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BELL SYSTEM TECHNICAL JOURNAL 



voltage, and Er , I and Rs are respectively the input voltage, current and 

 resistance. 



If the thermistor and associated resistor are placed in series between the 

 generator and load resistance, an expandor is obtained, and 



Eg = Ej — E. 



(24) 



As the resistance R in series with the thermistor is increased, the degree of 

 expansion is decreased and vice versa. 



4 8 12 16 



CURRENT IN MILLIAMPERES 



20 



Fig. 23. — Characteristics of a simple thermistor voltage regulator, limiter or com- 

 pressor circuit. 



The treatment thus far in this section assumes that change of operating 

 point occurs slowly enough to follow along the static curves. For a suffi- 

 ciently rapid change of the operating point, the latter departs from the static 

 curve and tends to progress along an ohmic resistance line intersecting the 

 static curve. For sufficiently rapid fluctuations,- control action may then 

 be derived from the resistance changes resulting from the r.m.s. power dis- 

 sipated in the thermistor unit. In speech volume limiters, the thermistor 

 is designed for a speed of response that will produce limiting action for the 

 changes in volume which are syllabic in frequency or slower, and that will 

 not follow the more rapid speech fluctuations with resulting change in wave 



