THE EARTHED EMITTER TRANSISTOR 



circuit. Figure 45.3b is therefore the transistor equivalent of a conventional 

 valve amplifier, and Figure 45.3c corresponds to a cathode follower. 

 Figure 45.3a is equivalent to a valve circuit which exists but with which we 

 have not dealt — the 'grounded grid'. 



At this point we anticipate the results of explanations which remain to be 

 given and assert that the transistor is, like the valve, an amplifying device 

 but which, unlike the valve, consumes power at the input. From a black-box 

 viewpoint {Figure 45.4), we can distinguish the current gain, (5/out/^^in> 



6L 



' out 



Figure 45.4 



the voltage gain, <5Kout/(5f^in, and the power gain (6Fout ^/out)/(^in ^^in). 

 In the earthed base configuration a transistor gives a voltage gain, but a 

 current gain less than unity. In the earthed collector configuration, a 

 current gain, but a voltage gain less than unity. In the earthed emitter 

 arrangement the device shows both voltage gain and current gain, so that 

 the power gain is maximal : power gains of over 40 dB's are possible. Further, 

 an earthed base transistor has a low input resistance and a high output 

 resistance, higher by about 3 orders, whilst in an earthed collector arrange- 

 ment the situation is reversed. With the emitter earthed, the input and 

 output resistance are more comparable (though the latter is still at least 

 10 times greater). This facilitates the coupling of transistors in cascade, 

 since the matching conditions for optimum power transfer are most nearly 

 met. 



The earthed emitter configuration is therefore of the greatest interest, 

 and will receive the bulk of our attention. With transistors occupying 

 their present low level of importance in biological work, a full treatment 

 involving all three configurations would produce a chapter of dispropor- 

 tionate length. There is no dearth of textbooks in which common base 

 and common collector configurations are properly discussed; our purpose 

 here is to introduce transistors and to cover sufficient mathematics to enable 

 simple transistor design work to be carried out. Before beginning it must 

 be mentioned that considerable confusion exists at present (September 1957) 

 about the meaning of transistor symbols. A number of groups of transistor 

 engineers have developed their own transistor symbolism and it is as yet 

 too early for standardizing authorities to decide upon, and lay down, the 

 best system. This confusion accounts for much of the difficulty in under- 

 standing transistors at the present time. 



THE EARTHED EMITTER TRANSISTOR 



Suppose we arrange an earthed emitter transistor in the test set-up shown 

 in Figure 45.5. We can vary the current flowing out of the base by the 

 variable resistor Ry, seeing what effect is produced on the collector voltage 

 and current, as indicated by the voltmeter and milliammeter. R^ and R2, 



679 



