PROPERTIES AND APPLICATIONS OF fl-p-fl TRANSISTORS 551 



stage. Suppose that the collector is connected through a transformer winding 

 to a fixed voltage supply as indicated in Fig. 17. Since Ve is always a very 

 small fraction of a volt (see Fig. 4 and Fig. 5) the collector voltage will be 

 approximately equal to the supply voltage. If no d-c. connection is supplied 

 to the base, it will float at a potential above ground equal (in magnitude) to 

 Ve — i.e., a very small fraction of a volt — and the collector current will be 

 exactly equal to the emitter current. To find out approximately what this 

 value of the current will be, consider the upper set of static characteristics 

 in Fig. 5. Note that, when the emitter current is zero, the collector current 

 is of the order of 20 microamperes (the exact value varies from 1 to 30 micro- 

 amperes in transistors tested so far). The collector current is then of the order 

 of 20 microamperes greater than the emitter current. If the emitter current 

 is now increased by Ale , the collector current will increase by aAIe . That is, 

 the increments of collector current will be slightly smaller than the incre- 



Fig. 17 — One practical arrangement of a grounded emitter stage. 



ments of emitter current and, as the emitter current is increased, the emitter 

 and collector currents will become more nearly equal. If a were perfectly 

 constant they would become exactly equal when 



/. = /c = ^-^ (47) 



1 — a 



where Ico is the collector current which flows when the emitter current is 

 zero. Equation (47), then, gives the value of emitter (and collector) current 

 which will flow in a grounded emitter stage if no d-c. connection is made to 

 the base. This current varies rapidly with a. For the transistor which has 

 been considered numerically, this current would amount to about 465 micro- 

 amperes which is certainly within the range of suitable values for the 

 transistor. In certain low level applications, however, it might be desirable 

 to work at a smaller current for the sake of decreasing battery power con- 

 sumption. This requires that a small current be drawn out of the base. The 

 required current is small because the collector current will decrease by 

 1/(1 — a) microamperes for each microampere drawn from the base. One 

 method of obtaining this base current is to provide a resistive path between 



