TRIGGERED PULSE GENERATORS 



a positive-going square wave applied from elsewhere until an output of 

 sufficient amplitude has been obtained (Figure 16.16). On cutting off anode 

 current once more, the anode tends to rise in potential taking the control 

 grid with it — ^via C — and causing grid current to flow. This current recharges 

 C and allows the anode to return to HT plus potential with time constant 

 R]^C. R is much greater than Rj^ and plays no significant part in the flyback 

 operation. 



HT* 



Suppressor 



Anode 



Figure 16.16 



Whilst it is clear that the forward stroke time constant RC{A + 1) is 

 enormously greater than the flyback time constant RlC, we usually use so 

 little of the former that the flyback time is often of the same order as the 

 forward stroke time (the two times are roughly in the ratio R^IR) which is 

 undesirable, since one wants the circuit to be available for a further forward 

 stroke, if required, as quickly as possible. We shall mention now two refine- 

 ments to the Blumlein integrator, one of which aims at improving matters 

 here. It is due to Attree^. 



At tree refinement'^ — Here a cathode follower is interposed between the 

 pentode anode and the upper plate of C. This has almost no effect on the 

 operation of the circuit during the forward stroke. On the flyback the anode 

 is at liberty to return quickly to HT plus, the charge on C being replaced 

 quickly through the low output resistance of the cathode follower, perhaps 

 500 ohms, instead of via Rj, perhaps 50,000 ohms. The flyback is thus 

 100 times quicker {Figure 16.17). 



Gibbs and Rushton refinement^ — When it is required to produce a triangu- 

 lar wave lasting for periods of the order of 5 minutes it is undesirable to 

 achieve such protracted forward strokes by using enormous values of R and 

 C, since large capacitors are bulky and expensive, and high resistances of 

 upwards of 1 k megohms are also expensive and possibly of dubious stability, 

 becoming comparable with the circuit leakage resistances. Let us illustrate 

 with an example. Differentiating the expression for V^, 



we get 



dr 



AEt 



RC(A + 1) 

 242 



e ijcu+i) 



