COLD CATHODE GAS TUBES FOR TELEPHONE SWITCHING SYSTEMS 757 



tions, however, it is desired to apply the output voltage directly to other 

 tubes without impedance transformation. In this case, voltage gain is of 

 more interest than current gain. The maximum voltage gain per stage, 

 defined as the maximum output voltage divided by the minimum input 

 signal, is limited by variation in tube characteristics as will now be shown. 

 The bias voltage E of Fig. 1 is expected never to cause breakdo\\'n 

 during times when the input voltage is zero. This establishes the upper 

 limit of E as 



1^1^ V, ,„in (1) 



where Vb min is the minimum breakdown voltage at any point in the 

 life of any tube to be used in the circuit. As the bias voltage E approaches 

 breakdown, the input signal voltage required for triggering approaches 

 zero, and if there were no variation in breakdown voltage or bias voltage, 

 and no noise voltages, the gain could be made to approach infinity. 



The input signal, added to the bias, must be made large enough always 

 to cause breakdown. Thus the minimum input signal is determined by 

 Vb max , the maximum breakdown voltage at any point in the life of any 

 tube to be used in the circuit: 



Combining (1) and (2) 



or 



ein ^ Vb max " E (2) 



a = ' B max ' B min 



ein ^ AFb (3) 



where AVb is the maximum variation in breakdown voltage among all 

 tubes to be used in the circuit. 



The output signal is the difference between the bias voltage and the 

 sustaining voltage of the tube: 



gout = E - T^sVlS (4) 



The minimum output voltage corresponds to the maximum sustaining 

 voltage, Fsus max . It is this value that must be used in calculating the 

 maximum gain per stage as limited bj^ the tube characteristics. The gain 

 is then calculated as 



f, , E—V Vr. ■ — V 



/~i «'Out -«-' ' sus max ' B mm ' sus max ff\ 



ein AT B AFb 



This gain cannot be realized in practice because additional allowances 



