Name: Vacuum Tubes and Semiconductors
Uploaded: 2019-07-19
Duration: 8 min 14 s
Description: Vacuum tubes were the first devices where the electronic discharge was realised. Further work on them gave birth to semiconductor electronics. Let's discuss...

Question

Describe the construction and working of a triode valve. Describe a circuit diagram for the study of its characteristics.

Accepted Answer

A triode valve is a vacuum tube device that contains three electrodes- cathode, anode and grid.

Construction- It is construction in such a way that the cathode lies in the centre with the grid surrounding it. Anode or plate then surrounds the grid. The entire structure is encapsulated in a glass tube.

Cathode is a nickel cylinder that is coated with barium and strontium oxide. Plate is made of nickel and surrounds the cathode. Grid is spiral of wire made of nickel or iron. It is placed very close to the cathode. The spiral wire serves a purpose such that the spacings in-between ensures that it does not entirely block the flow of electrons from cathode to the plate.

Working- When the cathode is heated, it emits electrons which reaches the plate. A third electrode called grid is placed between the cathode and the plate, which is used to control the flow of electrons between the cathode and the anode by applying the potential to the grid.

No voltage on the grid- In such a case, the electrons are attracted towards the plate and move through the openings of the grid. This action is similar to the diode.

Positive voltage on the grid- In this case, the grid will start to attract more electrons towards it. This results in the formation of grid current. Grid current is undesirable as it causes power losses and may even damage the grid structure.

Negative voltage on the grid- In this case, the electrons moving from cathode to anode will experience a repulsive force. This results in the reduction of the flow of current. If the negative potential on the grid is increased continuously then at a particular point, the current becomes zero.

Circuit diagram of the triode-

The above circuit diagram is used to study the characteristics of the triode. $P$ represents the plate, $K$ represents the cathode and $G$ represents the control grid. The plate is connected to battery $E_{p p}$ from where it is given positive potential whereas the cathode is heated. The grid is connected to the negative potential. Here, $I_{p}$ denotes the plate current, $E_{p}$ denotes the plate voltage and $E_{g}$ represents the grid voltage.

Describe the construction and working of a triode valve. Describe a circuit diagram for the study of its characteristics.

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