Different types of Vacuum Tubes and its uses.

An Electronic Device,in which the flow of Electrons is through a Vacuum is known as Vacuum Tube.

A Vacuum Tube usually contains a Cathode which is called as Electron Emitter,an Anode which is called as plate (which is electron Collector) and one or more Electrodes called as Grids,for controlling the flow of Electrons between the Cathode and Anode.

These Electrodes are housed in highly evacuated glass envelope.The Anode is held at Positive Potential with respect to Cathode,so that Emitted Electrons are attracted to Anode to provide Current in Vacuum Tube.

The application of Vacuum Tubes are so varied that this has won a place in the Industrial and Commercial fields.These Tubes have been found wide applications in Radio,Long Distance Telephones,Sound Motion Pictures,Televisions,Radar and Industrial Automation.

Though Vacuum Tubes have been replaced by Semi-Conductor Devices these days,they are still used at many places in Electronic Circuits.

According to the number of Electrodes,Vacuum Tubes are classified as below.

01)Vacuum Diode:-

An English Physicist,Sir J.A.Fleming has first invented Vacuum Diode in 1904.This Diode is known as Fleming's Valve.This Valve was so insensitive that it found little immediate applications.Many improvements have been made later.

02)Vacuum Triode:-

An American Scientist Dr.Lee De Forest,has placed a Third Electrode in the form of Wire Mesh,between the Cathode and Anode of Vacuum Diode.The resulting Device was called Triode.It ushered in the Electronics Industry as we know it today.

03)Vacuum Tetrode:-

Although Triode can make Amplification,it presents the major limitations that Plate-to-Grid Capacitance causes feed back particularly at higher Frequencies.

The Plate-to-Grid Capacitance of a Triode can be reduced by inserting an additional Grid,called the Screen Grid,between the Control Grid and Plate.Such a Four Electrode Valve is known as Tetrode.

04)Vacuum Pentode:-

A Suppressor Grid,is inserted in between the Plate and Screen Grid of Tetrode to eliminate the undesirable effects of Secondary Emission.This gives Five-Electrode Valve and known as Pentode.

A Pentode,contains a Cathode,a Anode and Three Grids.The Grid closest to the Cathode is Control Grid,next is Screen Grid and the third is Suppressor Grid.

The Suppressor Grid is connected to the Cathode and serves to Suppress the effects of Secondary Emission.

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