PROPERTIES AND APPLICATIONS OF fl-p-n TRANSISTORS 559 



the capacitance is seen to be approximately 7 micro-microfarads. In terms 

 of the equivalent circuit, this capacitance is in shunt with the series combina- 

 tion of fc and the generator, fmie , as shown in Fig. 26. This can be shown 

 to be equivalent to the circuit of Fig. 27 in which Tc has been replaced by 



r/ = rc/(l + jCcTcOi) (67) 



and fm has been replaced by 



rL = rj{\ + jCcTcOj). (68) 



The effect of collector capacitance can be computed by substituting 

 Tc and Ym for the values of r^ and Tc (implicitly contained) in equation (8). 

 In the sections which follow, this will be done for each of the three transistor 

 connections and the resulting collector cutoff frequencies jcc will be com- 

 puted. It will be shown that at least for the transistor on which data are pre- 

 sented collector capacitance tends to produce a cutoff frequency well below 

 those to be expected from emitter cutoff or alpha cutoff. For this reason, 

 only collector cutoff will be considered. 



Collector Cutoff in the Grounded Base Stage 



If the values of Tc and Tm from equations (67) and (68) are substituted 

 for Tr. and Tm in equations (16) and the resulting values of the i?'s are sub- 

 stituted into equation (8), the result is 



"''^"'^ ^ (re -\-H-\- R,)[\ -f k(l + icoC.r,)/rJ ^^^^ 



The cutoff frequency fc: is defined as the frequency at which the voltage 

 across the load has dropped 3 db compared to its low-frequency value. 

 This is the frequency at which the imaginary part of the denominator of (69) 

 is equal to the real part. Solving for fee gives 



f =-^[1 + 1 



I (70) 



i?L(re + r6+i?a). 



Substituting into this equation Cc = 7(lO)-i2 farad, numerical values 

 of the r's from (16), and the values Rg = 91 and Rl = 4.58 (10)« ohms, cor- 

 responding to maximum available gain gives 



/,, = 3390 cps. 



With these terminations, the low-frequency gain is 44.3 db. If Rg and Rl 

 are reduced to 25 and 200,000 ohms, respectively, fee is raised to 23,500 cps 

 and the gain is lowered to 37.2 db. A further reduction of Rl to 20,000 ohms 

 increases /cc to 0.22 megacycles and reduces the gain to 27.8 db. This corre- 



