TRANSISTORS 



transistor is a three-terminal device in which two such junctions are arranged 

 back-to-back (Figure 45.1). Clearly two dispositions are possible, and these 

 are described as N-P-N, or P-N-P. At present only the P-N-P type is 

 widely used in this country, and we shall concentrate upon this. However, 

 all the remarks which we make about the workings of P-N-P transistors 



Collector 



9 Collector 



N 



Baseo- 



N 



Baseo- 



P 



I 



Emitter 



(a) 



€) 







Emitter 



(b) 



Figure 45.1 



Figure 45.2 



apply equally to the N-P-N type, except that in N-P-N circuits the polarity 

 of all supply voltages is reversed, and all currents flow in the opposite 

 direction. 



The terminals of a triode* transistor are called the emitter, the base and 

 the collector {Figure 45.2a) and may be regarded as corresponding to the 

 cathode, grid and anode of a valve. The circuit symbol for the P-N-P type 

 is shown in Figure 45.2b. In the N-P-N type the direction of the emitter 

 arrowhead is reversed. If we regard the transistor as a 'black box' having 

 two input and two output terminals, since the device has actually only 

 three terminals altogether, clearly one of the real terminals must always 

 be common to the input and output circuits, and three configurations are 

 possible: 



common base {Figure 45.3a) 



common emitter (Figure 45.3b) 

 and common collector (Figure 45.3c) 



(a) 



(b) 



Figure 45.3 



(C) 



Since the common terminal is generally earthed so far as signals are con- 

 cerned, leaving the remaining two as 'live' input and output connections, 

 these configurations are also called 'earthed base, earthed emitter and 

 earthed collector'. In America the term 'ground' is used for 'earth'. Paren- 

 thetically it may be remarked that, in transistor terminology, an amplifier 

 valve is an earthed cathode, and a cathode follower is an earthed anode, 



* Tetrode transistors exist, but as yet are a rarity. Henceforward we drop the term 

 'triode' from the description. 



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