Input Admittance of Cathode-Follower Amplifiers

Abstract

General expressions are derived for the input admittance, conductance, and susceptance of a cathode-follower amplifier, and curves are given that show the effect of frequency and circuit parameters upon the input conductance and the effective input capacitance for typical values of plate resistance, transconductance, and interelectrode capacitances. The analysis shows that a capacitance shunting the load resistance of a cathode-follower amplifier may result in a negative input conductance. If the load capacitance is of the order of magnitude of the interelectrode capacitance or greater, this negative conductance is of the order of 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> mho at frequencies at which ωC <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gk</inf> is of the order of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> mho and above. For typical values of C <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gk</inf> this value of ωC <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gk</inf> corresponds to frequencies of the order of 50 megacycles and above. Negative conductance of this magnitude may readily cause oscillations in the input circuit unless preventive measures are taken. A number of such measures are discussed.