Negative Resistance or Dynatron Oscillators

Author: J.B. Hoag

Any electrical device which has a "negative resistance" can be used as an oscillator. An electric arc and a screen-grid tube have this characteristic. "Negative resistance" means that, as the voltage across the device increases, the current through it decreases, and vice versa. This is true over the region AB of Fig. 15 B, which shows the characteristic curve of a screen-grid tube. Throughout this region, the screen grid is more positive than the plate, and hence attracts not only electrons from the filament but also secondary electrons emitted from the plate. A tuned circuit may be connected in the grid circuit, as in Fig. 29 B.

Fig. 29 B. A negative-resistance or "transitron" oscillator

The grid electrons, accumulating on condenser C, lower the positiveness of the screen grid so that it captures fewer electrons. The condenser C discharges through L after which it refills, and the process starts over again. The rate at which the cycle occurs is determined by the resonant frequency of LC. The tuned circuit can also be in the plate circuit, but it is necessary in only one of the two positions. It might be said that, since the secondary electrons are alternately shifted back and forth from the grid to the plate, that this type of oscillator is secondary-electron-coupled.