Triode Working Principle

A triode is an electron tube that consists of three electrodes, an anode plate, a cathode filament, and a control grid. A triode is usually used as an amplifier for audio signals, in electrical circuits, and as an oscillator.

Small glass triodes are mainly used as audio amplifiers while the large triodes are used as radio transmitters for radio frequency.

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This segment will highlight the operation and explain the working principle of the triode.

The first process in the working of a triode is called thermionic emission. In this process, heat is applied to the metal cathode to release electrons into the tube.

A separate current flowing through the filament heats the cathode red hot. The filament is a thin metal that conducts electricity. In powerful triodes that generate radio frequency, the cathode is usually the filament itself. However, in most triodes, the filament is separate and heats the cathode electrode.

The air in the tube is removed virtually through electron transfer. This allows the electrons to move freely within the tube.

A positive DC voltage is applied to the anode. The voltage ranges from 20V to thousands. Once the anode is positively charged, the negative electrons are attracted to the anode. A flow of electrons from the cathode to anode is created as the positive and negative electrons attract.

How is the magnitude of currents controlled?

To control the magnitude of currents flowing between the cathode and anode, an appropriate voltage is applied between the grid and the cathode. The grid is the electrons’ gate. If the grid is more negative, it repels some electrons to reduce the current since fewer will get through the anode. A more positive grid attracts more electrons and increases the current since more electrons will get to the anode.


Amplification is a resulting process where a low power varying signal control a much powerful anode current.

If there is variation in the grid voltage, it will result in proportional variations in the anode current.

Voltage gain

This is the result of the placement of appropriate load resistance in the anode circuit to cause the voltage across the resistance to vary and it could be larger than the input voltage variations.

In operation, the triode is similar to the n-channel JFET. However, the difference is that the triode’s anode current depends on the anode voltage and grid voltage.

This is the basic working principle of the triode.