Microphonism in the Dynamically Operated Planar Triode

The theory of microphonism in planar triodes is extended to cover the practical case of simultaneous electrical and mechanical excitation. It is shown that, in addition to other components, microphonic plate current components result at the sum and difference frequencies of the two types of excitation for transverse motion of any of the three electrodes. Experiments providing verification of the theory, with a tube especially designed and constructed for this purpose are described. On the basis of the theory, (1) an electrical nondestructive method for determining which electrode of the tube is set into vibration at a given mechanical frequency is outlined, and (2) a method for considerably reducing the microphonic output of certain high-gain amplifiers of practical interest is described.