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BELL SYSTEM TECHNICAL JOURNAL 



is employed, and the optimum grid impedance at 60 mc as shown in Fig. 26 

 is approximately 3000 ohms. 



The vacuum tube employed in an input stage of an IF amplifier should 

 have the following characteristics to achieve the optimum performance. 

 The tube should be capable of providing sufficient gain to assure that the 

 noise contribution of the following stages is negligible. The input con- 

 ductance of the selected tube at the intermediate frequency should be as low 

 as possible indicating generally that physical size should be small to assure 

 small transit time effects and low lead inductance values. The noise output 

 of the tube itself should be a minimum, this characteristic being somewhat 

 controllable by proper emission characteristics. The characteristics of the 

 most desirable of the vacuum tubes available to the radar receiver designer 

 during the past war period for IF amplifier purposes is given in Table I. 



Table I. — Principal Characteristics of IF Amplifier Tubes 



The development of the 6AK5 pentode was the direct result of tlie necessity 

 for improved radar IF amplifier performance, and details of this develop- 

 ment and the performance of this tube have been described elsewhere. ^- 



The noise present in a pentode is greater than in a triode, primarily due to 

 the presence of additional grid structures. Because of this fact, a number 

 of attempts have been made to employ triodes in the input stage of the IF 

 amplifier. To prevent oscillation due to large positive feed-back present 

 through the plate-to-grid interelectrode capacitance, neutralization methods 

 have been employed. For moderate IF band widths at 60 mc such experi- 

 mental designs have shown an improvement in noise figure of slightly more 

 than 1 (lb over the pentode design; however, the criticalness of the neiilrali/a- 

 tion scheme and the difiiculties in extending the ])erformance to wider II'" 

 band widths has not allowed this design to be adojiled extensively to 

 military radar e([ui])ment during the ])ast war period. 



'2 "Characteristics of Vacuum TuIjcs for Radar Intermediate Frequencv .Xmijlifiers," 

 G. T. Ford, Bell System Teclmical Journal, Vol. XXV, July 1946. 



