PRINCIPLES OF TRANSISTOR ACTION 249 



le increases, corresponding to a negative input admittance. For given 

 values of I',, and ]',., there are two possible operating points. Thus for 

 W = (^-l and Vc = —20 one may have /« = 0.3 ma, Ic = —1.1 ma or 

 /, = 1.0, Ic = -2.7. 



The negative resistance and instahiUly result from the effect of the col- 

 lector current on the emitter current.^ The collector current lowers the 

 potential of the surface in the vicinity of the emitter and increases the 

 efTective bias on the emitter by an equivalent amount. This potential 

 drop is RfIc, where Rp is a feedback resistance which may depend on the 

 currents flowing. The effective bias on the emitter is then Ve — RfU, 

 and we may write 



7, = J{Ve - RfIc), (2.2) 



where the function gives the forward characteristic of the emitter point. 

 In some cases Rp is approximately constant over the operating range; in 

 other cases Rp decreases with increasing h as the conductivity of the ger- 

 manium in the vicinity of the points increases with forward current. In- 

 crease of le by a change of Ve increases the magnitude of Ic, which by the 

 feedback still further increases le. InstabiUty may result. Some conse- 

 quences will be discussed further in connection with the a-c. characteristics. 

 Also shown on Fig. 3 is a load line corresponding to a battery voltage of 

 — 100 in the output circuit and a load, Rl, of 40,000 ohms, the equation of 

 the line being 



Vc= - 100 - 40 X 10 Uc. (2.3) 



The load is an approximate match to the collector resistance, as given by 

 the slope of the solid lines. If operated between the points Pi and P2, 

 the output voltage is 8.0 volts r.m.s. and the output current is 0.20 ma. 

 The corresponding values at the input are 0.07 and 0.18, so that the over- 

 all power gain is 



Gain '-' 8 X 0.20/(0.07 X 0.18) -^ 125, (2.4) 



which is about 21 db. This is the available gain for a generator with an 

 impedance of 400 ohms, which is an approximate match for the input 

 impedance. 



We turn next to the equations for the a-c. characteristics. For small 

 deviations from an operating point, we may write 



AVe = Ru Me + i^l2 A/„ (2.5) 



AVc = ^12 Me + R22 Mc, (2.6) 



in which we have taken the currents as the independent variables and the 

 directions of currents and voltages as in Fig. 1. The differentials represent 



