LIMITS TO AMPLIFICATION 91 



The space charge which Umits the current between cathode and anode 

 consists of electrons in rapid progress toward the anode. A massive 

 ion placed in this region travels much more slowly and contributes 

 to the space charge for a much longer time than does an electron. 

 While its own charge contributes little to the current, one ion may 

 cause the current to change by the amount of hundreds of electrons 

 during its flight through the space charge region, and the action of 

 many ions would be additive. 



Probably most of the ions existing in a tube are positive. Some of 

 them are molecules of residual gas that have lost an electron by 

 collision with an electron of the space current. Residual gas has been 

 found to increase the noise of tubes, especially at the higher pressures. 

 Observations at very low pressures are not conclusive, and it is not 

 certain whether in any modern tubes the noise level is determined by 

 the presence of residual gas.^' ^^' ^^' ^''' ^^' ^^' '^^' ^^ 



Positive ions may be emitted also by the cathode. These never can 

 attain a high velocity because they remain in a region of low field 

 intensity. They may be trapped for a time in the region of the 

 potential minimum near the cathode before they finally pass to the 

 grid or possibly become neutralized by an electron. In modern tubes 

 with low temperature filaments the effect of these ions is reduced 

 greatly, yet still may account for a large part of the difference 

 between the observed tube noise and the theoretical thermal noise 



of tubeS.l^' 25. 26. 27, 28 



Noise in Commercial Tubes 



Noise generated in an amplifier should consist largely of the thermal 

 noise of the input circuit, to which is added the noise produced in the 

 plate circuit of the first tube. It is convenient to consider that 

 the tube noise comes, not from the plate circuit of the tube, but from a 

 fictitious resistance Rg in series with the resistance Re of the input 

 circuit. 2 The effective thermal noise of the input circuit then is given 

 by the expression 



72 = 4kTF{Re + Rg) = 1.64 X IQ-^'FiRc + Rg). (H) 



The tubes therefore may be rated conveniently in terms of Rg- The 

 transformation to volts or to watts can be accomplished readily by 

 equations (2) or (3). If, with a given tube and circuit, Rg approaches 

 or exceeds Re in value, the tube is responsible for an appreciable part 

 of the total noise. The choice of another tube in which the ratio of 

 Re to Rg is more favorable then may be considered. 



