756 



THE 15ELL SYSTEM TECHNICAL JOURNAL, MAY 1957 



formance. Finally, a description is given of a new talking path tube in 

 which improved switching and transmission performance have been 

 achieved. 



EXTERNAL SWITCHING PROPERTIES 



From the point of view of the external circuit, a gas diode may often 

 be treated as a device which is an open circuit so long as the applied 

 voltage is low, and which becomes a conductor if the applied voltage is 

 raised above a threshold or "breakdown" value for a sufficient length 

 of time. When the tube is conducting currents of the order of a few 

 tens of microamperes or higher, the voltage is relatively independent 

 of current and has a value, referred to as the "sustaining voltage", 

 which is less than the breakdown voltage. 



Although the details of actual circuits differ, it is possible to illustrate 

 some important switching principles by the simplified circuit of Fig. 1(a). 

 A gas diode is shown with the cathode connected to a bias voltage E 

 through a load resistor. A signal source, assumed to have zero internal 

 impedance is connected to the anode. The output voltage waveform 

 corresponding to a pulse input is shown in Fig. 1(b). Note that after a 

 time delay, t, the output rises to a voltage that differs from the total 

 applied voltage by an amount equal to the sustaining voltage of the 

 tube. The memory function is illustrated by the fact that the output 

 signal remains after the input signal is removed. 



It is often desired to use the output signal resulting from the triggering 

 of one tube to switch one or more additional tubes. Since the input signals 

 can be a few microamperes and the output signal can be tens of milliam- 

 peres, a large current gain is available from a gas tube. In many applica- 



(rye,^ 



OUT 



(a) 



6|N ■- 







-M 



Em 



kT 



I ---> I 



r^ 



r*— VsuSTAIN 



I ^OUT 



± 



TIME — 

 (b) 



Fig. 1 — Simplified gas diode switching circuit. 



