404 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



as open circuit elements in shunt with the load. As the instantaneous input 

 signal reaches half the permissible peak value, the transistors begin to dis- 

 tort because the collector current swing begins to approach the value of the 

 (current) supply. At this point the vacuum tubes begin to operate in two 

 separate ways to increase the power output. First they act as class C am- 

 plifiers delivering power directly to the load and second, they behave as 

 negative resistance elements in shunt with the load and thereby increase 

 the impedance into which the transistors work. This permits the transistors 

 to deliver more power without increased collector current swing. 



The circuits of Figs. 12 and 13 can both be adjusted to give reasonably 

 Unear performance. Perhaps the most interesting aspect of these circuits is 

 that the theoretical maximum efficiency (for sinusoidal signals) is 93%. 

 This should be a matter of importance in applications where the greatest 

 possible power output is desired from transistors and tubes of limited dis- 

 sipation rating. 



It has been pointed out by Ryder and Kircher^ that a transistor with a 

 just equal to unity behaves like a vacuum tube triode when operated with 

 the emitter grounded. If transistors can be made to operate satisfactorily 

 in this way with large signal swings then all the vacuum tubes in the circuits 

 discussed in this section can be replaced by grounded-emitter transistors. 



General Comments 



It is obvious that not all useful transistor circuits can be found in the 

 manner presented in this paper and, furthermore, not all of the circuits found 

 through the application of duaUty are useful. 



One limitation of the method is imposed by the fact that present day tran- 

 sistors correspond to rather low m vacuum tubes. On this account, vacuum 

 tube circuits which require high /x tubes for satisfactory performance will 

 lead to inferior transistor circuits. If further development of the transistor 

 produces higher values of a, this limitation will be reduced. 



Another limitation of the method comes from the failure of the transistor 

 to produce a phase reversal. Although this is not important in many cases, 

 and in other cases in which it is important a transformer provides a satis 

 factory solution, still the fact remains that transformers do not respond at^ 

 d.c. and because of this fact some transistor dual circuits are useless. 



In spite of these limitations, the methods presented in this paper have led 

 to a number of useful transistor circuits and may be expected to yield still 

 more in the future. 



Acknowledgment 



The authors wish to express their gratitude for the keen interest in 

 work shown by Mr. W. E. Kock and Mr. R. K. Potter under whose direction- 



» Ryder and Kircher, loc. cit. 



-.^^^i 

 ^ 



