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THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



computed from the values in the vacuum tube circuit by means of the rela- 

 tions of Table 1. 



Figure 5, (a) and (b), shows a different arrangement of power supplies 

 in the vacuum tube circuit and the resulting more convenient arrangement 

 of power suppUes in the transistor circuit. In this case, the ideal transformer 

 has been omitted but in all other respects the circuits are duals. 



The apphcation of duality in this case has led to the use of a series tuned 

 circuit in series with the load instead of the shunt tuned circuit in shunt 

 with the load, which the vacuum tube circuit might otherwise have sug- 

 gested. The series tuned circuit has the advantage when used with short- 

 circuit unstable transistors of insuring stability outside the pass band. 



The Dual or a Push-pull Class B Amplifier 



Figure 6(a) shows the circuit of a push-pull amplifier and the Kirchoff 

 equations which apply to it. Figure 6(b) shows the transformed equations 



(a) 



(b) 



Fig. 5 — An unconventional arrangement of power supplies in the vacuum tube circuit 

 leads to a convenient arrangement for transistors. The phase reversing transformer which 

 would make the transistor circuit strictly dual has been omitted. 



and the dual transistor circuit. In this case, not only the circuit configuration 

 but also the choice of operating point is important. For class B operation 

 the two tubes are given a high negative grid bias, so that in the absence 

 of an input signal the two plate currents are nearly zero while the plate 

 voltages are quite high. In the transistor case, the dual situation is that the 

 emitters are given a high positive emitter current bias so that in the absence 

 of an input signal the two collector voltages are nearly zero while the collector 

 currents are quite high. During a positive half cycle of input voltage the 

 upper vacuum tube plate circuit begins to conduct and the plate current 

 of the upper tube goes through a positive half cycle while the plate current 

 in the lower tube remains essentially at zero. During this half cycle the plate 

 current of the upper tube is coupled through the output transformer to the 

 load while the lower tube contributes nothing, behaving simply as an open 

 circuit element in shunt with the load and with the upper tube. The cor- 

 responding set of events in the transistor amplifier is that, in response to 



