The Plate Resistance

Author: J.B. Hoag

The opposition to the flow of electrons between the filament and the plate set up by the space charge and by the negatively charged grid is the equivalent of a resistance r_P, known as the plate resistance of the tube. A law, similar to that of Ohm, can be written for the case of a three-electrode tube. There is, however, a sharp difference between Ohm's law and the new law, in that the new one uses changes in current and changes in voltage rather than the currents and voltages themselves. Thus, the change of plate current i_p is given by the change of plate voltage (μe_g) divided by the total resistance of the circuit (r + r_p).

Fig. 12 E. The plate resistance rp of a triode. The symbol eg is used in the text in place of ΔEg in this figure. (From E. & N. P.)

As shown in Fig. 12 E this leads to the equation

where r is the external or load resistance and r_p is the internal resistance (or plate "impedance") of the tube, defined for zero load by the equation

Fig. 12 F. The "plate resistance" of a triode is given by ΔEp/ΔIp ( = ep/ ip in the text) rather than as the inverse slope of the dotted line. The latter is called the d.c. plate resistance. (From E. & N. P.)

We can better understand the meaning of the plate resistance of a tube by referring to Fig. 12 F which shows the plate current dependence on the plate voltage. Since this curve is not a straight line, r_p varies as the voltage of the plate changes. With a fixed voltage on the plate, r_p is a constant. Note that the d.c. resistance of the tube is given by E_p/I_p whereas r_p = e_p/i_p. The values, of the plate impedance r_p for normal operating voltages of different commercial triodes range from 800 to 150,000 ohms. They can be obtained from the curves of the tube, by means of tube testers, and from data published by the manufacturer of the tube.