VACUUM TRIODE
The basic vacuum diode is used to convert the alternating current into direct current. However they cannot amplify the electric signal.In other words , they cannot amplify the voltage or power. To amplify the electrical signal, an extra electrode is required.When the extra electrode is placed between the cathode and anode,the resulting electronic device is called vacuum triode.
As its name only indicates that ,its has three electrodes: cathode ,anode and the control grid.American electrical engineer Lee De Forest invented first electronic amplifying device in 1906 by adding an extra vaccum triode is a 3 electrode device that amplifies the electrical signal.
Electrodes of vacuum triode
Vacuum triode consists of three electrodes:anode ,cathode and control grid.The anode ,cathode and control grid are enclosed in an empty glass envelope.The cathode is surrounded by a control grid, which is an turn surrounded by anode. However ,vacuum triode contains of extra electrode.
cathode emits the free electrons when it is heated .Hence,cathode is also called as emitter, the process by which cathode emits the free electrons when it is heated is called thermonic emission.Anode collects the free electrons that are emitted by the cathode,hence anode or plate is also called as collector.
In between the anode and the cathode,control grid is present,control grid is placed more nearer to the cathode than anode to increase the electric current efficiency.Control grid will control the flow of electrons between the cathode and anode.Hence,controls also called as electron controller or electric current controller.
Control grid is made of network of wires that controls the electrons flow between the cathode and the anode.The space between the network of wires in the grid is very large.Hence,the free electrons move easily from cathode to anode through the opening of the control grid.Free electrons that are moving from cathode to anode will carry the electric current.
Electric field
electric field is the region around a charged particle within which other charged particle will experiences a force. Positively charged particles have positive electric field around them whereas negatively charged particles have negative electric field around them.
if two positively charged particles are placed close to each other, the get attracted. On the other hand, if two like or same charged particles are placed close to each other they get repelled.
In vacuum triode, if a positive voltage is applied to the anode or plate,it becomes positively charged.Hence, anode produces positive electric field towards the free electrons.On the other hand, free electrons emitted from the cathode are negatively charged.Hence free electrons produce negative electric field towards the anode.
The positive electric field of anode has more strength than the negative electric field of free electrons.Hence,free electrons are attracted towards the anode.However ,the distance between the anode and cathode is high.Therefore, if small voltage is applied,small number of free electrons is attracted towards the anode.
On the other hand, the distance between the control grid and the cathode is less (control grid is much closer to the cathode than anode).Hence, a small positive voltage applied to the control grid is enough to attract the free electrons.The free electrons that are attracted towards the control grid will easily move towards the anode.
What is meant by electrode?
The conductor through which free electrons enter or leaves is called electrode.In vacuum triode,cathode is an electrode, which emits the free electrons.In other words,free electrons leave or go away from cathode and enter into vacuum.Anode is an electrode, which collects the free electrons emitted by the cathode. In other words, free electrons that are emitted by the cathode are entered into plate or anode.Control grid is also called as electrode because, it increases the flow of electrons between the cathode and anode.
Directly and indirectly heated cathode
In the vacuum triode, the cathode is heated to emit the free electrons.This can be done in two ways: by directly heating the cathode or indirectly heating the cathode.
if the heat is supplied directly to the cathode,the cathode is said to be directly heated.In this method, the cathode itself is a heater or heating element.Hence, a small amount of heat energy will provides enough energy for the free electrons to escape from the cathode.
The free electrons that are escaped from the cathode will enter into vacuum.These free electrons in the vacuum are attracted towards the anode.In the directly heated cathode, the amount of heat energy required to emit the free electrons is less compared to te indirectly heated cathode.
If the hear is supplied indirectly to the cathode, the cathode is said to be indirectly heated.In the indirectly heated cathode, the cathode, there is no electrical connection between heater and the cathode.
When the heat is supplied to the heater, it gains heat energy. The heat energy gained by the heater is supplied to the cathode. Thus, heat is indirectly supplied to the cathode. When the free electrons in the cathode gain enough energy in the form of heat, they break the bonding with cathode and jumps into vacuum.
Electronics emitted from the cathode depends on the amount of heat applied and work function
The number of free electrons escaped from the cathode depends on the amount of heat applied to the cathode and the work function of the cathode.
If large number amount of heat energy is supplied to the cathode . Similarly, if small amount of heat energy is supplied to the cathode, less number of free electrons is emitted from the cathode.
Work function is the minimum amount of heat energy required to remove the free electrons from the metal. Low work function metals require less amount of heat energy to emit the free electrons. On the other hand, high work function metals require large amount of heat energy to emit the free electrons.
Vacuum triode with zero grid voltage
If no voltage is applied to the control grid and positive voltage is applied to the plate, the vacuum triode behaves like normal vacuum diode, because control grid will not shows any effect on the free electrons emitted from the cathode.
If voltage is applied to the control grid, it produces electric field. In this case, no voltage is applied to the control grid.Hence, control grid will not produce the electric field to attract or repel the free electrons. Therefore, the free electrons emitted from the cathode will easily moves towards the anode or plate from the openings of the control grid.
Vacuum triode with negative grid voltage
If the negative voltage is applied to the control grid without changing the positive plate voltage, no electric current flows in the vacuum triode, because the control grid opposes or repels the free electrons that try to move towards the anode.
Because of this supply of the negative voltage, the control grid becomes negatively electric field.On the other hand, free electrons emitted from the cathode are also negative charged.Hence, free electrons also produce negative electric field.
We know that, if two like or same charges are placed close to each other they get repelled. Hence, the control grid opposes or repels the free electrons emitted from the cathode. However, a small number of free electrons overcome the negative electric field of the grid and move towards the anode.
If the negative field applied to the control grid is increased, no electric current flows in the vacuum triode.
Vacuum triode with positive grid voltage
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