The bulk transformer working in k33-mode in both input and output circuits was studied. Theoretical model was developed to describe the transformation ratio as a function of the electrode size, material properties, frequency and impedance load in output circuit. Numerical simulation allows for the optimization of the primary electrode aspect ratio (l1/l=0.34) for the best transformation ratio at no-load condition. Transformation ratio, efficiency, input and output impedances are measured for the set of samples with variable input/output electrode size for the lowpower signal and compared with derived formulae. The maximum efficiency of 52% and transformation ratio of U2/U1=0.59 at the optimum load of 20 kOhm is obtained for the input part volume ratio l1/l=0.40 for the fundamental resonance. The higher efficiency of 60% and the transformation ratio of 0.36 at the optimum load of 10 kOhm is observed at the first overtone for l1/l=0.69 input part volume ratio. The proposed piezoelectric transformer is therefore suitable for step-down transformer applications with low output power density (
0.09W/cm3) and relatively low efficiency (
50%).