TRANSISTORS AND JUNCTION DIODES 837 



2.33. Current Gain 



Now when a second relatively small potential is connected between the 

 base and emitter rectifier (Eb in the sketches) additional current, !« 

 will flow through the emitter rectifier in the forward direction and Tc 

 w ill also increase. This increase in /^ caused by the increase in le is 

 transistor action. The increase in /^ is related to the increase in /« by 

 the factor alpha (a) as written below: 



Ale = (xAle. (1) 



llie application of lvirchoff''s current law to the sketches in Fig. 7 gives 

 the change in lb as follows 



Ah = Ale - Ale . (2) 



By combining equations (1) and (2), Ale can be written as a function of 

 Ah only 



^h=j.r^Ah. (3) 



(1 - a) 



The usual value of a for junction transistors is near but slightly less than 

 unity. In a typical case a might be 0.98. This value when substituted in 

 equation (3) shows the current gain of the transistor, Ah/ Ah to be 49. 

 Most of the circuits discussed in this paper are based on equation (3). 



It has been shoAvn how a small change in base to emitter potential 

 with a small change in base current effects a large increase in collector 

 current at a higher voltage. This explains how large power gains, of the 

 order of 60 db, can be obtained from the junction transistor. 



The sketches in Fig. 7 do not show why this transistor action takes 

 place. The reasons for it involve the use of such solid state physics terms 

 as the migration of electrons and holes through a crystal lattice, and the 

 interposition of junction barriers. The "why" for transistor action is 

 very important in the manufacture of transistors, and it has been thor- 

 oughly covered in the literature.^' '^ For present purposes it is only 

 necessary to examine the static characteristics of an n-p-n transistor as 

 sho^\^l in Fig. 8. This figure presents transistor characteristics in a 

 manner which simpUfies the explanation of the operation of the transistor 

 control circuits covered later in this paper. 



Referring to the curves in Fig. 8, it will be seen that in the straight 

 portion of the 1.5- volt curve, a change of 50 microamperes in the base 

 current will result in a change of about 2 miUiamperes in the collector 

 current. This illustrates the current amplification of transistors and the 



