As the name suggest, vacuum tetrode consists of four electrodes:cathode,anode,control grid and screen grid. The ameruican physicts and electrical engineer Albert Wallance Hull invented the tetrode vacuum tube in 1926.
Electrodes of vacuum tetrode
vacuum tetrode consists of four electrodes:cathode, anode, control grid and screen grid. the cathode,anode , control grid, and screen grid are enclosed in an empty glass envelope.The cathode is surrounded by control grid and the control grid is surrounded by screen grid. The screen grid is surrounded by the anode or plate.
The construction of vacuum tetrode is similar to vacuum triode. However, vacuum tetrode contains an extra electrode called screen grid.
Screen grid
Screen grid is placed between the control grid and plate. This grid is placed closer to the control grid than plate to reduce the capacitance efficiently. Screen grid acts as an electrostatic shield to protect the control grid from the positive electric field or influence of the plate when its potential changes.
Main functions of screen grid
The main functions of screen grid are:
- To accelerate and attract the free electrons to the plate or anode
- To reduce the capacitanes between the plate and control grid
- To increase the control over the electron flow
- To increase the vacuum tube efficiency
- Reduce distortion
- increase gain
The control grid and the screen grid are made of network that controls the electron flow between the cathode and the anode. The space between the network
of wires in the control grid and screen grid is very large. Hence, the feree electrons move easily from the cathode to anode through the opening of grids.
Vacuum tetrode with zero grid voltage and positive plate voltage
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