Dynamic Curves

Author: J.B. Hoag

"Static" characteristic curves, such as those shown in Fig. 12 B, are obtained when there is no resistance or impedance in series with the plate battery. Dynamic curves take account of the effect of a plate load and hence show the behavior of the tubes in actual operation. Consider the case of a pure resistance load, as in Fig. 12 D.

Fig. 12 D. A triode with a resistance load; and its dynamic curve

The voltage E_p across the tube itself is less than that of the plate battery E_b, by an amount equal to the I_pR drop in the load resistance R. For example, if R = 10,000 ohms, I_p = 10 ma. (milliamperes) and E_b = 200 volts, then the voltage lost in R will be 0.01 · 10,000 = 100 volts and only one-half the battery voltage is applied to the tube, i.e., E_p = 200 — 100 = 100 volts. With a fixed bias voltage on the grid, the plate current I_p is less when the plate voltage is less. Thus, in Fig. 12 D (b), the static or no-load current would be 15 ma. when the full 200 volts was on the plate. With R in the plate circuit, it would be 10 ma., corresponding to 100 volts on the plate.

Any change of the grid voltage will alter the plate current. Suppose an input signal should make the grid less negative. In the absence of R, the plate current would increase along the upper static curve. When R is present, however, the plate current would increase along the dotted static curve [Fig. 12 D (b) ], provided the voltage on the plate remained fixed at 100 volts. But this is not true. When the plate current increases, the I_PR drop increases to a value greater than 100 volts, say to 110 volts, leaving only 90 volts on the plate. With this lower plate voltage, the plate current cannot be as large. Eventually, the current attains a value such as that indicated at C in the figure. From a succession of such points for various voltages on the grid, the dynamic curve is obtained. Note that it is straighter and has less slope than the static curve.