TRANSISTOR CIRCUITS FOR ANALOG AND DIGITAL SYSTEMS 



311 



100 



LU 



m 

 u 



LU 

 Q 



<^ 



z 

 < 



H 

 Z 

 UJ 



a. 

 a. 



D 



o 



Q. 

 O " 



o 



_l 



80 



60 



40 



20 



20 



-40 



■160 



-200 



to 



LU 

 -240 ^ 



O 



LU 



Q 



-280 7 



•320 



-360 



-400 



■440 



10= 

 FREQUENCY IN CYCLES PER SECOND 



10^ 



10' 



Fig. 10 — Loop current transmission of the summing amplifier. 



below db. The amplifier has a 68° phase margin and 27.5 db gain margin. 

 In order to insure sufficient feedback at dc and adequate margins against 

 instability, the transistors used in the amplifier should have alphas in 

 the range 0.98 to 0.99 and alpha-cutoff frequencies equal to or greater 

 than 2.5 mc. 



3.2. Automatic Zero Set of the dc Summing Amplifier 



The application of germanium junction transistors to dc amplifiers 

 does not eliminate the problem of drift normally encountered in vacuum 

 tube circuits. In fact, drift is more severe due principally to the varia- 

 tion of the transistor parameters alpha and saturation current with 

 temperature variation. Even though the amplifier has 80 db of negative 

 feedback at dc, this feedback does not eliminate the drift introduced by 

 [the first transistor stage. Because of the large amount of dc feedback, 

 the collector current of the first stage is maintained relatively constant. 

 The collector current of the transistor is related to the base current by 

 the equation 



Ic = 



/c 



+ 



a 



I — a 1 — a 



(18) 



[The saturation current, Ico , of a germanium junction transistor doubles 

 (approximately for every 11°C increase in temperature. The factor 

 a/(l — a) increases by as much as 6 db for a 25°C increase in tempera- 



