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BELL SYSTEM TECHNICAL JOURNAL 



Laboratories 1553 triode^^ the theoretical analysis no longer represents a 

 valid approximation. 



Let us consider curve c in greater detail. The fact that electrons are emitted 

 with a Maxwellian velocity distribution, instead of being emitted at zero 

 velocity as in the Child's law or complete space charge case, means that more 

 electrons are introduced in the space between the electrodes than can flow 

 to the anode in accordance with Child's law. The surplus electrons depress 

 the potential in front of the cathode to a value below that of the cathode. 

 This potential minimum is indicated by Vm in the figure. Electrons which 

 have insufficient energy to cross this barrier return to the cathode. 



In the space between the cathode and the potential minimum, electrons 

 are found traveling with various velocities in both directions. Between the 

 potential minimum and the anode, electrons travel in one direction only, 



Fig 1 — Potential distributions in a diode 



toward the anode, but with multiple velocities. With close spacings and 

 higher frequencies the distance between the cathode and the potential 

 minimum may be an appreciable part of the total cathode-anode spacing, 

 with the result that the electrons returning to the cathode may absorb a 

 substantial amount of power from the high-frequency held. 



This argument also applies to the cathode-grid region of a microwave 

 triode such as the 1553. In order to increase the transconductance of the 

 triode, it is desirable to locate the grid as close to the cathode as possible. 

 The close spacing, however, leads to a greater loss of power to the returning 

 electrons, which prevents a realization of the full benefits expected from the 

 reduced spacing. All of these difficulties are a result of the Maxwellian veloc- 

 ity distribution of the emitted electrons. 



In view of the importance of electron stream admittances in the design 

 of microwave amplifiers and of the need for a better understanding of the 

 performance of the 1553, a program was initiated to investigate some of 



