THE EARTHED EMITTER TRANSISTOR 



Now if this were all we had to do to specify the transistor, the subject 

 would be no more difficult than valves; for we have two parameters, R^^ 

 which corresponds to r^ of a pentode, and A, which corresponds (but not 

 dimensionally) to g^. Unfortunately we have to discover what is happening 

 at the input circuit. We keep the investigation symmetrical if we set up 



Figure 45.9 



Figure 45.9, seeing how the base voltage and current are related to the 

 collector current. On plotting the results we find a characteristic like 

 Figure 45.10. When /<. = we have merely the base-emitter junction as 

 a diode biased in the forward direction; voltage across it is low, and the 

 resistance is represented by the reciprocal of the characteristic slope, about 

 1 YD. in the small transistor we are considering. The effect of allowing 



200mY 



Base voltage 

 Figure 45.10 



collector current to flow is to reduce the base current at a given base voltage, 

 and we can define a 'backward current amplification factor' 





K = const 



and call it B. Its value is less than 1 , about 0-04 in the case considered ; the 

 transistor does not amplify in both directions at once. Our equivalent 

 circuit now has the appearance of Figure 45.11, and is complete. It is 

 only one of many possible equivalents for a transistor, but has the merit 

 that the four parameters required follow from the characteristic curves. 

 Another equivalent circuit, simpler in that it contains only one generator, 



Perhaps its greatest merit lies in the fact that it is 



681 



is shown in Figure 45.12 



