204 BELL SYSTEM TECIIMCAL JOlh'XAL 



mistors have proven to be very valuable as automatic gain controls, es- 

 pecially for use with negative feedback ampliliers. This arrangement has 

 seen extensive use in wire communication circuits for transmission level 

 regulation, and has been described in some detail elsewhere.^-- ^^' ^^ In 

 one form, a directly heated thermistor is connected into the feedback circuit 

 of the amplifier in such a way that the amount of feedback voltage is varied 

 to compensate for any change in the output signal. By this arrangement, 

 the gain of each amplifier in the transmission system is continually adjusted 

 to correct for variations in overall loss due to weather conditions and other 

 factors, so that constant transmission is obtained over the channel at all 

 times. In the Type K2 carrier sj^stem now in extensive use, the system 

 gain is regulated principally in this way. In this system the transmission 

 loss variations due to temperature are not the same in all parts of the pass 

 band. The loss is corrected at certain repeater points along the transmission 

 line by two additional thermistor gain controls: slope, proportional to fre- 



H EATER T^'PE 

 /T HERMISTOR 



constantI /;t\ ipRi I^CCt^ to" 



CURRENTS (^) ^f^2 (Nif) CONTROLLED 



SOURCE T Vp^t I rV^W CIRCUIT 



DISC 

 THERMISTOR 



HEATER THERMISTOR 



Fig. 21. — Circuit employing an auxiliary disc thermistor to compensate for effect of 

 varying ambient temperature on a control thermistor. 



quency, and bulge, with a maximum at one frequency. These thermistors 

 are indirectly heated, with their heaters actuated by energy dependent upon 

 the amplitude of the separate pilot carriers which are introduced at the send- 

 ing end for the purpose. 



In this type of application, the thermistor will react to the ambient tem- 

 perature to which it is exposed, as well as to the current passing through it. 

 Where this is important, the reaction to ambient temperature can be elimi- 

 nated by the use of a heater type thermistor as shown in Fig. 21. The 

 heater is connected to an auxiliary circuit containing a temperature com- 

 pensating thermistor. This circuit is so arranged that the power fed into 

 the heater of the gain control thermistor is just sufficient at any ambient 

 temperature to give a controlled and constant value of tejnjjerature in the 

 vicinity of the gain control thermistor element. 



Another interesting form of thermistor gain control utilizes a heater 

 type thermistor, with the heater driven by the output of the amplifier and 

 with the thermistor element in the input circuit, as shown in Fig. 22. In 

 this arrangement the feedback is accomplished by thermal, rather tiian 

 electrical coupling. A broad-band carrier system, Type LI, is regulated 



