TRANSISTORS 



are safety resistors to limit the currents and prevent accidents to the tran- 

 sistor. Then the graph which emerges has the form shown in Figure 45.6. 

 Probably the first thing which strikes us about this 'collector characteristic' 

 for the transistor is the resemblance to the anode characteristic of a pentode. 

 Above a certain 'knee' collector voltage, the collector current /^ is determined 



Figure 45.5 



Collector \«)ltage 



Figure 45.6 



almost entirely by the base current 4 and is much larger than it. We can 

 define A, the 'forward current amplification factor' as 



L<54 



F. 



const 



It is generally between 20 and 60. This suggests the beginnings of an 

 equivalent circuit for the transistor, Figure 45.7. However, above the knee 



Figure 45.8 



Figure 45.7 



the characteristics are not perfectly flat; the reciprocal of their slope 

 represents a resistance R^-c seen looking into the transistor between 

 emitter and collector, which might in a typical small transistor be 50 kO. 

 Our equivalent circuit is modified to Figure 45.8. 



680 



