474 



BELL SYSTEM TECHNICAL JOURNAL 



d-c plate currents of Fig. 3 over a complete cycle of modulation it is 

 found that the effective average efficiency at 100 per cent modulation 

 is also 63 per cent. The d-c plate current of the amplifier therefore 

 rises 50 per cent with full modulation, as does the output power. 



The necessity for careful adjustment of the relative excitation and 

 bias on Tube 2, to obtain a linear characteristic in the amplifier, is 

 eliminated when the feedback principle ^ due to Black is employed. 

 Negative feedback may be used in radio transmitters at either radio 

 frequency or audio frequency. The resulting improvements in line- 

 arity are useful in noise reduction as well as in distortion correction. 



O 3 



I 5S9 



50 100 200 300 400 500 1000 2000 3000 5000 



MODULATION FREQUENCY IN CYCLES PER SECOND 



Fig. 4 — Distortion measurements on a 50-kilowatt transmitter. 



Figure 4 gives the results of distortion measurements on a complete 

 50-kilowatt transmitter built in the Laboratories and operating at a 

 plate circuit efficiency for the final stage of 60 per cent. The use of 

 28 db of audio-frequency feedback, besides permitting alternating- 

 current filament heating for all of the tubes, resulted in a distortion 

 level less than 1 per cent at any frequency between 50 and 1000 cycles. 

 At the higher audio frequencies the feedback is less effective because 

 of the cumulative phase shifts in the various stages. The percentage 

 modulation actually occurring in a broadcast program at these high 

 frequencies, however, is so small that the distortion measured at high 

 percentages of modulation is not of practical significance. The test, 

 moreover, was made at the low-frequency end of the broadcast 



' H. S. Black, "Stabilized Feedback Amplifiers," Electrical Engineering, January, 

 1934; Bell Sys. Tech. Jour., January, 1934. 



