevents the influx of further electrons.

The control grid current is undesirable because it only flows when the control grid is more positive than the cathode. Then it represents a finite resistance of this route, which loads the mostly high-resistance control voltage source and therefore leads to distortions. Furthermore, the ideal of powerless control of the anode current is no longer given.

The sum of the components of the grid current shown below and other components together give the grid current.

Electron flow

The electrons emerging from the heated cathode form a space charge and are accelerated towards the anode on average by the potential gradient. Some electrons of this cloud also hit the grid and cause a positive grid current component. This grid current component I _ge represents an exponential function for negative grid voltages U _g <0 V :

${\displaystyle I_{\mathrm {ge} }=I_{\mathrm {ge0} }\cdot e^{\frac {U_{\mathrm {g} }}{U_{\mathrm {T} }}}}$

I _ge0 expresses the maximum grid current component at U _g = 0 and U _T corresponds to the temperature voltage:

${\displaystyle U_{\mathrm {T} }={{k\cdot T} \over q}}$

with the Boltzmann constant k , the elementary charge q of an electron and T the absolute temperature .

The point at which the electron flow begins is almost independent of the anode voltage and, depending on the type of tube, has values ​​between U _g = −3 V to −0.1 V.

Secondary electrons

Secondary electrons knocked out of the anode also contribute to the control grid current. At high anode voltages, they can have enough energy to counter the negative field of the grid bias and land on the control grid. This causes a positive grid current. For this reason, too, screen grids are inserted between the control grid and anode in some electron tubes (among other things with tetrodes ). This screen grid is connected to a relatively high and low-resistance positive direct voltage source and thus shields the control grid from the secondary electrons of the anode. This type of grid current can therefore only occur with triodes .

Likewise, electrons emanating directly from the cathode and accelerated by the anode can have sufficient energy to release secondary electrons from the control grid itself, which are accelerated further to the anode. This effect also ultimately represents a current flow that corresponds to a negative grid current.

Both effects increase with increasing anode current.

Thermal lattice emission

Since the control grid is arranged close to the cathode, heating of the grid cannot be avoided. As a result of slight evaporation of the oxide cathode layer, which condenses on the cooler control grid, electrons are emitted by the control grid, which are accelerated in the direction of the anode and thus also cause a negative grid current.

The thermal grid emission is primarily dependent on the tube construction, the heating voltage, which indirectly determines the grid temperature, and the manufacturing quality.

Ion current

In contrast to the electron current , the ion current represents a negative part of the grid current. The cause of this part is the inadequate vacuum inside the electron tube. Due to the electron flow from the cathode to the anode, with a correspondingly high potential difference, individual electrons are accelerated so strongly that their kinetic energy is sufficient to ionize them in the event of accidental collisions with gas molecules. The positively charged ions are then attracted by the negatively charged grid and thus represent part of the negative grid current.

The ion current is a function of the anode current, the anode voltage, the gas pressure in the tube and the mean free path, which corresponds to the distance between anode and cathode and is therefore essentially determined by the tube construction.

Isolation current

The insulation current is also a negative grid current, but with only a small proportion. Parasitic insulation resistances occur between the electrical connections of the electron tube, especially when using inferior types of glass as insulators with relatively high electrical conductivity. The insulation resistance between the anode and the grid is dominant, as the greatest potential difference usually exists between these connections. The insulation grid current I _gi can be described to a good approximation as a linear function of the anode-grid voltage U _ag and the insulation resistance R _ag between anode and grid as:

${\displaystyle I_{\mathrm {gi} }={\frac {U_{\mathrm {ag} }}{R_{\mathrm {ag} }}}}$

Ultimately, the insulation current also represents a negative grid current.

Photocurrent

Among the residual grid currents, which are negligible in most applications, the negative photocurrent, which is independent of the grid voltage, takes up the largest share. The underlying physical effects are used in photocells , but are generally undesirable with conventional electron tubes. The photocurrent is partly triggered by external light from the outside and by photons , starting from the glowing cathode, on the grid. It is essentially a function of the heating capacity of the tube. Short-wave light in particular can trigger electrons from the surface of the grid. The photocurrent also represents a negative grid current.

Causes of the screen grid current

The screen grid of tetrodes and pentodes is operated with a positive voltage in order to achieve a uniform attraction of the electrons from the cathode. Since not all electrons are accelerated through the turns of the grid, but some hit the turns themselves, a screen grid current results . This can be done by constructive measures such. B. "hide" the grid windings from the cathode point of view behind the exactly similarly manufactured control grid can be reduced, but it cannot be completely suppressed in practice. (Example: EL90)

The screen grid current is undesirable because

• he needs power from the power supply, but otherwise has no positive benefit,
• through him the power distribution noise comes about.

Dynamic operation

When the grid is controlled with a time-varying voltage, a time-varying current flows into the grid, which is a consequence of a displacement current through the electrical capacitance of the spatial electrode arrangement. It depends, among other things, on the rate of change of the voltage and the geometric structure of the tube and occurs with every capacitor in dynamic operation. When the tube is controlled with harmonic alternating voltage, a phase shift between the control voltage and the displacement current can be measured, which is also referred to as reactive current .

Another effect that only occurs in dynamic operation is related to the finite transit time of the electrons between the cathode and the control grid. If the grid voltage changes while an electron passes the control grid, an interaction with the charge of the control grid occurs. The electron is decelerated or accelerated. These changes in speed can only be achieved by using appropriate energy. Outwardly, this is shown by a decrease in the input resistance with increasing frequency. One consequence is that amplifier tubes on VHF, despite the cathode base circuit, only have an input resistance of a few kiloohms, although the control grid is never positive.

With selected tube types (e.g. PC86, PC88) the grid-cathode distance was reduced to very small values ​​in order to minimize the runtime effect. These still achieve sufficient gain in UHF band IV (up to 860 MHz).

These currents are not counted directly to the grid currents, which are line currents , but to the reactive currents , which only occur in dynamic operation.

Literature sources

• Josef Schintlmeister: The electron tube as a physical measuring device . 4th edition. Springer Verlag, Vienna 1944.
• Curt Rint (ed.): Handbook for high frequency and electrical technicians . tape I. . Verlag für Radio-Foto-Kinotechnik, Berlin-Borsigwalde (1949/1953).
• F. Bergtold: Röhrenbuch for radio and amplifier technology . Weidmannsche Buchhandlung, Berlin 1936.