Patent GB-799891-A (GB)
Title: Improvements in electro-mechanical transducer
IPC: H01L41/107
Filed: 1955-01-04
Abstract: 799,891. Piezo-electric elements. GENERAL ELECTRIC CO. Jan. 4, 1955 [Jan. 4, 1954; June 29, 1954], No. 249/55. Class 40 (8). An electrical voltage-transformer device comprises a piezo-electric resonator made of a prepolarized ferro-electric material having input and output electrodes arranged on its external surface. The resonator may be in the form of a tube 2 of a polycrystalline ferro-electric ceramic such as a titanate or niobate. The body is polarized in the direction of its major axis and an input is applied across the inner pair of circumferential electrodes 4, 5 at a frequency equal to the frequency of vibration of the body in the fundamental length mode. An output signal is derived from the outer circumferential electrodes 6, 7. The ratio of the output to the input voltage is shown in the Specification to depend upon the spacing of the inner electrodes and this ratio is shown to increase as the spacing decreases. The resonator of Fig. 3 comprises a body 20 having a rectangular cross-section. The body is transversely polarized between input electrodes 26 centrally situated on opposite broad faces. The outer portions 22 of the body are longitudinally polarized and an output is derived from electrodes 31, 32 on the end faces when the body is excited in its fundamental length-resonance mode by the inner electrodes 26. Additional electrodes 35, 36 encircling the resonator near the boundaries of the input electrodes are connected in common. The Specification contains a theoretical discussion of the manner of operation of this construction and it is shown that maximum voltage transformation is obtained when the distance between the input electrodes is 1/ 3  the distance between the output electrodes. A modification of this construction (Fig. 5, not shown) comprises input electrodes placed near one end of the body instead of centrally. The output is derived from an output electrode on the opposite end face and an input electrode used in common. The tubular barium titanate resonator 57 of Fig. 6 is fixed at one end to a mass 59 of material having a mechanical impedance much geater than barium titanate. Under these circumstances the length of the body will be equal to one quarter of the acoustic wavelength for the fundamental length resonance. Alternatively, the body 59 may be designed to form a half-wave resonator with the barium titanate tube. The tube is polarized longitudinally and is provided with an input electrode 61, an output electrode 60 and a common electrode 62. The resonator of Fig. 7 is in the form of a disc having an axially polarized central region provided with circular input electrodes 69, 70. The remaining peripheral portion of the disc is radially polarized and the output is derived between the central electrode 70 and a circumferential electrode 71. The disc resonates in a radial mode. The resonator of Fig. 8 is in the form of a broken ring equally divided into an axially polarized input region 85 and a circumferentially polarized output region 86. The input is applied across semicircular electrodes 88, 89 on the upper and lower faces of the body whilst the output is derived between electrode 89 and an electrode 91 on the end face of the broken ring.