The Bell System Technical Journal 



April. 1924 



High Frequency Amplifiers 



By H. T. FRIIS and A. G. JENSEN 



IN this papiT. ii simplilU'd inallienidtiral trt-.itment of the thec>ry 

 of hii;h frec|iuMuy aiuphticrs is presented, and the theory is verified 

 by experiment. This method of mathematical analysis provides a 



ERRATA 



ISSUE OF JANUARY, 1924 



On page U)2, hne 11 from bottom of page, and on 

 page 163, line 4: 



read Kt instead of Kb. 



On page 173, line 3 from bottom of page: 

 read emerging instead oj engineering. 



at its natural frequency, i.e., the transformer inductance and dis- 

 tributed capacity must be in resonance. We have therefore, next 

 treated the simplest type of resonance circuit amplifier, namely, a 

 single tuned circuit amplifier, and it is shown that exactly the same 

 method can be used for a choke coil amplifier or a close coupled 

 transformer am|)lifier. Finally, it is shown that a loosely coupled 

 transformer amplifier can l)e treated like two coupled tuned circuits. 

 Considering a low frequency transformer-coupled amplifier, in 

 Fig. 1 (a) there is shown an amplifier tube I with its output trans- 

 former 7 working into another tube II and in Fig. 1 (b) is given the 

 corresponding equivalent circuit. The equivalent circuit is obtained 

 by the theorem, that the plate circuit of a vacuum-tube may be 



181 



