SINGLE SQUARE WAVE GENERATORS 



not seem to the author to be electrobiologically interesting and so has not 

 been described, but the triggered version is important and will be dealt with 

 now. 



In the transitron a pentode valve is used, but not in the classical pentode 

 manner, where the screen-cathode and suppressor-cathode potential dif- 

 ferences are constant. Instead, the screen is used as a kind of subsidiary 

 anode and the suppressor as a second control grid. Not many pentodes 

 make good transitrons because the controlUng effect of the suppressor of 

 most pentodes is insufficient. At the time of writing the Mullard EF 50 and 

 the Mazda 6F 33 are conspicuously good for transitron action. 



Figure 16.6 shows the circuit. When it is quiescent, the suppressor poten- 

 tial is such as to encourage a substantial anode current and a much smaller 



HT+ 



Figure 16.6 



screen current, as in normal pentode usage. If now a negative trigger pulse 

 be apphed to the suppressor the anode current is reduced and electrons flow 

 instead to the screen, increasing the screen current and reducing the screen 

 potential. This reduction is transferred via C to the suppressor, producing a 

 further fall in anode current and increase in screen current. Cumulative 

 action then occurs as a result of which all the cathode current goes to the 

 screen and the anode current is cut off. This state of affairs persists until, 

 as a result of re-adjustment of the charge on C, the suppressor potential 

 returns exponentially to a point at which anode current once more begins to 

 flow. The screen current then falls, the screen potential rises and a reverse 

 cumulative action occurs. The waveforms are similar to Figure 16.4, except 

 that screen potential should replace V-^ anode potential, and suppressor 

 potential the V^ grid potential. 



Uses for single square waves — ^The pulses produced by the flip-flop and 

 transitron may be used for applications where their shape is specifically 

 applicable, as in square wave stimulation, or use may primarily be made 

 of the duration of the wave for the production of delays. If the square wave is 

 passed through a CR network of short time constant*, then a pip is generated 

 at the beginning and end of the wave, and the second pip selected by an 

 appropriately connected diode, as in Figure 16.7. The second pip is then 

 used to trigger further circuits. 



Flip-flop circuits are so commonplace that there is little point in trying 

 to refer the reader to all possible uses for them. The triggered transitron is 



* Sometimes called a 'differentiating' circuit, since the output is responsive to rates of 

 change at the input, rather than to the input itself. 



237 



