758 THE BELL SYSTEM TECHNICAL JOIJRXAL, MAY 1957 



must be made for the variation in power supply voltages and protection 

 against noise. In some cases the need for higher speed reduces the gain 

 still further, as will now be shown. 



In Fig. 1 a delay, t, is indicated between the application of the trig- 

 gering signal and the appearance of the output signal. Part of the delay 

 is statistical in nature and part is occasioned by the building up of ioniza- 

 tion within the tube. As will be discussed later, the delay can be reduced 

 by tube design technicjues. Howe\'er, for any gi\'en tube, the delay is a 

 function of the excess of the tiggering voltage over the breakdown volt- 

 age. The larger this overvoltage, Fov , the shorter is the breakdown delay. 

 Since this overvoltage must be added directly to the input signal, the 

 gain is reduced. 



Although not shown in Fig. 1, the tube is turned off by applying a sig- 

 nal that reduces the anode-to-cathode voltage below the sustaining value. 

 This turn-off signal must have sufficient duration so that the tube does 

 not again break down at the return to normal bias conditions. If the turn- 

 off pulse duration is less than that needed for complete recovery, the ef- 

 fective breakdown voltage is reduced. Ecjuation (5) can be modified to 

 show the effect of this reduction in turn-off time by defining a quantity 

 Vr , the reduction in breakdown voltage resulting from incomplete 

 recovery of the tube. The combined effects of Vr and Fov are then 



/-> \' B min • r) ' sus max /^.x 



AT B + Fov 



Equation (6) shows that faster turn-on obtained by increasing the 

 over voltage Fov and faster turn-off obtained by allowing for decrease in 

 breakdown voltage by an amount Vr , both result in a reduction in volt- 

 age gain. Thus the familiar trade of speed for gain extends to gas tube 

 switching circuits. Summarizing, it can be seen by (5) that constant 

 breakdown voltage and large difference between breakdown and sustain 

 are desirable switching properties. Also, as shown in (6), the tube should 

 be designed so that the overvoltage needed to cause fast breakdo\m is 

 small and the recovery of breakdown voltage after the tube is turned off 

 is fast. It is useful to consider now the internal physical processes of a 

 cold-cathode glow-discharge tube in order to see how the desired external 

 properties can be obtained. 



PHYSICAL PROCESSES OF A COLD CATHODE GLOW DISCHARGE 



Since the gas particles are neutral and the cathode does not spon- 

 taneously emit electrons, current flow requires an auxiliary supply of 

 charged particles. A small amount of radioactive material to ionize some 



