VACUUM TUl'ES FOR RADAR I.F. AMPLIFIERS 



389 



load resistor and any loading effects due to high-frequency losses in the 

 tubes or circuit elements, and C is the total shunt capacitance of the tubes, 

 the circuit elements and the wiring. If it is assumed that R, L and C are 



BLOCKING 

 CONDENSER 



Fig. 5 — Band-pass interstage, 

 independent of frequency, the magnitude of the impedance of this network is 



! (1) 



z = 



+ [^ - ZL 



1/2 



1 



When o) = ojo = /jj; the impedance is a maximum and the frequency /o 



= — is the resonant frequency. The maximum value of the impedance is 



I Zo I = ^. If this type of interstage is used in a grounded-cathode type 

 of circuit employing a pentode tube, the small-signal voltage amplification 

 A V from the control grid of the tube to the plate is 



A„ = G,n\Z\ ' (2) 



where G„, is the grid-plate transconductance of the tube. It is assumed that 

 1 Z I is small compared to the plate resistance of the pentode and that there 

 is no feedback. The voltage amplification .1 ,o at the resonant frequency is 

 then 



A vO = (^rmR w) 



If the bandwidth is defined as IF = J\ - f-i , where J\ and /o are the two 

 frequencies where /U = y 



, it can be shown that 

 1 



AF =/,-/, = 



2xRC 



(4) 



The product of the mid-band voltage gain times the bandwidth can be 

 called the band merit, Bo , and we have 



B, ^ (AFKA.) = ^-^^(G.R) = ^^ 



(5) 



