Transconductance of electron tubes

ALL OF the functions of an electron tube when used as an amplifier, modulator, frequency doubler, or rectifier, can be formulated in terms of a plate-current-grid-voltage (i-e) characteristic expressed as the sum of constant, linear, and square terms in e. The characteristics of actual tubes often have such parabolic regions, which can be identified easily on a transconductance-grid-voltage (g-e) characteristic by observing that the part corresponding to the parabola on an i-e characteristic is a straight line on a g-e characteristic.