Interelectrode-capacitance

You may know, capacitance exists when two pieces of metal are separated by a dielectric. You may also remember from your studies that a vacuum has a dielectric constant of 1. As the elements of the triode are made of metal and are separated by a dielectric, capacitance exists between them. This capacitance is called interelectrode capacitance, and is schematically represented in the illustration below.

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Notice that there are three interelectrode capacitances involved in a triode. The capacitance between the plate and grid, designated Cpg, is the largest, because of the relatively large area of the plate, and therefore has the greatest effect on triode operation.

The grid-to-cathode capacitance is designated Ckg. The total capacitance across the tube is designated Cpk. As we said earlier, Cpg has the greatest effect on the tube operation. This is because this capacitance will couple part of the ac signal from the plate back to the grid of the tube.

The process of coupling the output of a circuit back to the input is called FEEDBACK. This feedback affects the gain of the stage. It may be desirable in some applications. In others, the effects must be neutralized. The effects of Cpk are greater at higher frequencies where Xc is lower.

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