men VREQUENCY .IMri.lllF.RS 203 



111 ili'tiTiniiiink; A"', it is neccssiiry to kimw tlie .ictii.il ii>i>t,in( c R 

 of the loop and this may be conveniently obtained 1)\' the reactance 

 variation niethoil using a tube voltmeter across half of the loop as the 

 voltage indicating device. It has been found that the loss introduced 

 by such a tube voltmeter is negligible, a fact which can be easily 

 checked by connecting two similar tube voltmeters across the loop 

 and determining the maximum reading of one of them. When the 

 other one is then disconnected and the loop condenser slightly re- 

 adjusted so as to again give maximimi reading of the first tube \(ilt- 

 meter, it will be found, that the two rcadiiii;'^ oljiaiiicd are exactly 

 the same. 



The tliscussion of the two types of amplification measurements of 

 high frequency amplifiers may be summarized as follows: 



The ordinary voltage amplification K is defined as the ratio of the 

 amplified signal voltage impressed on the grid of the low frcquenc>' 

 detector and the signal voltage impressed on the grid of the first 

 amplifier tube. This amplification is measured by using a resistance 

 input to the amplifier and includes the effect of "feed-back" action 

 between the stages in the amplifier. This "feed-back" action between 

 stages can naturally be analyzed by a method similar to the one used 

 to determine the "feed-back" action between the amplifier and its 

 tuned input circuit. 



The "feed-back" amplification A" is defined as the increase (or 

 decrease) of signal voltage due the "feed-back" action between 

 amplifier and its tuned input circuit. The "feed-back" amplification 

 depends upon the selecti\ity of the input circuit and will only vary 

 slightly from unity when the resistance of this circuit is very large, 

 while large variations, as shown in Fig. 12, may be found when a 

 selecti\e input circuit is used. 



The total amplification is defined as the product of the ordinar\' 

 amplification A" and the "feed-back" amplification K' . 



