PRINCIPLES OF TRANSISTOR ACTION 251 



and the impedance of the generator is equal to the emitter resistance, 

 Ru, is: 



Gain = a^R22/4.Riu (2.13) 



which is the ratio of the collector to the emitter resistance multipUed by 

 1/4 the square of the current amplification factor. This gives the a-c. 

 power delivered to the load divided by the a-c. power fed into the tran- 

 sistor. Substituting the values Usted above (Eqs. (2.11)) for the unit whose 

 characteristics are shown in Fig. 3 gives a gain of about 80 times (or 19 db) 

 for the operating point Pq. This is to be compared with the gain of 21 db 

 estimated above for operation between Pi and P^. The difference of 2 

 db represents the increase in gain by feedback, which was omitted in Eq. 

 (2.13). 



Equations (2.5) and (2.6) may be solved to express the currents as func- 

 tions of the voltages, giving 



A7e= FnAF.-f 7i2AF, (2.14) 



Ale = Yn AVe + F22 AF. (2.15) 



where 



Fii = R22/D, F12 = -Rn/D , . 



F12 = -i?2i/£>, F22 = RxxlD ^ ■ ^ 



and D is the determinant of the coefficients 



D = Rn R22 - R12 R21. (2.17) 



The admittances, Fn and F22, are negative if D is negative, and the tran- 

 sistor is then unstable if the terminals are short-circuited for a-c. currents. 

 Stability can be attained if there is sufiicient impedance in the mput and 

 output circuits exterior to the transistor. Feedback and instability are 

 increased by adding resistance in series with the base electrode. Further 

 discussion of this subject would carry us too far into circuit theory and 

 applications. From the standpoint of transistor design, it is desirable to 

 keep the feedback resistance, Rn, as small as possible. 



Variation with Spacing 



One of the important parameters affecting the operation of the transistor 

 is the spacing between the point electrodes. Measurements to investigate 

 this effect have been made on a number of germanium surfaces. Tests 

 were made with use of a micro-manipulator to adjust the i^ositions of the 

 points. The germanium was generally in the form of a slab from .05 to 

 0.20 cm'thick, the lower surface of which was rhodium plated to form a low 

 resistant contact, and the upper plane surface ground and etched, or other- 



