SOFT VALVES 



and the ballast resistance is 210 V/17 mA = 12,350 ohms for a F„ of 510 V. 

 The forward stabiUzation ratio will be 



250 



50 



250 + 12,350 



and the variation in output voltage between and 100 per cent full load will 

 be 7 mA X 250 ohms = 1-75 V. Clearly both backward and forward stabi- 

 lization are much improved by reducing the maximum current demands on 

 the device. 



Difference diode 



In the design of voltage reference and voltage stabihzer tubes every effort 

 is made to make the striking voltage as little above the running voltage as 

 possible, as this facilitates the design of the ancillary circuit, as we have seen. 

 In the difference diode, on the other hand, the striking voltage and running 

 voltage are deliberately made widely different. This property can be made 

 use of in various ways, of which two will be mentioned. 



Bistable circuit — A bi-stable circuit is one which has two distinct stable 

 conditions and may be altered from one to the other by appropriate electrical 

 pulses. The difference diode bi-stable circuit is extremely simple and appears 

 in Figure 7.9a and b. Its two states are simply 'aglow' and 'not aglow'. 



/ , 



■ ^ ^ 





V^Vi 



-It- pulses 

 II 



n '°"' 



-I Lpulses 



V^^V^V, 



(a) 



Figure 7.9 



(b) 



In Figure 7.9a, we have a difference diode and ballast resistance fed from a 

 direct supply voltage which is greater than the running voltage but less than 

 the striking voltage of the tube. Control pulses are introduced (via a capaci- 

 tance) with respect to earth by a pulse generator of some kind, connected at 

 A, as shown. 



Suppose the tube be at first extinguished. Then if a positive pulse be 

 applied to the anode sufficient to carry it above the striking potential, the 

 glow is initiated and the anode falls to the running potential and the circuit 

 remains in this 'aglow' condition indefinitely. However if now a negative 

 pulse is fed in at ^4, the anode is carried below the running voltage and the 

 glow is extinguished. On the cessation of the pulses the anode potential 

 returns to + F and the circuit is once more stable in the 'not aglow' condition 

 until a further positive pulse arrives at A. 



124 



