PROPERTIES AND USES OF THERMISTORS 205 



with this type of thermistor. In this system a pilot frequency is suppHed, 

 and current of this frequency, selected by a network in the regulator, actu- 

 ates the heater of the thermistor to give smooth, continuous gain control. 

 By utilizing a heater thermistor of diflferent characteristics, the circuit 

 and load of Fig. 22 may be given protection against overloads. In this 

 application the sensitivity and element resistance of the thermistor are 

 chosen so that the thermistor element forms a shunt of high resistance 

 value so as to have negligible effect on the amplifier for any normal value of 

 output. However, if the output power rises to an abnormal level, the 

 thermistor element becomes heated and reduced in resistance. This 

 shunts the input to the amplifier and thus limits the output. Choice of a 

 thermistor having a suitable time constant permits the onset of the limiting 

 eflfect to be delayed for any period from about a second to a few minutes. 



LOAD 



THERMISTORS"^ ^HEATER 



HEATER nPE THERMISTOR 



Fig. 22. — Thermal feedback circuit for gain control purposes. This arrangement has 

 also been used as a protective circuit for overloads. 



Regulators and Limiters 



A group of related applications for thermistors depends on their steady 

 state nonlinear volt-ampere characteristic. These are the voltage regulator, 

 the speech volume limiter, the compressor and the expandor. The com- 

 pressor and expandor are devices for altering the range of signal amplitudes. 

 The compressor functions to reduce the range, while the expandor increases 

 it. In Fig. 23, Curve 1 is a typical thermistor static characteristic having 

 negative slope to the right of the voltage maximum. Curve 2 is the charac- 

 teristic of an ohmic resistance R having an equal but positive slope. Curve 

 3 is the characteristic obtained if the thermistor and resistor are placed 

 in series. It has an extensive segment where the voltage is almost inde- 

 pendent of the current. This is the condition for a voltage regulator or 

 limiter. If a larger value of resistance is used, as in Curve 4, its combination 

 with the thermistor in series results in Curve 5, the compressor. In these 

 uses the thermistor regulator is in shunt with the load resistance, so that 

 in the circuit diagram of Fig. 23, 



E = Eo = Ei- IRs. (23) 



Here E is the voltage across the thermistor and resistor R, Eo is the output 



