470 BELL SYSTEM TECHNICAL JOURNAL 



nearly to the cut-off point so that the plate-current pulse width is 

 approximately a half cycle. Under these conditions the plate efficiency 

 is proportional to the amplitude of the radio-frequency plate voltage. 

 It is possible to obtain large outputs from tubes with radio-frequency 

 plate voltage amplitudes of 0.85 to 0.9 of the applied d-c potential, 

 i.e., with the plate voltage swinging down to a minimum value as low 

 as 10 to 15 per cent of the d-c potential. The corresponding plate 

 efficiency for the tube and its tuned circuit is approximately 67 per 

 cent. This condition, however, prevails only at the peak output of 

 the amplifier, and since the amplitude of the plate voltage wave, in a 

 transmitter capable of 100 per cent modulation, is only half as great 

 for the unmodulated condition as for the peaks of modulation, the 

 efficiency with zero modulation in the conventional amplifier does not 

 exceed half this peak value, or about 33 per cent. Even during com- 

 plete modulation the effective efficiency over the whole audio cycle is 

 only 50 per cent, and for the average percentage modulation of broad- 

 cast programs the all-day efficiency is only slightly greater than the 

 efficiency for unmodulated carrier. 



In order to improve this situation it is necessary to devise a system 

 in which the amplitude of the alternating plate voltage wave is high 

 for the unmodulated condition, and in which the increased output 

 required for the positive swings of modulation is obtained in some 

 other manner than by an increase in this voltage. 



A simple and fundamental means is available for achieving this 

 result. One embodiment of the scheme is illustrated in Fig. 1. Each 



TUBE 2 



Fig. 1 — Form of high-efficienc\- circuit. 



of the two tubes shown in this figure is designed to deliver a peak 

 power of E^/R watts into an impedance of R ohms. The total peak 

 output of the two tubes being lE^/R watts, the tubes are suitable for 

 use in an amplifier whose carrier output is one-fourth of this value, 

 or E^jlR watts. If the tubes were to be connected in parallel in a 

 conventional amplifier circuit the load impedance used would be 

 i?/2 ohms, and each tube would w'ork effectively into R ohms by virtue 

 of the presence of the other tube. The same load impedance Rjl is 

 used in the new circuit, but between the load and one of the tubes 



