498 BELL SYSTEM TECHNICAL JOURNAL 



When this Hmitation became clear several years ago, a study was under- 

 taken to determine which particular type of electron tube amplifier then 

 known had the best possibilities of being pushed to greater gain-band 

 products. The results of this study indicated that a very promising pros- 

 pect was to build, for operation at 4000 megacycles, an improved planar 

 triode, that is, one in which the active elements are on parallel planes. 



In arriving at this conclusion, two general types of device were con- 

 sidered: velocity-modulated, as in a klystron, and current-modulated, as 

 in a triode. (Nowadays, such a study would of course include traveling- 

 wave tubes.) The conclusions were reached with the aid of the gain-band 

 figures of merit, along the following lines: 



Gain-Band Product 



The system performance requirements demand amplifiers capable of 

 reasonable gains and power outputs over prescribed bandwidths. How- 

 ever, it is known that bandwidth can be increased by complicating the 

 circuits (double-tuning, stagger-tuning, etc.). Such factors, being common 

 to whatever tube may be used, are extraneous to a discussion of tube 

 performance, and accordingly the tubes are rated by their performance 

 with simple, synchronous resonant circuits. Furthermore, even then the 

 bandwidth can be increased at the expense of a corresponding reduction 

 of gain, by simply depressing the impedance levels of the interstages. 

 Since the product of gain and bandwidth remains constant, it is a suit- 

 able figure of merit, independent of the particular choice of bandwidth, 

 provided the definition of gain is suited to the device. 



Unfortunately there is more than one possible gain-band product, the 

 appropriate form depending on how many simple resonant circuits shape 

 the band of the amplifier stage. For example, a conventional pentode or a 

 velocity-variation tube is usually used in conjunction with two high-() 

 resonant circuits, one each on input and output. If these are adjusted to 

 give the same Q, then it is well known that, no matter what the band- 

 width, the product of voltage gain and bandwidth is constant. (See 

 Appendix 1) 



I To I 5 = I I'2i I /2-K\/C~CZ, (1) 



Here To is the mid-band voltage gain, B the bandwidth 6 db down (3 for 

 each circuit), F21 the stage transadmittance, and C\n and Com the total 

 effective capacitances of the resonant circuits, including the contribu- 

 tions of the tube*. It is assumed that the stage is matched into trans- 

 mission lines of some suitable constant admittance level Go . 



In amplifiers using triodes such as the B.T.L. 1553 (or tetrodes) in 



* As shown in Ai)pendix 1, all (luantilics in equations (1) and (2) are ihe values effective 

 at the electrodes adjacent to the electron stream. 



