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BELL SYSTEM TECHNICAL JOURNAL 



characteristic to be simulated with sufficient precision. For example, 

 we are obviously in physical difficulties if the cutoff characteristic 

 specifies a net gain around the loop at a frequency so high that the 

 tubes themselves working into their own parasitic capacitances do not 

 give a gain. 



This limitation is studied most easily if the effects of the parasitic 

 elements are lumped together by representing them in terms of the 

 asymptotic characteristic of the loop as a whole at extremely high 

 frequencies. An example is shown by Fig. 11. The structure is a 



Fig. 11 — Elements which determine the asymptotic loop transmission characteristic 



in a typical amplifier. 



shunt feedback amplifier. The /3 circuit is represented by the T com- 

 posed of networks N^, Ne and Nt. The input and output circuits are 

 represented by Ni and Ni and the interstage impedances by N2 and N3. 

 The C's are parasitic capacitances with the exception of C5 and Ce, 

 which may be regarded as design elements added deliberately to TVs and 

 A''6 to obtain an efficient high frequency transmission path from output 

 to input. At sufficiently high frequencies the loop transmission will 

 depend only upon these various capacitances, without regard to the 

 N's. Thus, if the transconductances of the tubes are represented by 

 Gu Gi, and Gz the asymptotic gains of the first two tubes are Gi/coCi 

 and G2/C0C3. The rest of the loop includes the third tube and the 

 potentiometer formed by the capacitances Ci, d, C5 and Ce- Its 

 asymptotic transmission can be written as GzjoiC, where 



C = Ci + C4 + 



C1C4 



(Ca + Ce). 



Each of these terms diminishes at a rate of 6 db per octave. The com- 

 plete asymptote is GiG^Gzloi^CdCz. It appears as a straight line with 

 a slope of 18 db per octave when plotted on logarithmic frequency 

 paper. 



