PUSH-PULL POWER STAGES FOR LOADS 



A reasonable procedure for feeding penwriters is to make Ri = i?^. Then 

 if K = 250 V and R^ = 2,500 Q., the standing current consumed by each valve 

 is 125/2,500 = 50 mA, and the maximum possible load current is 25 mA. 

 To have to provide a continual 100 mA to a stage capable of supplying only 

 25 to the load is clearly rather wasteful. If the load is a meter, the maximum 

 load current required is hkely to be much less, and 7?^, can safely be made 

 much greater than /?j. 



Valve-bridge output stage. 



The power inefficiency of the preceding system is overcome by the bridge 

 scheme shown in Figure 12.45. These valves are arranged so that an increase 

 in the current through V-^ and F4 coincides with a decrease in the current 

 through V.2 ^"^ '^s- ^^ ^he limiting case where Fj and V^ approximate 

 to short circuits and V^ and Kg are nearly cut off the arrangement 

 reduces to Figure 12.46 so that all the current consumed passes through the 



HT + 



HT* 



-^WTWwr*- 



Figure 12.45 



Figure 12.46 



load. Ki and V^ act as cathode followers having V^ and F4 as cathode loads. 

 F2 and K4 act as amplifiers having Kj and V.^ as anode loads, and are biased 

 by Rj^ such that HT is evenly divided between upper and lower valves. In 

 order that upper and lower valves have equal effect on the currents flowing, 

 the lower valve grids are fed from potential dividers. The division ratio is 

 correct when a signal applied in-phase to the stage input terminals increases or 

 decreases all the valve currents equally, such that no change of potential 

 occurs either across the load, or between load and earth. 



REFERENCES 



1 Wireless World 61 (1951) 389 



2 Uttley A. M. and Williams F. C. J. Instn. elect. Engrs. 93, Pt IIIA, No. 7 (1946) 

 1256 



195 



