1226 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 



negative and low when the emitter current is positive. Experimentally, 

 it is found convenient to give three values to r^ and hence to Rn , one 

 for each region as shown in Fig. 14. This recognizes the non-linearity 

 with It in the forward direction and assumes that a single value in the 

 reverse direction is sufficient. As the circuitry becomes more sophisti- 

 cated a more precise approximation will undoubtedly be required, 

 particularly near I^ = 0. 



It may be noted that in the functional relation Rn = /(/« , Ic) that 

 Rn is taken to be a function of It only. The contribution of Ic is to shift 

 the characteristic in voltage by VbAIc increments. Thus the relation- 

 ship of Ve = f(Ie , Ic) can be written very simply as 



7. = Rnle + Rnic (8) 



Since the problem has been linearized to first order terms only, the 

 currents and voltages are total instantaneous or dc values as indicated 

 by the capital letters. 



IDEALIZATION OF Roi 



As indicated previously, Rn will be small in Regions I and III and 

 large in Region II. Since Ru = Vm -\- n , Rn can be no less than rb ; the 

 defining approximations will be applied to r^ . In Region I when the 

 emitter current is negative, Vm is taken to be zero and reflects the device 

 approximation that the emitter current under this condition is entirely 

 electron current. This is not always a true approximation, particularly 

 near 7^ = 0, and limiting tests are employed in transistor testing. 



REGION in 



Fig. 14 — Idealization and regional division of input characteristic (Rn). 



