TRIGGERED PULSE GENERATORS 



less common. Attree^ describes a stimulator employing a transitron delay, 

 stage and — incidentally — a flip-flop to form the stimulus pulses. 



Trig 



in 



Square 



wave 



generator 



B 



Hh 



D 



(a) 



Waveform at: A 



B 



Delay 

 period 



(b) 



Figure 16.7 



Eccles-Jordan circuit 



This circuit is included for lack of anywhere better to put it. It is a square 

 wave generator and it does require triggering, but whereas the flip-flop and 

 transitron bring their square waves to an end automatically after a time 

 determined by the charging of a capacitor, the Eccles-Jordan circuit requires 

 a further trigger pulse to return it to the initial condition. It is, in fact, a 

 bi-stable or two-state circuit, like the difference diode circuit in Figure 7.9. 



In its simplest form it appears as in Figure 16.8. Two valves are connected 

 to each other so that when both amplify there is much too much positive 

 feedback for stability, and by cumulative action one valve is cut off" and the 

 other conducts hard. The resistance chains are chosen so that if V^ is con- 

 ducting hard. Kg grid is held just negative enough to cut off" Kg and vice versa; 

 the circuit is quite symmetrical, and will remain indefinitely in either state 

 until the bias on the cut-off" valve is lifted by a positive trigger pulse on its 

 grid, when the circuit snaps over to the opposite condition. 



The change-over speed may be improved by shunting the resistance from 

 each anode to opposite grid by a capacitance, and by making the valves 

 pentodes {Figure 16.9). The negative supply may be dispensed with by the 

 use of automatic bias. This is not a cathode coupled circuit — either one valve 



238 



