FLUCTUATION NOISE IN VACUUM TUBES 651 



noise voltages produced in the plate circuit by factors 2.1, 3.7, and 16 

 respectively. These calculations show that each of these four types 

 of tubes approaches the requirements of an ideal low noise amplifying 

 tube although none of them is perfect in this respect. 



As stated above, the best signal-to-noise ratio in a high-gain amplifier 

 is obtained when thermal agitation in the input resistance is responsible 

 for most of the noise in the amplifier. This condition is met when 

 the resistance of the input circuit is higher than the value oi Rg for 

 the input tube. In case the resistance in the input circuit is less than 

 Rg the input signal and the thermal noise from the input circuit can 

 be raised above the noise of the tube by using an input transformer 

 having a sufficiently high voltage step-up. The voltage ratio of the 

 transformer, and in turn the possible ratio of input circuit thermal 

 noise to tube noise, is limited, especially at the higher frequencies, by 

 the dynamic grid-to-ground capacitance of the input tube and its 

 leads. In such a circuit the No. 259B tube with its lower inter- 

 electrode capacities and higher tube noise is often more desirable than 

 even the quietest three-element tubes. 



In those high-gain amplifiers in which unavoidably the resistance of 

 the input circuit is low, the tube rather than thermal agitation in the 

 grid circuit is responsible for most of the noise. Here the best signal- 

 to-noise ratio can be obtained by choosing a tube for the initial stage 

 having the lowest possible noise level. The above measurements 

 show that one of the three-element tubes, particularly the No. 102G 

 tube if sufficient shielding is used, is best suited for this purpose. 



The lower limits of noise obtainable with high gain amplifiers may 

 be estimated by means of Fig. 5, which shows the noise as a function 

 of input resistance and frequency band width when thermal agitation 

 in the input circuit is responsible for all the noise. The data for this 

 figure are obtained from the thermal noise relationship 



V? = 1.64 X 10-^'RF voh\ (16) 



R is expressed in ohms and the temperature has been taken at 300° K., 

 which is approximately room temperature. It must be remembered 

 that the attainment of these noise levels at low input resistances is 

 limited by the input transformer. 



The results of the noise measurements on the No. 259B tube with 

 floating grid may be compared with the value predicted by equations 

 (8) and (9). Inserting the tube characteristics obtained by experi- 

 ment (rg = 1.4 X 10^" ohms, ig = 9 X 10~^^ ampere, and c = 6 

 X 10~^^ farad), and integrating between the frequency limits 10 cycles 



