512 BELL SYSTEM TECHNICAL JOURNAL 



back values; accordingly the cathode current would have to be re- 

 duced below the maximum permissible from life considerations. 



d. The circuit degradation factor (1 -f CpijC-nf'^ becomes more un- 

 favorable as the active capacitance C22 is reduced by widening the 

 output spacing. For discussion and calculation of this factor, see 

 Appendix 2. 



e. A wider output spacing, by virtue of the reduced capacitance, per- 

 mits a higher maximum frequency limit on the tube. 



The actual choice of output spacing in the 1553 is .012". This com- 

 promise between the foregoing factors appears to be suitable at 4000 

 Mc/s. The output transit angle of 1.6 radians gives 78% of the theoretical 

 optimum intrinsic gain-band product. The anode dissipation is near the 

 maximum safe value for the maximum allowable cathode current. The 

 grid runs very close to cathode potential so that grid current is small. 

 The circuit degradation factor has a value of about 0.8, while the upper 

 frequency limit of the tube is satisfactory (about 5000 Mc/s). 



The optimum design just described is an attempt to get the best pos- 

 sible gain-band product in the resulting tube, and is based on a particular 

 electronic theory (that of Llewellyn and Peterson). Two points remain to 

 be discussed. (1) What would be the result of optimizing for other merit 

 figures such as power-band product or noise figure, and (2) how valid is 

 the theory? 



Power-Band Product 



The radio relay amplifier requires not only gain, but perhaps even more, 

 power output. In such a case, the design specification of greatest im- 

 portance is the bandwidth over which a certain power output can be 

 obtained with a specified maximum distortion, and is expressed by an 

 analogous figure of merit, the power-band product. 



Of the many methods of specifying distortion, one which is particu- 

 larly useful in this connection is the "compression", that is, the amount by 

 which the gain is reduced from the small-signal value. In an amplitude- 

 modulated system, the compression would be a direct measure of non- 

 linear amplitude distortion in the amplifiers. In the actual relay, using 

 FM, compression is an indication that the amplifier is approaching its 

 maximum limit of power output. 



The maximum power output depends not only on how much current 

 the tube can carry, but also on the magnitude of the load impedance 

 into which this current works, which in turn depends upon the band- 

 width of the load. To compare tubes without need of specifying any 

 bandwidth, one notes that the product of power output and band- 



