We use screen grid in tetrode to reduce capacitance between the control grid and plate (anode). However, tetrodes have one drawback. When the screen grid voltage is greater than the plate voltage, the secondary electrons emitted from the plate are attracted to the screen grid. Because of this, the electric current flows in reverse direction (from plate to screen grid) which is undesirable.
This drawback can be overcome by placing an extra grid called suppressor grid in between screen grid and the plate. The suppressor grid repels secondary electrons towards anode or plate.
As the name suggests, vacuum pentode consists of five electrodes: cathode, control grid, screen grid, suppressor grid, and plate or anode. Bernard D. H. Tellegen invented the pentode in 1926.
Electrodes of vacuum pentode
The pentode is made of evacuated glass envelope containing 5 electrodes. The air inside the glass envelope is removed completely. The 5 electrodes of the pentode include cathode, control grid, screen grid, suppressor grid, and plate.
The cathode is surrounded by control grid. The control grid is surrounded by screen grid. The screen grid is surrounded by suppressor grid. The suppressor grid is surrounded by plate or anode.
The construction of vacuum pentode is similar to vacuum tetrode. However, vacuum pentode contains an extra grid (suppressor grid).
Cathode
Cathode is a negatively charged electrode, which emits free electrons when heat energy is supplied. It has more number of negative charges (electrons) than positive charges (protons). Therefore, it is negatively charged. Cathode is also sometimes referred as emitter.
Plate or anode
Anode is a positively charged electrode, which collects the free electrons emitted by the cathode. It has lesser number of negative charges (electrons) than positive charges (protons). Therefore, it is positively charged. Anode is also sometimes referred as collector.
Control grid
As the name suggests, it is used to control the flow of electrons. Control grid is placed between anode and cathode. This grid is placed closer to the cathode than anode to increase the flow of electric current efficiently. Control grid is also sometimes referred as electron controller.
Screen grid
Screen grid is mainly used to reduce the capacitance between control grid and anode, and to increase the velocity of free electrons. Screen grid is a positively charged electrode placed in between control grid and plate or anode. It is placed closer to the control grid than anode to reduce the capacitance efficiently.
Uses of screen grid
The screen grid is mainly used to:
- Reduce the capacitance between control grid and plate.
- Increase gain
- Increase control over electron flow
- Increase velocity of free electrons.
- Reduce distortion
Suppressor grid
The suppressor grid is placed between the screen grid and plate. It is mainly used to repel the secondary electrons (emitted from anode) back to the anode. It is internally connected to the cathode. Therefore, a large number of free electrons are transmitted directly to the suppressor grid. Because of the gaining of excess electrons, suppressor grid acts as a negatively charged electrode and generates negative electric field. Therefore, it repels the secondary electrons (emitted from anode) back to anode.
Voltage applied across vacuum pentode
When positive voltage is applied to the control grid, screen grid and anode or plate, electric current starts flowing in the vacuum pentode. The suppressor grid is connected internally to the cathode.
The screen grid and control grid are made of network of wires that controls the electron flow between cathode and anode. The space between the network of wires in the screen grid and control grid is very large. Hence, the free electrons emitted from the cathode flows easily through the network of wires and attracted to the plate.
Because of the positive supply voltage to the control grid and screen grid, a large number of free electrons are pulled away from the grids towards the positive terminal. As a result, control grid and screen grid loses large number of electrons. Therefore, they become positively charged and generates strong positive electric field.
The electric field generated by the control grid and screen grid has greater influence on the free electrons than plate electric field. This is because, control grid and screen grid are closer to the cathode.
Because of the large space between the wires of screen grid, most of the free electrons attracted to the screen grid, will pass easily through the wires and reaches anode. However, some electrons attracted to the screen grid were observed by the wires of a screen grid. As a result, a loss of electric current occurs. However, this electric current is negligible compared to the electric current from cathode to anode.
The free electrons emitted from cathode travel at high speed. Therefore, when the electrons emitted from the cathode strike the plate surface at high speed, they transfer their kinetic energy to the valence electrons in the plate. As a result, the valence electrons in the plate gains sufficient energy and overcome the strong attractive force of the plate and escapes or emitted from the plate surface. These emitted electrons from the plate surface are called secondary electrons and the electrons, which hit the plate surface, are called primary electrons.
We know that suppressor grid is connected directly to the cathode. Therefore, the suppressor grid is also negatively charged. This negatively charged suppressor grid generates a negative electric field. The negative electric field generated by the suppressor grid is weaker than the anode electric field. However, this electric field is enough to control the secondary electrons emitted from anode or plate.
The secondary electrons emitted from the plate surface experience a repulsive force from the suppressor grid and attractive force from the plate. As a result, the secondary electrons return to the plate or anode.
Thus, the suppressor grid reduce the unwanted secondary electron current from plate to screen grid.
Applications of pentode
Pentodes are widely used in radios and televisions until 1960s. After 1960s they were replaced by transistors. However, they continued to be used in some applications such as electric guitar amplifiers, microphone preamplifiers, high-power radio transmitters, and professional audio applications.
Advantages of pentode
- Pentodes are able to operate at high frequencies
- Pentodes have high amplification factor than tetrodes
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