228 



BELL SYSTEM TECHNICAL JOURNAL 



12 to 14 db were obtainable for 65 mc amplifiers of the required bandwidth 

 with 6AK5 tubes and matched input transformers. This was much worse 

 than was desired. By using a 6J4 close spaced grounded grid triode in the 

 input stage, much lower noise figures were obtained, but the gain with 

 matched input and output circuits was so low (approximately 3 db for a 

 coupling coefficient of 0.5) that the noise from the following stage contributed 

 considerably to the overall noise figure. 



By removing the loading resistance on the output side of the first inter- 

 stage transformer and reducing the coupling coefficient to 0.3, the gain was 



K =0.5 



tu 



Q.-I 



(nLU 



UJCQ 



>^ 



0.5 



5 10 



_J 



UJ 



Q 



> ^ 

 »- 

 < 

 -I 

 "J ft 



35 



45 



50 55 60 65 70 75 80 



FREQUENCY IN MEGACYCLES PER SECOND 



95 



Fig, V-7. — Band pass and delay characteristics of the IF transformer. 



raised sufficiently to give an overall noise figure of the order of 7 db. Figure 

 V-5 shows the preamplifier developed for the New York-Boston circuit. 

 This amplifier employs a 6J4 and a WE 404-A and provides a gain of 23 db. 



The 6J4 tube, when used in a grounded grid circuit, has an input imped- 

 ance of approximately 85 ohms which is close to the desired 75 ohm imped- 

 ance. When a good match is required, it is necessary to use an input 

 transformer, but it should be noted that an improvement in the noise figure 

 can be obtained by deliberately producing a mismatch in the right direction 

 at the input. Noise figures as low as 4 db have been obtained in this manner 

 with recent experimental tubes. 



Figure V-8 is a schematic diagram of an amplifier with a very low noise 



