7-9] OVERALL AMPLIFIER GAIN 373 



Total capacitance at output of the second tube is then 3.0 + ^ = 6.0 mm/ 



n- 



and the conductance is 



gi - 27r(6.45)(10«) (6.0) (10-12) = 2.43 X 10-^. 

 From Equation 7-30 the noise figure of the first tube is 



V 1 -L IQ"' , 5.0 X 10-^ , 150 ,, . ^ ^ _ ^, , 



^^ = ^ + 6.3 X 10- + 6.3 X 10-^ + 6.3 X 10"^ ^^'^ >< ^^ '^'^ 



1.196 = 0.78 db 



The noise figure of the second tube is (from Equation 7-32) 



P ^ J . 10-^ 5.0 X 10-^ 2.43 X 10-^ 150(0.95) 



' "^ 4.55 X 10-* ^ 4.55 X 10"* "^ 4.55 X 10-* "^ 4.55 X 10-* 



(4.65 X 10-^)2 



4.55 X 10-* = ^ = g^ip. 



= 1.74 = 2.4 db. 

 From Equation 7-33, the available power gain of the first tube is 



(20XI0-y(6.3X10-)(g^^3) 



(6.5 X 10-*)2 ^^^ 



while the available power gain of second tube is approximately 

 4.55 X 10- 



2.43 X 10-* 



1.87. 



Assuming 10 db for the third tube F, the preamplifier noise figure is given 

 by Equation 7-7: 



1196+ ^-^^-^ + ^Q-^ ^1237 

 i.iyo^ 131 ^(131)(1.87) 



^ 0.92 db. 



7-9 OVERALL AMPLIFIER GAIN 



It is necessary that signal amplitudes corresponding to the thermal noise 

 level at the input of the receiver be amplified to a suitable level for detec- 

 tion. The level required at the detectors depends on the use of the signal. 

 For example, for signal detection on an intensity-modulated display 

 providing range and azimuth coordinates, signal voltages on the order of 

 50 volts are usually required. The total amplification that is required 

 depends on the signal bandwidth and the receiver noise figure. The equiv- 



