422 



THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1952 



The coefficients are simply the open-circuit driving point and trans- 

 fer impedances of the transistor, or the slopes of the appropriate 

 static characteristics at fixed dc operating currents. These equations 

 may be represented by any one of a large number of equivalent cir- 

 cuits of which the one shown in Fig. 11 is perhaps currently most useful. 

 In this circuit Vg is very nearly the ac forward impedance of the emit- 

 ter barrier, Vc is very nearly the ac reverse impedance of the collector 

 barrier, Vb is the feedback impedance of the bulk germanium common 

 to both, and a is the circuit current gain representing carrier collec- 

 tion and multiplication if any. It turns out this is very nearly equal 

 to the current multiplication factor a of the collector barrier mentioned 

 before. Average values of these elements for the type A transistor are 

 given in Fig. 11. In Fig. 12 are given the ranges of these parameters 

 for the type A as of September, 1949, and the control limits* for the 

 same characteristics for new point-contact transistors now under de- 

 velopment. For September, 1949, the ranges are taken about the average 

 values shown in Fig. 11 for the type A transistor. The control limits 

 given for the present situation apply to a number of different types of 

 point contact transistors so that the present average values of these 



Vg = LfZ|, + LcZ,2 

 V^ = Lg Z21 + Lq Z22 



Fig. 10 — The general linear transistor. 



equivalent circuit elements depend upon the type of transistor con- 

 sidered. In Fig. 13 are given the average values of the characteristics 

 of the M1729 point-contact video amplifier transistor which bears the 

 closest resemblance to the older type A transistor. By way of contrast 

 are given some typical values of the elements for the M1752 junction 

 transistor which is not yet far enough along in its development to have 

 design centers fixed nor reliable dispersion figures available. 



As Ryder and Kircher have shown, ^ transistors in the grounded-base 

 connection may be short-circuit unstable if a > 1 and ri, is too large, 



* A.S.T.M. Manual, "Quality Control of Materials," Jan. 1951, Part III, pp. 55- 

 114. 



