750 



BELL SYSTEM TECHNICAL JOURNAL 



video band of frequencies. At high frequencies the gain-frequency char- 

 acteristic of a \'ideo ampliiier stage employing pentodes as illustrated in 

 Fig. ii is gi\en by 



Vi + p/ll 



where fu represents the frequency at which the relative gain has been reduced 

 vS db over the value achieved at the video midband region. This cut-off 

 frequency relationship is similar in form to that encountered in the radar II'' 



(a) 



10 



10^ 



IC^ 10-^ 10^ 10-^ 

 FREQUENCY IN CYCLES PER SECOND 



Fig. 33. — Radar video amplifier gain vs. frequency relationships. 



amj)lilier design previously reviewed. In the video amplifier design the 

 vacuum tube band merit Z?,, again determines the limiting performance of the 

 amplifier, but since the associated video interstage circuit elements con- 

 tribute considerably to the total circuit parasitic capacitance by reason of 

 their large i)hysical size, the effective band merit of a vacuum tube for video 

 purposes must be considered in terms of the total tube and circuit capaci- 

 tances. The additional consideration in vacuum tube choice for radar Nideo 

 amplifiers is one of load capacity, since the output signal voltage required 

 for indicator use may range upward to several hundred volts. 



In a somewhat analogous manner to the relationships discussed in the 



