1230 THE BELL SYSTEM TECHNICAL JOURNAL, NOVEMBER 1952 

 THE ALPPL\ OR CURRENT GAIN FACTOR* 



The derivation just given has been in terms of the equivalent circuit 

 parameters, r, , rh , r^ , and r^ . Another circuit factor, alpha or the 

 short-circuit current gain, is also quite useful. Alpha has been defined 

 in Table 1 as the negative ratio of the incremental change in output 

 current to the incremental change in input current under the condition 

 of short-circuit output terminals. 



Thus alpha is restricted in interpretation to a specific device termina- 

 tion and care should be taken in the employment of alpha when other 

 terminations are involved. For example, the circuit current gain under 

 general conditions is given by Rix/R^i . The ratio R21/R22 has been 

 sometimes termed ac . Thus, 



^21 rb -{■ Rb + Tm (.^. 



Uc = —f = (lb; 



R22 n -\- Rb + r, -\- Re 



Depending upon the magnitudes of Rb and Re , the two current gain 

 ratios may be markedly different. In Region II where Vm and re are very 

 large the effects of Rb and Re in equation (16) often may be neglected. 

 The circuit current gain, ac , may then be taken as the device alpha. 

 In Region I, Vm has been taken as zero; hence the current gain will be 

 somewhat less than unity, given by (?'{, + Rb)/(rb + -Rs + ^c + Re), and 

 is definitely not zero. Eciually, in Region III, the circuit current gain is 

 not zero but rather approaches the ratio, (rj, + Rb)/{rb -\- Rb + Re)- 

 If Rb^ Re , the ratio is nearly unity, 



ANALYSIS OF NEGATIVE RESISTANCE CHARACTERISTIC 



The objectives of the circuit analysis, as stated previously, are: 



1. To determine operating conditions such as proper values of loads, 

 biases, trigger sensitivities and operating currents and voltages, 



2. To determine the relationships of the device parameters to the 

 circuit behavior in order that these parameters may be optimized, 

 properly characterized and controlled for required circuit performance. 



For example, the trigger sensitivity may be given by the voltage 

 difference between the load line intersection with the Region I portion 

 of the characteristic and the upper peak or turning point of the charac- 

 teristic as shown in Figs. 6, 7 and 9. The sensitivity A is thus determined 

 by the nearness of the bias point to the peak of the characteristic. 

 Since the bias is normally fixed, variations in the sensitivity will arise 



* This section is inserted parenthetically as clarifj'ing material due to the use 

 of the a-factor in subsequent discussion. 



