Effective Input Capacitance

Author: J.B. Hoag

Although the capacitance between the grid and filament of a triode is small, it is paralleled by the grid-to-plate capacitance and by the plate-to-filament capacitance and by the load, as shown in Fig. 13 E.

Fig. 13 E. The equivalent circuit of the triode of Fig. 13 A

The combination can be replaced by an effective input capacitance whose value is often many times that expected from the grid-to-filament capacitance alone. When very high frequencies are applied to the grid of the tube, the effective capacitance can shunt an appreciable percentage of them to ground and hence lower the voltage amplification very much.