We use screen grid in tetrode to reduce capacitance between the control grid and plate (anode). However, tetrodes have one disadvantage. Whenever the screen grid voltage is more prominent 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 unwanted.
This downside can be overcome by putting an additional a grid called suppressor grid in the middle of screen grid and the plate. The suppressor grid repulses secondary electrons towards anode or plate.
As the name recommends, vacuum pentode comprises 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 emptied glass envelope containing 5 electrodes. The air inside the glass envelope is taken out totally. The 5 electrodes of the pentode include cathode, control grid, screen grid, suppressor grid, and plate.
The cathode is encircled by control grid. The control grid is encircled by screen grid. The screen grid is encircled by suppressor grid. The suppressor grid is encircled by plate or anode.
The development of vacuum pentode is like vacuum tetrode. However, vacuum pentode contains an additional grid (suppressor grid).
Cathode is a negatively charged electrode, which radiates OR emits free electrons when heat energy is provided. It has more number of negative charges (electrons) than positive charges (protons). Therefore, it is negatively charged. Cathode is also referred as emitter.
Plate or anode
Anode is an positively charged electrode, which collects the free electrons radiated by the cathode. It has lesser number of negative charges (electrons) than positive charges (protons). In this manner, it is positively charged. Anode is also referred as collector.
As the name proposes, it is utilized to control the progression OR flow of electrons. Control grid is set among anode and cathode. This grid is put nearer to the cathode than anode to increase the progression of electric flow effectively. Control grid is also sometimes referred as electron controller.
Screen grid is basically used to reduce the capacitance between control grid and anode, and to increase the speed of free electrons. Screen grid is a positively charged electrode set in the middle of control grid and plate or anode. It is set nearer to the control grid than anode to decrease 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 speed of free electrons.
- reduce distortion
The suppressor grid is put between the screen grid and plate. It is used to repulse the secondary electrons (transmitted from anode) back to the anode. It is inside associated with the cathode. Therefore, an enormous number of free electrons are sent directly to the suppressor grid. Due to the acquiring of excess electrons, suppressor grid goes about as a negatively charged electrode and creates negative electric field. Therefore, it repulses the secondary electrons (emitted from anode) back to anode.
Voltage applied across vacuum pentode
Whenever positive voltage is applied to the control grid, screen grid and anode or plate, electric current starts in the vacuum pentode. The suppressor grid is connected inside to the cathode.
The screen grid and control grid are made of network of wires that controls the electron stream among cathode and anode. The space between the network of wires in the screen grid and control grid is exceptionally huge. Thus, the free electrons produced from the cathode streams effectively through the network of wires and attracted to the plate.
Due to the positive stock voltage to the control grid and screen grid, countless free electrons are pulled away from the grids towards the positive electrode. Therefore, control grid and screen grid loses enormous number of electrons. In this manner, they become positively charged and creates solid positive electric field.
The electric field created by the control grid and screen grid has more prominent effect on the free electrons than plate electric field. This is because, control grid and screen grid are nearer to the cathode.
As a result of the huge space between the wires of screen grid, the majority of the free electrons attracted to the screen grid, will go effectively through the wires and arrives at anode. However, a few electrons attracted to the screen grid were seen by the wires of a screen grid. Therefore, a deficiency of electric current happens. However, this electric current is negligible compared with the electric current from cathode to anode.
The free electrons emitted from cathode travel at rapid. Therefore, when the electrons emitted or radiated from the cathode strike the plate surface at fast, they move their kinetic energy to the valence electrons in the plate. Therefore, the valence electrons in the plate acquires adequate energy and beat the solid attractive force of the plate and get away or transmitted 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 realize that suppressor grid is connected directly to the cathode. Therefore, the suppressor grid is likewise negatively charged. This negatively charged suppressor grid produces a negative electric field. The negative electric field created 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 an repulsive power from the suppressor grid and attractive force from the plate. As a result, the secondary electrons return to the plate or anode.
In this way, the suppressor grid decrease the unwanted secondary electron current from plate to screen grid.
Applications of pentode
Pentodes are generally utilized in radios and TVs until 1960s. After 1960s they were replaced by transistors. However, they kept on being utilized in certain applications like electric guitar speakers, microphone preamplifiers, high-power radio transmitters, and professional sound applications.
Advantages of pentode
- Pentodes can work at high frequencies
- Pentodes have high amplification factor than tetrodes