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BELL SYSTEM TECHNICAL JOURNAL 



This expression for band merit has been derived from the product of the 

 bandwidth and the voltage amplification for a simple band-pass interstage. 

 Higher band merits can be realized with a given tube by using more compli- 

 cated coupling networks. 



Another interpretation of band merit is to say that it is the frequency at 

 which the voltage amplification is unity. This is the frequency at which 

 the product of the transconductance and the reactance of the shunt 

 capacitance is unity. 



From the foregoing expression for band merit it is evident that, in general, 

 the higher the band merit the fewer is the number of stages that are required 

 to obtain a given gain and bandwidth. It is highly desirable to keep the 

 number of stages small in order to save space, weight and power consump- 

 tion and to avoid the use of unnecessary components which reduce the 



Table I 



reliability in operation. In practice the total amplification in the receiver 

 is made high enough so that the system noise, with no signal, produces a fair 

 indication on the output device when the gain control is set for maximum 

 gain. For a bandwidth of 5 mc, the equivalent RF input noise power level 

 is of the order of 2 X 10" ^^ watt and the power level necessary for a suitable 

 oscilloscope presentation in a radar is about 20 miUiwatts. The net over-all 

 gain needed is then about 110 db. Making an allowance of, say, 15 db for 

 losses in the detectors and elsewhere, a total of about 125 db gain is required. 

 A minimum of about 110 db of this is usually in the IF part of the receiver. 

 With this amount of gain as a requirement it is easy to see the importance 

 of high band merit, small size, and low power consumption in the IF tubes. 



Table I, above, compares the salient characteristics of the 6AK5 with 

 those of other similar types of tubes. 



The tube design factors which determine the band merit will now be 

 examined by considering an idealized case. For an idealized plane parallel 

 triode structure in which edge effects are assumed to be negligible, the plate 

 current can be related to the tube geometr\- and the applied voltages 

 approximately as follows :- 



2 "Fundamentals of Engineering Electronics," \V. G. Dow, pp. 44, 102, et seq. 



