Wide-Band Amplifiers for Television

The maximum uniform amplification that can be secured over a wide frequency band by means of a single vacuum tube is much greater than that of the usual simple circuits. It can be secured by either of two arrangements, one using an individual filter coupling each tube to the next, and the other using degenerative feedback in each stage to make the stage behave as a section of a confluent filter. In either case, the shunt capacitance on each side of each tube is included in an individual full-shunt arm of a band-pass or low-pass filter. One end of each interstage filter, or of each filter including one or more feedback stages, is extended to a dead-end termination with resistance approximately matching the image impedance. The other end is terminated at one of the tubes in a full-shunt arm, where the filter presents the maximum uniform impedance that can be built up across the tube capacitance. These concepts in terms of wave filters lead to practical wide-band circuits adapted to meet any given requirements. The following general formula is shown to express the maximum uniform amplification that can be secured in one tube: gm A= 7rfwVCgCp in which A is the voltage ratio between input and output circuits of equal impedance, g
m
is the transconductance of the tube, C
g
, and C
p
, are the grid and plate capacitance of the tube, and f
w
is the width of the frequencv band.