HIGH FREQUENCY AMPLIFIERS 201 



i-.itioii iil)t.iiiH-(l witli .iii\ kiiul (if .111 .iiiiplitiiT will, in j^iiitral, lie 

 higher at the lowi-r frtviueiuics. diU' to the Itiwcr loss .iiid the lii^ilu-r 

 ratio of L over C obtainable. 



The width of the freciiiency band for a dioke coil aniplit'ier will be 

 smaller, the higher the frequency due to the decrease in o> L with 

 increasing frequency, and at broadcasting frequencies it will, there- 

 fore, in general be found a(l\'antageous to use loosely cou])led trans- 

 formers rather than choke coils, whenever a wide frequency band 

 is desired. In addition to giving a wider frequency band, lower 

 fre<|uency amplitiers ha\e the ad\antage of a smaller grid-plate 

 feet! back action. 



.\MI'l.IFIlATIO.V MkASIRKNIKNTS AT Hll.H I-'REQfENXIES 



111 order to make a thorough study of radio freciucncN' am[ilifiia- 

 tion, it is necessary to ha\e a dependable method of measurement. 

 Such a method developed in our laboratory and used very successfully 

 will be described here. 



In order to obtain an accurate comparison between ditTerent types 

 of amplifiers, in which any type of resonant coupling is used, it is 

 essential that these amplifiers be operated from a resistance input and 

 not from an input containing a tuned circuit. With a tuned circuit 

 it is not only very difficult to obtain an accurate measure of the 

 voltage impressed upon the amplifier but considerable regeneration 

 may occur between this input circuit and the output circuit of the 

 first amplifier tube. There is, naturally, also a feed-back action 

 in connection with a resistance input circuit, but its effect is negligible 

 when the resistance is only a few hundred ohms. When the charac- 

 teristic of a radio frequency ainplifier with a resistance input has been 

 accurately determined, its characteristic when used with a tuned 

 circuit input may be determined as will be described later. 



A schematic circuit diagram of the apparatus as used is shown in 

 Fig. 3. To the left is shown the input apparatus which consists of 

 an oscillator, a sensitive thermocouple and a potentiometer. The 

 drop across the resistance R\ of the potentiometer is used as the input 

 to the amplifier stage I. The output of the amplifier stage is meas- 

 ured by the tube voltmeter II shown to the right in Fig. 3. The tube 

 voltmeter II may be a low frequency detector in the case of amplifi- 

 cation measurements of an actual receiver set. 



It is necessary first to calibrate the tube voltmeter or detector II 

 which is done by disconnecting it from the amplifier and connecting 

 it directly across the potentiometer R\- Ri. Rt is then adjusted to. 



