POWER AMPLIFIER FOR MODULATED WAVES 471 



(Tube 1) there is interposed a network which simulates a quarter-wave 

 transmission line at the frequency at which the amplifier is operated. 



It is a well-known property of quarter- wave transmission lines and 

 their equivalent networks that their input impedance is inversely 

 proportional to the terminating impedance. The network of Fig. 1, 

 in particular, presents to Tube 1 an impedance of R ohms when its 

 effective terminating impedance is also R ohms, that is, when half of 

 the power in the load is being furnished by Tube 2; but should Tube 2 

 be removed from the circuit, or prevented from contributing to the 

 output, the terminating impedance of the network would be reduced 

 to R/2 ohms, with a consequent increase in the impedance presented 

 to Tube 1 from R to 2R ohms. Under this condition Tube 1 could 

 deliver the carrier power E^jlR at its maximum alternating plate 

 voltage E and consequently at high efficiency. 



The operation of the amplifier over the modulation cycle is as fol- 

 lows: The grids of both tubes are excited by the modulated output of 

 the preceding amplifier stage, but for all instantaneous outputs from 

 zero up to the carrier level Tube 2 is prevented by a high grid bias from 

 contributing to the output, and the power is obtained entirely from 

 Tube 1, which is working into 2R ohms, twice the impedance into which 

 it is to work when delivering its peak output. In consequence, the 

 radio-frequency plate voltage on this tube at the carrier output is 

 nearly as high as is permissible and the efficiency is correspondingly 

 high. Beyond this point the dynamic characteristic of Tube 1, un- 

 assisted, would flatten off very quickly because the plate voltage swing 

 could not be appreciably increased. Tube 2, however, is permitted 

 to come into play as the instantaneous excitation increases beyond the 

 carrier point. In coming into play Tube 2 not only delivers power of 

 itself, but through the action of the impedance-inverting network 

 causes an effective lowering of the impedance into which Tube 1 

 works, so that Tube 1 may increase its power output without increasing 

 its plate voltage swing, which was already a maximum at the carrier 

 point. At the peak of a 100-per-cent modulated wave each tube is 

 working for an instant into the impedance R most favorable to large 

 output and delivering E^jR watts, twice the carrier power, so that the 

 total instantaneous output is the required value of four times the 

 carrier power. Thus the required tube capacity is the same as in a 

 conventional linear power amplifier. 



Since it is usually desirable in power amplifiers to provide low- 

 impedance paths for the harmonic components of the radio-frequency 

 plate current wave, the reactive elements designated —jR in a practical 

 circuit ordinarily consist of a considerably larger capacity shunted by 



