Conventional Tubes - Vacuum Tubes

The vacuum tubes such as Triode, Tetrode, Pentode etc., these are the examples of conventional tubes.

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Vacuum tubes

• Vacuum tubes are the electronic devices in which electrons flow through vacuum from one electrode to another electrode. 
• Vacuum tubes contain 1 cathode, 1 anode and one or more than one grids.
• Grids are used for controlling actions.
• Since, vacuum tubes are voltage-controlled devices so, the grid voltages will be the controlling voltages.
• The vacuum tubes can be operated in very high voltages, and these can generate high powers also, but these microwave tubes are useful below microwave frequencies only because at microwave frequencies, these conventional microwave tubes have some limitations.

High Frequency Limitations of Conventional Tubes

1. Conventional tubes fails to operate above 1 GHz due to - 
1. Stray Reactance
1. Inter-electrode Capacitance
2. Lead Inductance
2. Transit Time/angle effect
3. Gain BW Product Limitation
4. Skin Effect: Conductor Losses
5. Radiation Losses
6. Dielectric Losses