COMPLETE PIECES OF TRANSISTOR APPARATUS 



to produce the requisite direct output. The system is clearly reminiscent of 

 that outlined earlier for the generation of EHT from HT by valves, but 

 the oscillator is not of the sine wave variety. Its operation is as follows: 

 Figure 45.40 shows the collector characteristic for the transistor. At each 

 cycle of oscillation the working point moves once round the locus formed 

 by the triangular figure, ABC. Consider the state of affairs at point A; 

 the voltage across the transistor is very low, hence nearly all the battery 



Figure 45.39 



voltage appears across the primary v/inding L^, and the current in it rises 

 linearly at a rate £/L^ amps per second. If the turns ratio between Lj, and 

 Lj, is «6 : 1 then the voltage induced across L^ by the current rising in Lj, is 

 En^ and the sense of connections is such that this drives a current 4 = 

 {Erif^jRy out of the base of the transistor. The working point therefore 

 moves towards B in Figure 45.40, along the characteristic corresponding 

 to a base current of this value. 



c 

 ft 



3 

 U 



U 



o 

 U 



Magnetic 



field 



grows 



Battery 

 voltage 



Magnetic field 

 «C collapses 



Collector voltage 



Figure 45.40 



The upper limit of collector current is defined by /?4 ; that is, when the 

 collector current reaches point B, it has a value of about (i . {En^)IRy and 

 can increase no further. The rate of rise falls away, and with it the voltage 

 across the base winding. The base current also falls, defining a lower 

 maximum collector current, and a cumulative action occurs as a result of 

 which the transistor is switched to a point such as C. Primary current 

 ceases abruptly. The magnetic field built up by the primary begins to 

 collapse and in doing so does three things. 



701 



