834 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1954 



^-HIGH RESISTANCE REGION 

 ->--*« SATURATION VOLTAGE REGION- 



10"^ 10" = 



CURRENT IN AMPERES 



Fig. 5- — Reverse characteristics of a reference voltage diode. 



range can be compared to the 60 to 150 volts range of cold cathode volt- 

 age regulator tubes which are also used as sources of reference potential. 



2.22. Saturation Voltage Utilized in Regulating Circuits 



In all check back (feedback) regulating circuits the potential to be 

 regulated is compared to a reference potential. This comparison is a 

 form of subtracting the two values so that the changes in the potential 

 to be regulated produce a large percentage change in the difference or 

 error voltage. The methods by which this is accomplished in direct ciu'rent 

 circuits are illustrated in Section 3. A stable source of reference potential 

 is required for this type of regulation. When the saturation voltage of a 

 silicon junction diode is used for this piu'pose, we have called the device 

 a "reference voltage diode." 



2.23. Effect of Temperature on Saturation Voltage 



In order to evaluate the stability of Diode I in its saturation voltage 

 region a small section of Fig. 5 has been redrawn in Fig. 6 using a linear 

 scale. Additional curves are included in Fig. 6 to show the change of 

 voltage with ambient temperature variations. The slope of the 30 degree 

 curve in Fig. 6 is equivalent to a resistance of 200 ohms in series with a 

 23-volt battery with current flowing through this combination from an 

 external source. The change of potential with ambient temperature is 

 equivalent to a 0.07 per cent change per degree C. It should not be in- 

 ferred that these are limiting values, for diodes hiive been tested which 

 exhibit slopes of less than 10 ohms and temperature coefficients of less 



