250 BELL SYSTEM TECHNICAL JOURNAL 



small changes from the operating point, and may be small a-c. signals. 

 The coefficients are defined by: 



i?U= (aFe/a/e)/,= const.. (2.7) 



Rn = (aFe/a/c)/. = const.. (2.8) 



R2I = {dVc/dIe)l, = const.. (2.9) 



R22= {dVjdh)l,=. const. (2.10) 



These coefl5cients are all positive and have the dimensions of resistances. 

 They are functions of the d-c. bias currents, /« and Ic, which define the 

 operating point. For le = 0.75 ma and /, = —2 ma the coefficients of the 

 unit of Fig. 3 have the following approximate values: 



Rn = 800 ohms, 



Rn = 300, ^2 in 



i?2i = 100,000, ^ ■ 



i?22 = 40,000. 



Equation (2.5) gives the emitter characteristic. The coefficient Rn 

 is the input resistance for a fixed collector current (open circuit for a-c). 

 To a close approximation, Rn is independent of h, and is just the forward 

 resistance of the emitter point when a current le is flowing. The coefficient 

 Rn is the feedback or base resistance, and is equal to Rf as defined by Eq. 

 (2.2) in case Rf is a constant. Both Rn and Rn are of the order of a few 

 hundred ohms, Ru usually being smaller than Rn- 



Equation (2.6) depends mainly on the collector and on the flow of holes 

 from the emitter to the coUector. The ratio R-n/Rn is just the current am- 

 plification factor a as defined by Eq. (2.1). Thus we may write: 



AFc = i?22 (aA/e + Mc). (2.12) 



The coefficient i?22 is the collector resistance for fixed emitter current (open 

 circuit for a-c), and is the order of 10,000-50,000 ohms. Except in the 

 range of large h and small h, the value of R^a. is relatively independent of Ic. 

 The factor a generally is small when h is small compared with h, and 

 increases with h, approaching a constant value the order of 1 to 4 when 

 Ic is several times /«. 



The a-c. power gain with the circuit of Fig. 1 depends on the operating 

 point (the d-c bias currents) and on the load impedance. The positive 

 feedback represented by R^ increases the a\-ailable gain, and it is possible 

 to get very large power gains by operating near a point of instability. In 

 giving the gain under such conditions, the impedance of the input generator 

 should be specified. Alternatively, one can give the gain which would 

 exist with no feedback. The maximum a\ailable gain neglecting feed- 

 back, obtained when the load R^ is equal to the collector resistance R^, 



