Abstract: A 3-terminal oscillating element has a square plate-like piezoelectric body, first and second electrodes provided on one surface of the piezoelectric body and a common electrode provided on the other surface of the piezoelectric body. The piezoelectric body is polarized in one direction between the first and common electrodes and in the opposite direction between the second and common electrode. The 3-terminal oscillating element, when provided in place of a combination of 2-terminal oscillating element and two externally provided capacitors C.sub.1 and C.sub.2 in a Colpitts circuit, substantially satisfies the following equations: ##EQU1## wherein C.sub.01 is an electrostatic capacitance between the first electrode and the counter electrode, C.sub.02 is an electrostatic capacitance between the second electrode and the counter electrode, and n is an transformation ratio of an ideal transformer; and ##EQU2## wherein S.sub.1 and S.sub.2 are areas of first and second electrodes, respectively.

Abstract: A feedback circuit for use in an oscillator has a piezoelectric oscillating unit constituted by a square plate-like piezoelectric body, first and second electrodes provided on one surface of the piezoelectric body and a common electrode provided on the other surface of the piezoelectric body. The piezoelectric body is polarized in one direction between the first and common electrodes and in the opposite direction between the second and common electrodes.