THE L3 SYSTEM — AMPLIFIERS 



897 



magnitude of main pilot level at the output of the amplifier. The whole 

 forms an AVC circuit which acts to keep the level of the main pilot 

 constant at amplifier output, under the assumption that any changes in 

 this pilot level are caused by line temperature or length deviations. The 

 invariant arms of the regulating network are designed to give the ac- 

 curate shaping of network impedance versus frequency required so that 

 the amplifier gain change will have the wanted square-root of frequency 

 shape. The design of the netw^ork is fundamentally based on the variable 

 equalizer theory developed by H. W. Bode,^ but the process of finding 

 physical networks which mil give the desired performance to a very 

 high order of accuracy makes use of newer methods developed by S. 



OUTPUT 

 AMPLIFIER 



INPUT 

 AMPLIFIER 



Zi 



i- 



Fig. 11(a) — Regulating network block schematic. 



Darlington.^' ^ Since these methods were also used in the design of several 

 other amplifier networks, and because the precision thus made possible 

 is essential to the system, a brief recapitulation of the steps involved 

 is of interest. 



Fig. 11(a) shows the basic configuration of the regulating network. 

 It was selected because it is one of the simplest that gives symmetrical 

 gain changes controlled by a single resistor. It is also capable of absorb- 

 ing the parasitic interamplifier capacity, and gives some advantageous 

 gain shaping at normal setting, although this shaping is not under de- 

 sign control. 



We need to find the impedance of the two arms Zi and Z2 , that will 

 give the required square-root of frequency shape of gain change (with 

 a value of 6 db at 8 mc) as the thermistor is varied by a factor of three. 

 For this circuit the change in gain may be expressed as a ratio of two 

 impedances. 



