296 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1956 



I 



4.0. The integrator can be used to generate a voltage ramp which is 

 linear to within one part in 8,000. By means of an automatic zero set 

 (AZS) circuit which uses a magnetic detector, the slope of the voltage 

 ramp is maintained constant to within one part in 8,000 throughout a 

 temperature range of 20°C to 40°C. 



The voltage comparator, described in Section 5.0, is an electrical de- 

 vice which indicates the instant of time an input voltage waveform 

 passes through a predetermined reference level. By taking advantage 

 of the properties of semiconductor devices, the comparator can be de- 

 signed to have an accuracy of ±5 millivolts throughout a temperature 

 range of 20°C to 40°C. 



In Section 6.0, the system application of the transistor circuits is 

 demonstrated by assembling the summing amplifier; the integrator, and 

 the voltage comparator into a voltage encoder. The encoder can be used J 

 to translate an analog input voltage into an equivalent time interval 

 with an accuracy of one part in 4,000. This accuracy is realized through- 

 out a temperature range of 20°C to 40°C for the particular circuits 

 described. 



2.0. FUNDAMENTAL CONSIDERATIONS IN THE DESIGN OF OPERATIONAL 

 AMPLIFIERS 



The basic active circuit used in dc analog computers is a direct coupled 

 negative feedback amplifier. With appropriate input and feedback net- 

 works, the amplifier can be used for multiplication by a constant coef- 

 ficient, addition, integration, or differentiation as shown in Figure 1 

 The accuracy of an operational amplifier depends only on the passive 

 components used in the input and feedback circuits provided that there 

 is sufficient negative feedback (usually greater than 60 db). The time 

 that is required for the amplifier to perform a calculation is an inverse 

 f miction of the bandwidth over which the feedback is maintained. 

 Thus a fundamental problem in the design of an operational amplifier 

 is the development of sufficient negative feedback over a reasonably 

 broad frequency range. The associated problem is the realization of 

 satisfactory stability margins. Finally there is the problem of reducing 

 the drift which is inherent in direct coupled amplifiers and particularly 

 troublesome for transistors because of the variation in their character- 

 istics with temperature. 



The first step in the design is the blocking out of the configuration 

 for the forward gain circuit (designated A in Fig. 1). Three primary re- 

 quirements must be satisfied: 



(1) Stages must be direct coupled. 



