Vacuum Tetrode- Definition, Working, Uses, Behavior According to Grid Voltage

We use triode to enhance the electrical signal, however at higher frequencies, it will acts as an oscillator rather than as an amplifier. Adding the additional grid (screen grid) between the control grid and the plate or anode diminishes OR reduces the undesirable capacitance among plate and the control grid.

As the name recommends, vacuum tetrode comprises of four terminals: cathode, anode, control grid, and screen grid. The American physicist and electrical engineer Albert Wallace Hull created the tetrode vacuum tube in 1926.

Electrodes of vacuum tetrode

Vacuum tetrode comprises of four terminals: cathode, anode, control grid, and screen grid. The cathode, anode, control grid, and screen grid are encased in a vacant glass envelope. The cathode is encircled by control grid. The control grid is encircled by the screen grid. The screen grid is encircled by the anode or plate.

symbol of vacuum tetrode

The development of vacuum tetrode is like vacuum triode. However, vacuum tetrode contains an additional electrode called screen grid.

Cathode

Cathode is an electrode that transmits the free electrons when warmed. In some cases referred as emitter. Cathode has more number of negative charges (electrons) than positive charges (protons). In this manner, it is negatively charged.

Anode or plate

Anode is a positively charged electrode that gathers the free electrons emitted from the cathode. It is referred as collector. Anode has lesser number of negative charges (free electrons) than positive charges (protons). In this way, it is positively charged.

Control grid

Control grid is set between the cathode and plate. This grid is set nearer to the cathode than the plate to increase the electric current proficiently. Controls grid present between the cathode and plate controls the progression of electrons. Hence, control grid is also called electron controller.

Screen grid

Screen grid is put between the control grid and plate. This grid is set nearer to the control grid than plate to reduce the capacitance efficiently. Screen grid acts as an electrostatic shield to protect the control grid from the positive electric field or impact of the plate when its potential changes.

Main functions of screen grid

The main functions or primary elements of screen grid are:

  • To speed up and attract free electrons to the plate or anode
  • To reduce the capacitance between the plate and control grid
  • To increase the control over electron flow
  • To increase the vacuum tube efficiency
  • Decrease distortion
  • Increase gain

The control grid and screen grid are made of network of wires that controls the electron flow among cathode and anode. The space between the network of wires in the control grid and screen grid is extremely huge. Hence, the free electrons move effectively from cathode to anode through the opening of grids.

Vacuum tetrode with zero grid voltage and positive plate voltage

Assuming no voltage is applied to the screen and control grids, and positive voltage is applied to the plate, the vacuum tetrode behaves like a vacuum diode. Since control grid and screen grid doesn’t show any impact on the free electrons emitted from the cathode.

Vacuum tetrode with zero grid voltage and positive plate voltage

Vacuum tetrode with negative grid voltage and positive plate voltage

Assuming negative voltage is applied to the control grid and screen grid without changing the plate voltage, no electric current flows in the vacuum tetrode. Since control grid and screen grid goes against or repulses the free electrons that attempt to move towards the anode.

As a result of the stock of negative voltage, the control grid and screen grid produces a solid negative electric field. On the other hand, free electrons emitted from the cathode are negatively charged. Hence, free electrons additionally create negative electric field. However, the electric field created by the control grid and screen grid is a lot more stronger than the electric field of free electrons.

Vacuum tetrode with negative grid voltage and positive plate voltage

We realize that, when two same charges are put near one another, they get repulsed. Hence, in light of this solid negative electric field, the electrons discharged or emitted from the cathode are repulsed by this solid negative electric field.

Thus, no electrons flow from cathode to anode. Therefore, no electric current flows through a vacuum tetrode, when negative voltage is applied to the grids.

Vacuum tetrode with positive grid voltage and positive plate voltage

Assuming positive voltage is applied to the control grid and screen grid without changing the positive plate voltage, electric current flows in the vacuum tetrode.

As a result of the supply of positively voltage, both control grid and screen grid will turn out to be positively charged and produces solid positive electric field.

Vacuum tetrode with positive grid voltage and positive plate voltage

The positive voltage applied to the screen grid is rather lower than the plate voltage. However, the electric field of the screen grid has more prominent impact on the free electrons. Due to the enormous space between the wires of the screen grid, the vast majority of the free electrons attracted to the screen grid will pass to the plate. Thus, screen grid acts as an electrostatic force pulling electrons from cathode to anode. Therefore, the screen grid acts as the primary anode and plate goes about as the secondary anode.

However, a few electrons pulled in towards the screen grid were seen by the screen gird. As a result, an undesirable screen grid current happens. However, this electric current is negligible compared with the electric current flow from cathode to anode.

As long as the plate voltage is higher than the screen grid voltage, plate current in the vacuum tetrode exceptionally relies or depends upon the screen grid voltage and significantly less on the plate voltage.

The screen grid is a lot nearer to the cathode than anode or plate. Hence, the electric field created by the screen grid has more prominent effect on the cathode space chare (free electrons).

If positive voltage applied to the screen grid is increaseed to a worth higher than the plate voltage, the electrons transmitted to anode or plate will return towards the screen grid. This is because, if we increase the screen grid voltage to a worth higher than the plate voltage, screen grid creates more stronger electric field than the plate electric field. Thus, electrons from the plate are attracted towards the screen grid. Hence, a reverse current streams in the vacuum tetrode, which isn’t attractive or desirable.

Numerous huge TV stations, radios, and industries utilize giant power tetrodes which works proficiently when utilized as RF power amplifiers.

Secondary electron emission in tetrode

The secondary electron emission is the emission of secondary electrons from the metal surface when primary electrons strike the metal surface at high speed.

In tetrode, the secondary electron emission happens mainly at anode or plate. Whenever the electrons strike the anode or plate at high speed, a few secondary electrons are emitted from the plate surface. What happens to these secondary electrons transmitted or emitted from plate surface is relies or depends upon voltage applied across screen grid and plate.

Case 1: If voltage applied across screen grid is more prominent than plate voltage

For this situation, screen grid has higher potential than plate. Hence, screen grid produces greater electric field than plate.

Whenever primary electrons strike the plate at high speed, they move their kinetic energy to the electrons in the plate. Hence, the electrons in the plate gain sufficient energy and defeat the solid attractive force of the plate and escapes from the plate surface.

The electrons got away from the plate surface experience a more prominent attractive force from the screen grid than the plate. Hence, the secondary electrons emitted or transmitted from the plate surface are attracted the screen grid. In this way, electric current flows in reverse direction (from plate to screen grid), which isn’t desirable.

Case 2: If voltage applied across plate is more prominent than screen grid

For this situation, plate or anode has higher potential than screen grid. Hence, plate produces more prominent electric field than screen grid.

At the point when electrons strike plate at high speed, a few secondary electrons are transmitted from the plate surface. The electrons emitted from the plate surface experience more prominent attractive force from the plate than the screen grid. Thus, the secondary electrons emitted from the plate surface return of anode. Therefore, electric current flows in forward heading (from screen grid to anode).

We generally need to keep the plate potential higher than the grids potential (screen grid and control grid), to efficiently increase the electric current.

Leave a Reply