Input Admittance of Cathode-Follower Amplifiers

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^-4mho at frequencies at which ωCgkis of the order of 2 × 10^-3mho and above. For typical values of Cgkthis value of ωCgkcorresponds 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.