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BELL SYSTEM TECHNICAL JOURNAL 



a coil, and in the tuning process either the coil or the condenser is 

 adjusted so that the impedance of the combination is the required 

 value of —jR ohms. The effective shunt load Rjl is then usually 

 obtained by coupling the radiating system of the transmitter to the 

 necessary extent into the parallel circuit associated with Tube 2. 



The presence of a quarter-wave network in the output circuit of 

 the amplifier causes the plate potentials on the two tubes to be 90 

 degrees apart in phase. This requires that the voltages impressed on 

 the two grids be 90 degrees apart in order that each may be opposite 

 in phase to the related plate potential, as is necessary in any power 

 amplifier. In addition to this phase requirement, there arises from 

 the variation in load impedance for Tube 1 the requirement that the 

 excitation on this tube shall rise considerably less than 100 per cent on 

 the positive peaks of modulation. Without some limiting action on 

 this excitation the grid current in Tube 1 would be excessive and would 

 result in a diminution of its output at modulation peaks. 



Both of these requirements concerning the input to the amplifier 

 are satisfied by the use of the input circuit shown in Fig. 2. With the 



x2 



rt — - r 





TUBE 2 I I TUBE 1 



Fig. 2 — Input circuit for a high-efficiency amplifier. 



output of the previous stage applied directly to the grid of Tube 2, 

 the resulting excitation Ei on Tube 1 is proportional to the terminating 

 resistance Ri of the quarter-wave network. With a suitable value of 

 Ri the input conductance of Tube 1 arising from the flow of grid 

 current at high excitations causes an effective lowering of R\ which 

 gives the desired limiting action on the excitation. At the same time 

 the input impedance X^JRi of the quarter-wave network is increased, 

 compensating to a large extent for the shunting effect of the grid 

 current in Tube 2, so that the previous stage is assisted in maintaining 

 the proper excitation on the amplifier. 



In a preliminary study of the behavior of an amplifier under these 

 new conditions of operation, the results shown in Fig. 3 were obtained 

 with a pair of small tubes. The radio-frequency plate potential of 

 Tube 2 is the potential across the load circuit and is required to be 

 linear with excitation. The short dotted portion halfway up on this 



