ULTIL4.-HIGH-FREQUENCY POWER AMPLIFIER 11 



It is the purpose of this paper to discuss the problem of amplification 

 at ultra-high frequencies and to describe one form of amplifier tube 

 designed for moderate power in that frequency range. 



The Triode as an Amplifier at Ultra-High Frequencies 



A simple triode amplifier as used at low frequencies becomes un- 

 stable as the operating frequency is increased, exhibiting a tendency 

 to oscillate or "sing" because of the interaction between the input and 

 output circuits. This interaction or "feedback" is, in the main, pro- 

 duced by the grid-plate capacitance of the tube. It may be overcome 

 either by the introduction of a compensating capacitance somewhere in 

 the circuit or by the introduction of an electrostatic shield or screen 

 within the tube envelope. The first expedient, known as neutraliza- 

 tion, is employed in the case of a triode. The second expedient results 

 in the screen-grid tetrode. At moderately high frequencies either 

 arrangement may be used. 



The conventional triode is unsatisfactory at very high frequencies. 

 The usual capacitance neutralization scheme fails, partly because of 

 the inductance of the tube leads which makes difficult the correct loca- 

 tion of the neutralizing capacitance. The appreciable time required 

 for the electrons to traverse the interelectrode spaces within the tube 

 structure makes neutralization more difficult by introducing a shift in 

 the phase of the necessary compensation. 



A more serious effect of electron transit time is the marked increase 

 at high frequencies in the input conductance of a tube over the value 

 observed at low frequencies. This effect has been the subject of con- 

 siderable study .^•^' *• '' Theory and experiment both agree in relating 

 the input conductance loss to the tube geometry and the applied 

 electrode potentials. The conductance depends upon the electron 

 transit time and increases rapidly with increasing frequency. The 

 transit time may be reduced either by decreasing the electron paths or 

 by increasing the electron velocities. Decreasing the path calls for 

 smaller interelectrode spacings, and increasing the velocity calls for 

 higher electrode potentials. On the other hand, practical considera- 

 tions limit both the dimensions and the potentials. An optimum de- 

 sign may utilize special mechanical arrangements to combine both 

 expedients. 



* J. G. Chaffee, "The Determination of Dielectric Properties at Very High Fre- 

 quencies," Proc. I.R.E., vol. 22, pp. 1009-1020, August, 1934. 



^ F. B. Llewellyn, "Operation of Ultra-High-Frequency Vacuum Tubes," BellSys. 

 Tech. Jour., vol. 14, pp. 632-665, October, 1935. 



^ W. R. Ferris, "Input Resistance of Vacuum Tubes as Ultra-High-Frequency 

 Amplifiers," Proc. I.R.E., vol. 24, pp. 82-104, January, 1936. 



' D. O. North, "Analysis of the Effects of Space Charge on Grid Impedance," 

 Proc. I.R.E., vol. 24, pp. 108-136, January, 1936. 



