840 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1956 



between various transistors, 18 tetrodes were measured in the first stage 

 of the amplifier. If the measured gain of each transistor is rounded off 

 to the nearest db and the number of transistors having this gain plotted 

 as the abscissa, the results shown on Fig. 21 are obtained. Of the 18 

 transistors measured, 11 have a gain of 8 db or greater. Similar data has 

 been obtained on the noise figure of the same 18 transistors, the results 

 being shown on Fig. 22. In general, the transistors having the highest 

 gain also have the lowest noise figure. The noise figure depends to some 

 extent on the source impedance but a 75-ohm source results in a noise 

 figure which is within a few tenths of a db of the minimum. The value 

 of the noise figure does not vary a great deal as the collector voltage and 

 emitter current are changed except that if the collector voltage is lowered 

 below 6 or 8 volts the gain decreases and in general the noise figure in- 

 creases. 



Noise Figure at 10 Mc. 



Although not described here, bandpass amplifiers centered at 10 mc 

 with a 200-kc pass band have been constructed using tetrode transistors. 

 A gain of slightly over 20 db per stage can be realized at this frequency. 

 The noise figure of transistors tried in this circuit is shown on Fig. 23, 

 the data being shown in the same manner as described above. At 10 mc 

 the noise figures are lower than at 70 mc. The remarks made above con- 

 cerning variation of noise figure with operating conditions also apply to 

 this case. 



ACKNOWLEDGMENTS 



We are happy to acknowledge the advice and encouragement given 

 us by R. L. Wallace, Jr., and others in the Laboratories. We also wish 

 to express our thanks to E. Dickten who fabricated the transistors used 

 to obtain the experimental data presented. W. F. Wolfertz made the 

 transistor parameter measurements used in the computations. R. H. 

 Bosworth and C. E. Scheideler were responsible for construction of the 

 circuits and some of the gain measurements. We also wish to thank W. R. 

 Bennett for his aid in preparing the manuscript. 



