TRANSMISSION-LINE DISTRIBUTED AMPLIFIERS UTILIZING FIELD-EFFECT TRANSISTORS.
HARVARD UNIV CAMBRIDGE MASS DIV OF ENGINEERING AND APPLIED PHYSICS
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The purpose of this investigation is the determination of operating characteristics of a distributed amplifier utilizing field-effect transistors as active elements and constructed in the form of helical transmission lines. A review of the properties of field-effect transistors is carried out, and the amplifier is analyzed in terms of transmission line equations, taking the input and output lines to be loaded by the FET input and output impedances, respectively. After equations are obtained which characterize such loaded transmission lines, and suitable terminations are described, an overall gain expression is developed, forming the basis of a computer simulation of the amplifier. In addition, general relations concerning design specifications and final amplifier performance are obtained from simple models and checked against the more elaborate simulation. Problems associated with transistor gate-drain feedback capacitance are examined in a variety of ways. Pulse response and variation of gain with frequency are investigated by means of the computer simulation for a variety of transistor and amplifier specifications. The construction and testing of experimental amplifiers utilizing both junction and MOS depletion FETs is described and shows satisfactory agreement with theory. Bandwidths exceeding 200 Mc. at a gain of two have been achieved, using transistors with a gain-bandwidth product of 50 Mc. The principal limitations found at present are the low transconductance -- leading to low gain -- and cutoff frequency of currently available FETs. Author
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