12 BELL SYSTEM TECHNICAL JOURNAL 



The electron transit time limitation becomes of particular importance 

 at frequencies above one hundred megacycles and sets an upper fre- 

 quency limit on the useful operation of the usual triode as an amplifier 

 just as it sets the limit at which the tube will operate as an oscillator. 

 Because of the similarity in the special high-frequency requirements, 

 negative grid tubes designed for use primarily as ultra-high-frequency 

 oscillators are good amplifiers at somewhat lower frequencies. The 

 necessity for very careful circuit design and for critical adjustment of 

 the neutralization becomes particularly pronounced when triodes are 

 used as ultra-high-frequency amplifiers. 



The Multi-element Tube as an Amplifier at Ultra- 

 High Frequencies 



Conventional screen-grid tetrodes and pentodes are also unsatis- 

 factory at very high frequencies. Two factors are again primarily re- 

 sponsible, the one set by the circuit requirements, the other set by the 

 electron transit time. These limitations will be considered in detail. 



In the usual radio-frequency amplifiers using tetrodes or pentodes 

 the input and output circuits are tuned to the desired frequency. For 

 most practical purposes the upper limit to the frequency for which 

 these circuits may be tuned is set by the natural period of the circuits 

 formed by the corresponding lead inductances and interelectrode ca- 

 pacitances. Even before this limit is reached the major portions of the 

 tuned circuits are within the tube envelope. Their inaccessibility 

 makes it difficult to obtain efi'ective coupling between amplifier stages. 



Interaction between the input and output circuits if excessive may 

 cause "singing." Such interaction is usually due to the residual value 

 of the grid-plate capacitance. Not only must this capacitance be 

 made very low by the proper design of the screen and suppressor grids, 

 but its effective value must remain low at the operating frequency. 

 This latter is realizable only if the screen and suppressor grids can be 

 coupled to the cathode by leads having extremely small inductances. 

 A further desirable feature is that there be no appreciable circuit im- 

 pedance in the form of lead inductance common to both input and out- 

 put circuits. The use of short leads is thus seen to be just as important 

 in the design and use of the multi-element tube as it is in the design of 

 the triode. 



As in the case of the triode, the electron transit time is effective in 

 limiting the useful frequency range of the multi-element tube. The 

 increase in the input conductance which it introduces is again primarily 

 responsible. 



In considering the design of an amplifier tube for ultra-high fre- 

 quencies, it appeared desirable to select frequency and power levels 



