EARTHED COLLECTOR CONFIGURATION 



Approximate equations for the earthed collector stage are very simple. 

 The current gain is roughly j3, but the voltage gain is less than unity. The 

 input resistance is merely f^Rj^ and the output resistance RqI^- Thus if 

 i?^ is 10 k and /5 = 30, the input resistance is about 300 kQ. If the earthed 

 collector transistor is being used as an output stage, Rq is the output 

 resistance of the penultimate stage as a whole and is substantially the 

 resistance of its collector load, perhaps 3 kQ. The output resistance of 

 the earthed collector stage is then 3 kQ/30 = 100 Q.. 



In point of fact, high input resistance is not of first importance with 

 grounded-collector stages. The power gain is so low that, as Jones and 

 Hilbourne^ point out, the performance is no better than that of an earthed 

 emitter stage with the input resistance raised by the simple expedient of 

 connecting a resistance in series with it (Figure 45.28). The low output 

 impedance therefore remains its most valuable feature. 



Gain control 



In potentiometric gain control between valve stages V-^ and Fg {Figure 

 45.29), the potentiometer is working between a generator of resistance, 



Low^ I 



Intermediate 



Figure 45.29 



Valve 2 



the output resistance of F^, perhaps 50 kQ, and a load which is much 

 higher, perhaps many hundreds of megohms. The potentiometer's own 

 value is intermediate, say 2 MQ, and we saw in Chapter 2 that such an 

 arrangement produces a satisfactory control characteristic. In transistor 

 Intermediate 



I 1 I 



Low 



Transistor 1 



Transistor 2 



Figure 45.30 



circuits the position is reversed. The output impedance of a transistor 

 stage is larger than the input impedance of the stage which follows it, and 

 under these conditions the arrangement of Figure 45.29 gives poor control ; 

 a better scheme is shown in Figure 45.30, and this is the one usually used. 

 It has another important advantage: the transistor is essentially a current- 

 controlled device. It is the input current, not the input voltage, which is 

 accurately amplified. The input resistance is that of a diode biased in the 

 forward direction and is highly non-linear. Thus if a transistor be fed from 



695 



