. Radio-frequency amplifiers . fier. responding radio-frequency current flows in the grid circuit of thesecond tube through the condenser mentioned. The resistancechosen should be such as to make the voltage drop between thegrid and filament in the latter circuit as high as possible. Thismethod is, however, limited to amplification of relatively lowradio frequencies and will not usually give satisfactory results onfrequencies much above 150 kilocycles per second with the tubesat present ordinarily available. In the second method—tuned-plate coupling—the plate circuitof the first tube is actual
. Radio-frequency amplifiers . fier. responding radio-frequency current flows in the grid circuit of thesecond tube through the condenser mentioned. The resistancechosen should be such as to make the voltage drop between thegrid and filament in the latter circuit as high as possible. Thismethod is, however, limited to amplification of relatively lowradio frequencies and will not usually give satisfactory results onfrequencies much above 150 kilocycles per second with the tubesat present ordinarily available. In the second method—tuned-plate coupling—the plate circuitof the first tube is actually tuned to the signal frequency eitherby means of an inductance shunted by a variable condenser orby a continuously variable inductor inserted in series with theplate circuit. The circuit is similar to that of Figure 1, exceptthat the coupling resistance shown in Figure 1 is replaced by acontinuously variable inductor or by an inductance coil shuntedby a condenser. A condenser permits the radio-frequency cur- 720°—22 2. 33§ Scientific Papers of the Bureau of Standards. [Vol. iS rents to flow from this plate to the grid of the next tube, as inthe resistance-coupled method. The tuned-plate method is moreefficient than resistance coupling at frequencies above 150 kilo-cycles per second. With special tubes having low capacitiesbetween the elements and by careful design the tuned-platemethod can be used for frequencies as high as about 1500 kilo-cycles. The effective range of a tuned-plate amplifier is rathernarrow unless each stage is separately tuned, which requires aconsiderable number of additional adjustments. The third method—transformer coupling—is the most effectiveand therefore most desirable, and it is this method for Avhich cir-cuits were developed and which is discussed in this
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