The Use of Vacuum Tubes as Variable Impedance Elements

The magnitude and phase angle of an impedance may be varied by means of circuits incorporating vacuum tubes. The analyses of several circuits show that they may be considered to be equivalent to a parallel combination of reactance and resistance. In certain circuits the effective resistance may be made infinite, these circuits acting like pure reactance, the magnitude of which may be controlled by means of electrode voltages. Vacuum-tube circuits containing only resistances and capacitances may act like an inductive reactance shunted by a negative resistance. Some types of "resistance-tuned" oscillators are based upon such circuits and may be readily analyzed from this point of view. By the use of an inverse-feedback amplifier it is possible to obtain very large effective capacitance or very low negative resistance, the magnitudes of which may be varied by means of the amplifier gain.