TRANSISTORS 



transistor during the sweep (0-2 V) must be small compared with the battery 

 voltage. A battery voltage of —6 is about the minimum. Thus we have 

 Fj ^ 6, and if the transistor has a maximum permissible collector voltage 

 of 30, we have Kg = 24. Thus TJT^ = 24/6 = 4. This is not a very good 

 figure for a continuous-running time base, but the position will improve as 



Primary 

 current 



Collector 

 voltage 



Figure 45.44 



transistors having higher maximum collector voltages become available. In 

 setting up the circuit /?^ is adjusted so that the voltage across it is 30; 

 T-^jT^ then has its highest possible value. It is hoped in future to use the 

 recovered energy, at present wasted in i?^, to power other transistors, or to 

 provide HT for input cathode follower valves. 



The circuit may be modified for triggered operation by the addition of a 

 single dry cell {Figure 45.45). This drives current into the base of the transis- 

 tor and the emitter-collector current has a very low value at which ^ is 



Sweep > I f% \ 

 amplitude "^X^ \ ^-*y 



Triggered 



I 



Rep. 



o 



I- 



Figure 45.45 



much reduced. The loop gain of the oscillator is insufficient to maintain 

 oscillation, and the circuit gets 'stuck' at point A in the cycle. On the arrival 

 of a pulse of trigger current, a voltage is induced in L^ which series-opposes 

 the dry cell. The circuit performs one stroke, then waits for a further pulse. 

 It seems probable that future, more sophisticated, transistor time-bases 

 will contain a number of stages, some transistors involved in generating 

 accurate triangular current waves, and others acting as power amplifiers. 

 See, for example, Nambiar and Boothroyd^^. 



Transistor multi-vibrators 



A transistor multi-vibrator is strongly reminiscent of its valve counterpart 

 {Figure 45.46). The subject has been discussed by Jackets^^. 



704 



