788 



BELL SYSTEM TECHNICAL JOURNAL 



start of the sweep. After this short interval of time, the feedback becomes 

 effective and the output current and input voltage corresi)ondence obtains. 

 It is essential in this ty{)e of circuit tliat the feedback be determined en- 

 tirely by the deflection coil current if optimum oj)eration is to be obtained. 

 It should be observed that this condition requires that the screen current 

 which also normally flows through the feedback impedance does not con- 



+ 300 VOLTS I 



' + 600 VOLTS 



DEFLECTION 

 COIL 



= LPR4+L2R4-L, R3 



L2R4 = i-iRs 



\^K-G = '-PR4 

 WHICH IS DEPENDENT ON PLATE 

 CURRENT ONLY 



Fig. 56.— Radar range sweei) amplifier cmiiloying negative feedback and screen grid 

 bridge circuit — simplified schematic. 



tribute to the net feedback. Figure 56 illustrates the bridge circuit which 

 has been devised to accomi)lish this. .\ xoltage from the screen of the third 

 stage is directly a{)])lied to the cathode of the iirst stage, this voltage being 

 equal in magnitude and of opposite jiolarity to that which appears across 

 the feedljack impedance due to the third stage screen current (low. This 

 bridge circuit operation is independent of tlie absolute screen \'ollage value. 

 To insure identical starting potential conditions regardless of the duration 

 of the range sweeps in use, d-c restoration is emplo^-ed in the grid circuit ot 

 the last stage. The action here is similar to the o|)eration described pre- 

 viously in connection with \ideo am])liner design. The delay inherent in 



