Battery-less sensors for IoT require energy harvesting from the ambient environment. RF-DC conversion with a rectifying antenna (so-called rectenna) is generally used to obtain energy from ambient electromagnetic waves. However, RF-DC conversion efficiency of the diode significantly decreases for ambient electromagnetic waves due to their weak RF power. We previously reported bulk acoustic wave (BAW) piezoelectric transformers based on c-axis zig-zag ScAlN multilayer films in order to amplify the RF signal in the rectenna. In this study, we report simpler BAW transformers based on monolayer thin films with a different dielectric constant (?). The theoretical voltage gain was simulated by Mason's equivalent circuit model. We also derived the simple formula describing the voltage gain of the BAW piezoelectric transformers without using the circuit. This simpler BAW piezoelectric transformers using the combination of high and low ? thin films were demonstrated with the combination of epitaxial PZT family films and ScAlN films. S parameters of the BAW piezoelectric transformers are measured by a network analyzer. As a result, the voltage gain approaching + 18 dB in 100 MHz range was obtained. Also, the voltage gain approaching + 5 dB in 1 GHz range was obtained. This new type of BAW transformer is promising for a high voltage gain transformer in the rectenna.