204 



BELL SYSTEM TECHNICAL JOURNAL 



plate circuit of the first amplifier in a high frequency amplifier set, 

 it will not be possible to measure this with a resistance input to the 

 amplifier since in this case the "feed-back" has no appreciable effect. 

 In order to get the correct value for the "feed-back" amplification, 

 the set must be connected up to the same loop with which it is going 

 to be used and the measurements can then be made in the following 

 way. 



A resistance box is inserted in llu" iiiiddle (if the loop and a tube 

 voltmeter is connected across half of the loop in addition to the 

 receiving set as shown in Fig. 14. With the filament circuit of the 

 set open, a strong high frequency cmf. is induced in the loop and the 

 loop circuit is tuned until the tiilic \oltmeter reads a maximum. 



# 



TUBE VOLT- 

 METER 



RECEIVING 5ET 



Fig. 14 — Method of Measuring "Feed Back" .•Xniplifuation 



A "feed-back" action in the set will then iiroduce a change in tiie 

 tube voltmeter reading when the filament current is switched on. 

 If the "feed-back" action is positive, i.e., if the resistance introduced 

 in the loop is negative, then the tube voltmeter reading will increase, 

 and in order to bring it back to its former value, the resistance of the 

 loop is increased by an amount R' by means of the resistance box. 

 If, on the other hand, the "feed-back" action is negative, the resistance 

 of the loop must be decreased in order to obtain the former value of 

 the tube voltmeter reading. 



R' represents the equivalent series resistance introduced in the 

 loop circuit by the "feed-back" action, and the apparent resistance 

 of the loop is, therefore, R — R', where R is the actual resistance of the 

 loop. The voltage impressed upon the grid of the first tube is in- 

 versely proportional to the apparent resistance of the loop, and the 

 amount of "feed-back" amplification is, therefore, defined as the ratio 

 K' = R/{R — R') where R' must be taken with the proper sign. This 

 ratio is seen to be a direct measure of the increase (or decrease) in 

 input voltage due to the "feed-back" action in the set and the total 

 amount of amplification in a set at a certain frequency will then 

 be given by the product of the ordinary voltage amplification factor 

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



