TRANSISTORS 



With practical transistors it is usually sufficiently accurate to write these as : 





Rc + Rl 

 Rb + Yi^e 



-Ti 



R 



m 



Rc + 



^ReRc 



^b + ^e + Rg 



Thus in the transistor we have been considering {R^ = 40 Q, i?^ = 1 kQ, 

 Re = 50 kQ, /? — 30) the current gain moves from 30 to as we vary the 

 load resistance from to infinity, and the input resistance goes from 1 kD. 

 to 2-2 kQ. The output resistance rises from 50 kQ, when the generator 

 resistance is infinite, to llOkQ, when the generator resistance is zero. 

 A typical value of Rj^ is 5 kO. With this value the current gain is 30(50 k/ 

 (50 k + 5 k)} — 27. The resistance seen looking in at the input of the 

 transistor is 2-1 kQ. The voltage gain is 27 X (5 k/1-9 k) = 71. The power 

 gain = voltage gain X current gain = 71 X 27 = 1,900 = 32-8 dB's. 



Simple design procedure 



The dependence of the input resistance of a transistor on the load used, 

 and of the output resistance on the generator used, means that in multi- 

 transistor amplifiers the stages cannot be designed in isolation; rather must 

 the circuit be regarded as a whole. We have here another reason for the 



Input 



Figure 45.15 



greater difficulty in working with transistors as opposed to valves. As an 

 example, consider the simple R-C coupled two-stage audio amplifier in 

 Figure 45.15, in which two transistors in the earthed emitter mode feed an 

 electromagnetic earpiece. We design around two Mullard OC 71 transistors. 

 Output stage — Starting at the output stage, we begin with the collector 

 characteristic of TRg in a manner very reminiscent of valve practice. The 

 0C71 has a maximum permissible emitter-to-collector voltage of —5, 

 a maximum permissible collector current of 10 mA, and a maximum 

 permissible collector dissipation of 25 mW. Since the 'knees' of the charac- 

 teristic curves are at a collector voltage of about —0-25, we have four 

 figures to define a 'working region' {Figure 45.16). A working point on 

 this diagram for which the makers of the transistor give parameters is 



684 



