5. Pentode

Pentode was developed from tetrode by inserting the fifth electrode called as supressor grid or grid No 3. The purpose of inserting suppressor grid is avoiding secondary emission occurs in tetrode that reduce plate current, as seen on b-c curve from tetrode characteristic.
Suppressor grid has negative voltage with respect to the others electrode because it connected directly to cathode. When the emitted electron from cathode come to plate and strike electrons within plate, secondary emission will occur. In tetrode application, electron produce by this secondary emission will attracted to screen grid to be screen grid current. By applying suppressor grid, its negative voltage will push back secondary electron going back to plate and by this way dicreament of plate current will avoid to produce flat plate characteristic as seen of Figure 24.
Test circuit to achieve pentode plate characteristic seen on Figure 17, except the tube is pentode not tetrode

 

Pentode plate characteristic performs such manner as :
1) Over the major portion of the characteristic plate current is largely
    independent of plate voltage,  therefore pentode may be considered as constant
    current device. Pentode is generally operated in  this  region.
2) The dip found in the plate characteristic of tetrode eliminate by pentode
    characteristic.
3) The characteristic are non linier only bellow the knee of the curve
 

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