132 



BELL SYSTEM TECHNICAL JOURNAL 



5 50 



730 M 



720 < 



710 o 



5.0 5.5 6.0 6.5 



FIELD ANGLE IN DEGREES 



Fig. ZZ — Relation of output and frequency to field angle in magnetron oscillator of 



second type. 



tion will also, no doubt, be dependent upon the development of 

 suitable amplifiers for this frequency range. Although certain forms 

 of pseudo-amplification are possible with tubes of the Barkhausen and 

 magnetron types, the negative grid triode and multi-element tubes 

 derived therefrom are the only devices available for very high fre- 

 quencies which will amplify in the sense that the output is an enlarged 

 undistorted replica of the input. 



As the frequency of operation of the negative grid triode is increased, 

 difficulties in securing stable operation as an amplifier and in realizing 

 the full gain indicated by the tube constants are encountered. These 

 difficulties, as is well known, are in the main due to the tendency of 

 the amplifier to oscillate or "sing" because of feed-back through the 

 grid-plate capacitance. This may be overcome either by the intro- 

 duction of a compensating capacitance somewhere in the circuit, so- 

 called neutralization, or by the introduction of an electrostatic shield 

 or screen within the tube envelope between the grid and plate, giving 

 the screen-grid tetrode. Neutralization schemes fail at very high 

 frequencies because of the inductance of the tube leads which makes 

 difficult the correct location of the neutralizing capacity and because 

 of transit-time effects which shift the phase of the needed compensa- 



