A feedback circuit and amplifiers using a vibrator comprising a piezoelectric body having a bismuth layered compound as the main ingredient thereof are provided so as to generate oscillation. Load capacitors comprising the feedback circuit together with the vibrator are formed. These load capacitors each have, as the main ingredient thereof, a dielectric body which decreases in the relative dielectric constant with the increase in temperature in the region not less than one half the temperature range from -20 to 80° C., and in which the epsilon-TC is not less than 5000 ppm/° C., wherein the epsilon-TC is an average change rate of the relative dielectric constant in the above-described temperature range and is represented by (Cmax-Cmin)/(C20.100), where Cmax is the maximum value of capacitance in the above-described temperature range, Cmin is the minimum value of capacitance in the above-described temperature range, and C20 is the capacitance at 20° C.