7-7] IF AMPLIFIER DESIGN 365 



A parameter of great importance is the gain bandwidth product. Taking 

 the simplest interstage circuit as a reference network, the gain bandwidth 

 product is 



This equation shows that the quantities which determine the gain and 

 bandwidth are so related that high gain can be obtained only at the ex- 

 pense of reduced bandwidth. GB is determined by the tube and the circuit 

 physical layout since it affects C. If two identical circuits are cascaded, 

 then the 3-db bandwidth of the cascaded circuits is the 1.5-db bandwidth 

 of a single circuit. For a given overall bandwidth, the individual stage 

 bandwidths must be increased. With flat staggering of the circuits, this 

 bandwidth shrinkage does not occur, and the GB product can be used to 

 determine overall gain of the amplifier. 



The normalized attenuation characteristic of the IF coupling circuits 

 employed in conventional pulse radar receivers can be expressed by the 

 equation 



Attenuation (db) = 10 log to [1 + x^^] (7-22) 



where X =/„( ///- /°// )^ ^C/-/.) 



n = number of poles in the circuit (low pass equivalent) 



Bi = 3-db bandwidth 



/ = frequency at which attenuation is to be evaluated. 



If m groups of such circuits are cascaded, then the overall amplifier 

 selectivity is 



Attenuation (db) = 10m logio [1 + x^""]. (7-23) 



The overall bandwidth of the amplifier is therefore 



B = (21/- - l)i/2«5i. (7-24) 



For a given overall IF bandwidth it is desirable that the principal 

 selectivity occur in low-level stages. The input stage selectivity, however, 

 is governed by noise figure considerations. The input stages are therefore 

 designed first and then the remaining amplification and selectivity intro- 

 duced. 



The selectivity of the IF amplifier is provided by one-pole or two-pole 

 networks between the stages in order to realize the maximum dynamic 

 range. With «-pole configurations in which the poles are distributed 

 through the amplifier, the dynamic range is usually not the same at all 

 frequencies within the pass band; such designs are therefore avoided. 



