Pentriode Amplifiers

Conventional video-amplifier design is complicated by low-frequency phase shift and degenerative decrease in gain caused by inefficient screen-grid and cathode by-pass circuits. This paper describes two amplifier circuits in which phase shift and degeneration in those by-pass circuits may be eliminated throughout the entire range to zero frequency. Small paper or mica capacitors may be substituted for bulky electrolytic units, with consequent improvement in electrical and physical amplifier characteristics. These circuits are applicable to amplifiers with stage gains less than the control-grid to screen-grid amplification factor of the tube. Inasmuch as common tubes have factors ranging from approximately 10 to 50, the circuits described are primarily applicable to video amplifiers. In practice, the interstage circuits are designed in accordance with conventional methods involving either two-terminal or four-terminal networks. The by-pass circuits herein described may then be incorporated into the design to provide the advantages outlined above with no loss in stage gain.