372 



BELl, SYSTEM TECIISICM. JOIRXAL 



In the choice among various equivalent circuits, it appears that tlie oji- 

 timum of convenience is also the one which most closely approaches the 

 underlying physical situation. In agreemg to use the hlack box approach we 

 have resolutely ignored the physical details, but here they are presenting 

 themselves in a new way, having sneaked in the back door after we barred 

 the front. Now, however, having chosen an equivalent circuit, we shall 

 continue pursuing the circuit analysis in resolute ignorance of the physics. 

 In what follows various equivalent circuits may be used, depending on the 

 convenience of the moment. 



Figs. 4 — Some equivalent circuits. 



CONSTANT- 

 CURRENT 

 GENERATOR 



HIGH- 

 IMPEDANCE 

 VOLTMETER 



Principle of measurement method. 



The principle of a method used for rapid measurement of the transistor 

 impedances is shown in Fig. 5, illustrating the measurement of forward 

 transimpedance. A pair of terminals of the transistor is driven by a small 

 alternating current of a few thousand cycles from a high impedance gen- 

 erator; the voltage developed is read by a high-impedance voltmeter. By 

 calibrating the meter directly in ohms, one can read off the open circuit 

 resistances of the unit as rapidly as one can switch and read meters. 



Average values found by this method for the Type A transistor are shown 

 on Fig. 6, together with data on the direct-current operating point. Since 



