The Gas-Type Phototube

Author: E.E. Kimberly

When a suitable gas is introduced in a phototube, the passage of electrons causes ionization by collision the same as in a gas-filled triode, nullifies the space charge, and produces greater current than it would if the gas were not present.

Fig. 27-46. Sample Characteristics of Gas-Type Phototube

Fig. 27-47. Photoelectric Relay

The gas-filled phototube is therefore not space-charge limited as the vacuum type is, and the plate current can be increased by increasing the plate voltage even above that which will draw all emitted electrons from the cathode. The increase in plate current with increase in plate voltage occurs because, in the migration of an electron from cathode to anode, the higher the plate voltage the greater the acceleration ot the electron alter a collision and so the more ionizing collisions it can have before reaching the anode. Fig. 27-46 shows a family of characteristic curves of the type 918 gas-filled phototube for different intensities of illumination. In applications where linearity of response is unimportant, such as "on" and "off" relay operation, the gas-filled phototube is preferred over the vacuum type because of its greater current and sensitivity.

Anode voltages of gas-filled phototubes are generally limited to 90 volts or less because higher voltages are likely to lead to a sustained uncontrolled discharge. Fig. 27-47 shows a photoelectric relay using a gas-filled phototube.