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THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



ponents of the signal are zero because of the symmetries of the circuit. Notice 

 that the dual of two vacuum tubes in parallel is two transistors in series. 

 Figure 28 shows a modulator which bears the same relation to the mod- 

 ulator of Reise and Skene (U. S. Patent 2,226,258) that the amplifier of 

 Fig. 11 of the test bears to the Doherty amplifier. The carrier wave is fed 

 into the tube and the transistor in the same way that the signal is fed into 

 the amplifier, and the modulating signal is fed into the grid and the emitter 

 through the transformers AFi and AF2 . The effect of the modulating signal 

 i«; to vary the biases of the active elements. Inasmuch as both elements are 

 used as class 5 or C amplifiers, their outputs are dependent on their biases 



Fig. 28 — High efl&ciency modulator. 



(b) 



Fig. 29 — Hartley oscillator and dual. 



K the RF signal is large enough, and if the phases and turns ratios of the 

 transformers are carefully chosen, the ampUtude of the output will be nearly 

 proportional to the modulating signal. A similar modulator can be based 

 on the circuit of Fig. 10. 



Figure 29 shows a Hartley oscillator and its dual. The configuration of 

 elements in the Hartley oscillator may seem unfamiliar, but is chosen de- 

 liberately to emphasize the point of view that the Hartley oscillator is an 

 amplifier with feedback through a coupling network. The part of the circuit 

 enclosed in dotted lines is the coupling network. The dual circuit is also an 

 amplifier with a coupling network, but because of the fact that the vacuum 



