THE EARTHED EMITTER TRANSISTOR 



Vg = —2, 1^ = 3 mA. If we were to use this working point the load line 

 for the 1,000 Q. earpiece would be as shown (LLl) and cuts the voltage axis 

 at the required battery voltage, in this case 5. If we power our amplifier 

 from dry cells, we cannot have 5 volts, but we can have 4-5 or 6. Let us 

 choose 6. Then we have a new load line LLl, and a reasonable working 

 point might be V^ = —2-5, 4 = 3-5 mA. Then the maximum possible 

 working point excursion is from approximately —0-25 V to —4-75 V 



;, 



c max 



< lOrr------ -^ — 



-1 -2 -3 -4 -5" 

 Collector voltage 

 Figure 45.16 



(a swing of 4-5 V) and from 1-25 mA to 5-75 mA (a swing of 4-5 mA) so 

 the maximum power output for a sinusoidal signal is 



4-5 V X 4-5 mA 

 8 



2-5 mW 



The power supplied to the output stage is 6 V X 3-5 mA = 21 mW, so 

 the efficiency of the stage is 2-5/21 X 100 per cent = 12 per cent. Thus far 

 the procedure is exactly as for valves. 



In using a working point V^ = —2-5, Z,. = 3-5 mA, our transistor 

 parameters are probably not far from those quoted by the makers for 



-2, L — 3 mA, which are 



/5 = 





47 



500 a 

 13 kQ 



To secure a standing collector current of 3 mA we need a base 'bias' current 

 defined by the bias resistor R^. The quickest way to find the proper value 

 of 7?3 is by experiment with the particular transistor in use. As a pointer 

 to the approximate value required, if we regard /? as a measure of IJIj, 

 (actually, of course, ^ is dljdl^) then to get a collector current of 3 mA we 

 need a base bias current of 3 mA/47 = 64 juA. Since the base and emitter 

 are practically at the same potential, the whole battery voltage appears 



685 



