226 



BELL SYSTEM TECHNICAL JOURNAL 



(1) Cp/Gm is a quantity that is difi&cult to evaluate numerically, (2) the 

 magnitude of PI is measured in terms of the ratio of the two voltages, d 

 and gp, and (3) the measurement is dependent upon the gain of a vacuum 

 tube amplifier, V-1. These difficulties may be materially reduced in their 

 consequence by an internal calibration circuit. 



The internal calibration circuit (Fig. 15.3B) consists of a capacitor, Ca , 

 and resistor, R.a. , in series. If the reactance of Ca is very much greater than 

 Ra , and the plate resistance of V-1 is very much greater than the reactance 

 of Cp , the calibration is essentially independent of frequency. 



The internal calibration circuit (Figure 15.3B) enables the evaluation of 

 Cp/Gm to be carried out. The additional subscript, c, indicates "calibrate" 

 conditions. 



^Oc — IcR.i 



eicCoCAR.i 



1 + 



_XcJ 



= e„ 



(15.22) 



(15.23) 

 (15.24) 



Equation (15.15) remains the same for both "operate" and "calibrate' 

 conditions with the exception of the second subscript reserved for the 

 "calibrate" operation. Therefore, by solving for Cp/Gm we find 



^Pc^ \ 1 + 



Xc. 



(15.25) 



Equation (15.25) may be rewritten as (15.26), if (15.23) and (15.24) are sub- 

 stituted in (15.25) 



Cic Ra Ca 



^^-my^-it 



(15.26) 



If (15.26) is substituted in (15.19), it is found that PI may be expressed 

 as follows: 





a 



.(Ct - c.y. 



1 



v^-[a ''■'' 



