THE EARTHED EMITTER TRANSISTOR 



transistor, which is 25 k. R^ is much too high to have any appreciable 

 effect on the parallel resistance of ^3 and Rin, and may be ignored. 



For the amplifier to operate down to a 3 dB point at co^ radians per second, 

 we have l/(C/?y) = oj^, so for our amplifier to go down to 50 cycles, 

 CO = 27r X 50 = 310, and 



C = 



1 



106 



McRt 



f^F 



310 X 4-3 X 103 



= 0-75 /nF 



Notice the large value required compared with in valve circuits. Fortunately 

 the working voltages involved are low, and physically small electrolytic 



C2 



(a) 



ofTRI 



R2 



.R3 



'in 



Of TR2 



(b) 



«1 



[(ignore) 



R\n for 

 TRl 



Signal source 



Figure 45.17 



capacitors may be used in this position. The value of the input capacitor 

 Ci may be worked out in similar fashion, from the diagram in Figure 

 45.17b. 



Transformer coupling — The total power gain for the amplifier is 29 + 

 34 = 63 dB's. The low figure obtained from the first stage is attributable 

 to the low value of load presented to the first stage by the second. Matters 

 can be greatly improved by using a transformer to match the stages together, 

 though it should be observed that three R-C coupled transistor stages may 

 have the same performance as, but be lighter, smaller and cheaper than, 

 two transformer-coupled stages. 



The output resistance for TR^ is given by 



^0 + 



^ReRc 



R, + Re + R 



a 



and lies between R^ (when the source resistance Rq is 00) 



^ReRc 



and 



/?. + 



^6 + Re 



687 



(when Rq = 0) 



