TRIGGERED PULSE GENERATORS 



If the capacitance is merely charged via a resistance, instead of a constant- 

 current device, the waveform across it rises exponentially rather than linearly. 

 We have already discussed three circuits — the difference diode relaxation 

 oscillator, the thyratron relaxation oscillator and the blocking oscillator — 



Forward stroke 



Flyback 



(rapid discharge) 



Figure 16.11 



which produce waveforms of approximately triangular — in fact exponential 

 — form. The degree to which the exponential produced resembles the 

 ideal form depends on how much of it is used; if the capacitance is 

 allowed to charge only to 1 per cent or so of the total charging voltage available 



HT-i- 



HT* 



Grid potential , 

 controls charging 

 rate 



Figure 16.12 



before discharge occurs, then the departure from linear rate of voltage rise 

 is usually quite negligible {Figure 16.13). In order to produce a triangular 

 wave — ^using resistance charging — of amplitude 200 V, we should need to 

 be able to call upon a charging e.m.f. of 20,000 V; in general this is not 



HT+- 



First part of 

 exponential 

 used; nearly 

 straight 



HT* 



I 



Appreciable proportion 

 of exponential used; 

 far from straight 



Time 

 Figure 16.13 



possible. However, we could produce a 2 V wave from a 200 V supply, 

 and amplify it with a valve giving a gain of 100 times; but in practice there 

 is a better way, which is to use a Blumlein integrator. 



240 



