Editing Vacuum tube (section)

===Characteristic curves===
All tubes with one or more control grids are controlled by an AC ([[Alternating Current]]) input ''voltage'' applied to the control grid, while the resulting amplified signal appears at the anode as a ''current''. Due to the high voltage placed on the anode, a relatively small anode current can represent a considerable increase in energy over the value of the original signal voltage. The ''space charge'' electrons driven off the heated cathode are strongly attracted by the positive anode. The control grid(s) in a tube mediate this current flow by combining the small AC signal current with the grid's slightly negative value.  When the signal sine (AC) wave is applied to the grid, it ''rides'' on this negative value, driving it both positive and negative as the AC signal wave changes.

This relationship is shown with a set of ''Plate Characteristics curves,'' (see example above,) which visually display how the output current from the anode ({{math|''I''<sub>a</sub>}}) can be affected by a small input voltage applied on the grid ({{math|''V''<sub>g</sub>}}), for any given voltage on the plate(anode) ({{math|''V''<sub>a</sub>}}).

Every tube has a unique set of such characteristic curves. The curves graphically relate the changes to the instantaneous plate current driven by a much smaller change in the grid-to-cathode voltage ({{math|''V''<sub>gk</sub>}}) as the input signal varies.

The V-I characteristic depends upon the size and material of the plate and cathode.<ref>[https://www.indiastudychannel.com/resources/144164-V-I-characteristic-vacuum-tubes.aspx indiastudychannel.com/]</ref>
Express the ratio between voltage plate and plate current.<ref>Basic theory and application of Electron tubes Department of the army and air force, AGO 2244-Jan</ref>
* V-I curve (Voltage across filaments, plate current)
* Plate current, plate voltage characteristics
* DC plate resistance of the plate{{snd}}resistance of the path between anode and cathode of direct current
* AC plate resistance of the plate{{snd}}resistance of the path between anode and cathode of alternating current