TRANSISTOR CIRCUITS FOR ANALOG AND DIGITAL SYSTEMS 303 



band. In addition, it is desirable that the feedback fall off at a rate equal 

 to or less than 9 db per octave in order to insure that the dc aniplifier 

 has a satisfactory transient response. 



Three methods of shaping are described in this paper; local feedback 

 shaping, interstage network shaping, and (3 circuit shaping. Local feed- 

 back shaping will be described first. The analysis starts by considering 

 the current transmission of a common emitter stage, ecjuivalent circuit 

 shown in Fig. 2(b). If the stage operates into a load resistance Rl , then 

 to a good approximation the current transmission is given by 



where 



Gr = r" = ^ ~ ^° +/ (13)^ 



^^ 1 + ^ + '^ 



wi a)aCOc(l — ao -\- 8) 



RL+Te 



8 = 



COl = 



(1 - ao + 8) 

 1 + 5,1 



-^ ^ alpha-cutoff frequency 

 Ztt 



1 



Uc 



(7?x, + re)Cc 



It is apparent from expression (13) that if (1 — Oo + 8) is less than 0.1, 

 then the current gain of the common emitter stage falls off at a rate of 

 6 db per octave with a corner frequency at wi .f A second 6 db per octave 

 cutoff with a corner frequency at [co^ + (1 + 5)aJc] is introduced by the 

 p" term in the denominator of (13). A typical transmission characteristic 

 is shown in Fig. .5. The current gain of the common emitter stage is unity 

 at a frequency equal to 



ao 



1 +5 I 1 



* Expressions (13) and (14) are poor approximations at frequencies above 



' coo/27r. 



' t Strictly speaking the corner frequency is equal to 01/2 tt. However, for sim- 

 plicity, corner frequencies will be expressed as radian frequencies. 



