TRANSISTORS 



generator resistance is infinite, and is our R^ ; and rout' is the output resistance 

 when the generator resistance is zero, and is our 



Rc + 



where Rq = 



Working entirely in terms of Mullard parameters, the resistance seen 

 looking in at the input of a transistor in earthed emitter feeding a load Rj^ is : 



/ Rl H" ''out 



11 



Rl + ''22' 



The resistance looking into the output of a transistor fed from a signa 

 generator of resistance Rq^^ 



, Rq + ''in' 



The current gain is 

 and the voltage gain is 



''lli^L + ''out' 



The power gain is, of course, the product of the voltage and current gains. 



EARTHED COLLECTOR CONFIGURATION 



We now mention briefly the performance of the earthed collector stage, 

 an a.c. coupled version of which is shown in Figure 45.27 and which, it will 



o AAAAA/^ 



Input 



Figure 45.27 



Figure 45.28 



be remembered, corresponds to the cathode follower in valve practice. 

 The fact that the input resistance can be arranged to be high (100 kD) makes 

 it appear at first sight that this configuration is the most suitable for the 

 input stage of a transistorized biological amplifier; there would be reason- 

 able efficiency of signal transfer from the lower resistance type of electrode 

 system (e.g. nerve lying over silver-wire hooks, large surface electrodes, 

 etc.). Further, the low output resistance makes the configuration appropriate 

 for driving low resistance loads, such as meters. 



694 



