ASSAY OF RADIOACTIVITY 



point at which conduction begins in Fg due to valve aging produce a much 

 more serious effect in the latter case. We see then that our flip-flop requires 

 that R be returned to a moderately positive potential when the circuit is 

 at rest, in the interests of high input resistance, and to a higher potential 

 during the flip period, in the interests of precision of timing. 



Our simple circuit is open to a third objection — that it is not perfectly 

 quiescent and ready for a further flip immediately after it has flopped. 



Variation in flip 

 time when returned 

 to HT+ 



/ 



/ 



Returned to 



Returned to 

 less positive 



Vai-ialion In 

 grid voltage 

 at which 

 conduction 

 begins 



Variaton in flip time 

 when returned to less 

 positive 



Figure 31.12 



This is because of the time taken for the charge on C to reach its resting 

 value via grid current in V^ (t in Figure 31.13). t may be reduced by pro- 

 viding an additional charging path via a diode. 



We are now in a position to discuss the operation of the circuit actually 

 used {Figure 31.5). The flip-flop {V^ and V^ is not of the cathode-coupled 

 variety. The positive feedback path is from anode of V^ to grid of V^ and 

 from anode of V^ via a cathode foflower to the screen of V^- When the 



^grid 



Figure 31.13 



circuit is quiescent K4 conducts and V^ is cut off. Q, R^ and R^ form a 

 compensated direct coupling and R^ and R^ are chosen such that cathode- 

 follower grid and cathode are approximately at earth potential. Since 

 i?i, corresponding to R in Figure 31.10, is returned to this line, V^ conducts 

 but not too hard and the input resistance is high. Since the screen of Kg is 

 connected to this line and the grid of V2 is returned via Kg^ to a yet more 

 negative point at P, V^ is cut off". 



The magnitude of input pulse required to flip the circuit is determined by 

 the setting of P. On the arrival of a pulse of sufficient size, V^ is cut off 

 and V2 is turned on, via the cathode-follower, both by a 100 V positive- 

 going change applied to its screen and by the removal of the holding-off 

 bias hitherto applied to the control grid via F3 4. The circuit remains 

 flipped for a time determined by the time-constant R^C, and because R^ is 



444 



