Equivalent Networks Of Negative-Grid Vacuum Tubes at Ultra-High Frequencies

H E N the equivalent network of a vacuum tube is mentioned, it brings to the m i n d of practically every radio engineer a certain combination of resistances and capacitances together with an internal generator which has become familiar through years of use. Historically, this equivalent network did not spring into being full grown like Athena from the forehead of Zeus, but was the result of a slow and painful development. The beginnings of the equivalent network of negative-grid vacuum tubes are to be found in the work of Nichols where it was pointed out that a non-linear resistance is the equivalent of a fixed resistance in series with a generator. As a second step, Van der Bijl's relation states that the plate current in a vacuum tube is a function of the plate voltage plus a constant times the grid voltage. This constant was identified with our well-known amplification factor n and it was an easy step thereafter to combine the Van der Bijl and Nichols relations and represent the vacuum tube by the equivalent network shown in Fig. 1. Here the cathode is located at C and the plate at P. Between them the vacuum tube is represented by the internal plate resistance r p acting in series with the fictitious generator iqV 0 . This equivalent naturally represents conditions between the cathode and plate at very low frequencies only, because the low-frequency impedance between the grid element and the other electrodes is so high that it can safely be disregarded.