The co-sintered functionally graded PZT ceramic transformers working in a kp-mode in both input and output circuits were studied. Theoretical model was developed to describe the transformation ratio as a function of the frequency. Transformation ratio, efficiency and power density are measured for the set of samples with homogenous PZT and variable input/output PZT compositions for the low-power signal and compared with the derived formulae. The maximum efficiency is observed for the homogenous hard PZT compositions (type APC840). Contrary to the theoretical calculations, co-sintering hard PZT ceramics along with the soft PZT were not experimentally proven to produce a noticeable increase in the transformation ratio, efficiency and output power density probably due to the difference in optimal sintering temperature.