308 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



shaping the loop current transmission. If the feedback impedance Zk 

 (Fig. 4) consists of a resistance Rk and condenser Ck in parallel, then 

 the loop current transmission is modified by the factor 



1 + 



CO; 





1 + ^ 



COS 



(17) 



where 



C07 = 



C08 = 



RkCk 



(Rl_±_Rk) 



RlRkC K 



Since Zk affects the external voltage gain of the operational amplifier, 

 (11), the corner frequency C07 must be located outside of the useful fre- 

 quency band. Usually it is placed near the gain crossover frequency in 

 order to improve the phase margin and the transient response of the 

 amplifier. 



In Sections 3.0 and 4.0, the above shaping techniques are used in the 

 design of specific operational amplifiers. 



3.0. THE SUMMING AMPLIFIER 



3.1. Circuit Arrangement 



The schematic diagram of a dc summing amplifier is shown in Fig. 8. 

 From the discussion in Section 2.0 it is apparent that each common 

 emitter stage will contribute more than 90 degrees of high-frequency 

 phase lag. Consequently, while the magnitude of the low-frequency : 

 feedback increases with the number of stages, this is at the expense of , 

 the bandwidth over which the negative feedback can be maintained. 

 It is possible to develop 80 db of negative feedback at dc with three 

 common emitter stages. This corresponds to a dc accuracy of one part 

 in 10,000. In addition, the feedback can be maintained over a broad 

 enough band in order to permit full accuracy to be attained in about 

 100 microseconds. Thus it is evident that the choice of three stages repre- 

 sents a satisfactory compromise between accuracy and bandwidth ob- 

 jectives. 



The output stage of the amplifier is designed for a maximum power 

 dissipation of 75 milliwats and maximum voltage swing of ±25 volts 



I 



