25^ 



BELL SYSTEM TECHNICAL JOURNAL 



Variation with Frequency 



Equations (2.5) and (2.6) may be used to describe the a-c. characteristics 

 at liigh frequencies if the coefficients are replaced by general impedances. 

 Thus if we use the small letters ie, Ve, ic, Vc to denote the amplitude and 

 phase of small a-c. signals about a given operating point, we may write 



Ve = Zii ie + Znic, (2.19) 



Vc = ^21 ie + ^22^c. (2.20) 



0.2 



10^ I06 10' 



FREQUENCY IN CYCLES PER SECOND 



Fig. 12 — Current amplification factor a vs. frequency 



Measurements of A. J. Rack and others,-^ have shown that the over-all 

 power gain drops off between 1 and 10 mc,' sec and few units have positive 

 gain above 10 mc/sec. The measurements showed further that the fre- 

 quency variation is confined almost entirely to Z21 or a. The other coeffi- 

 cients, Zu, Zn and Z22, are real and independent of frequency, at least up 

 to 10 mc/sec. Figure 12 gives a plot of a versus frequency for an experi- 

 mental unit. Associated with the drop in amplitude is a phase shift which 

 varies approximately linearly with the frequency. A phase shift in Z21 of 

 90° occurs at a frequency of about 4 mc/'sec, corresponding to a delay of 

 about 5 X 10"* seconds. Estimates of transit time for the holes to flow 

 from the emitter to the collector, to be made in Section V, are of the same 

 order. These results suggest that the frequency limitation is associated 

 with transit time rather than electrode capacities. Because of the difference 



