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Smooth regulation of output voltage of piezoelectric transformers can significantly widen the application range of piezoelectric transformers. So far the driving frequency of piezoelectric transformers has been used to regulate the output voltage at a matching load. However, the regulation range of voltage gain achieved by the method is usually very narrow within the acceptable range of efficiency. In this work, we investigate the possibility to regulate the output voltage of a k15 mode piezoelectric transformer by an external L/C component. The effects of an L/C component in series or parallel with the input and output ports on the voltage gain are investigated theoretically and experimentally. It is found that the voltage gain can be smoothly regulated in a relatively wide range by a tunable inductor that is in series with the input port. At a matching load of 80 Ω, the voltage gain can be regulated between 0.31 and 0.94 with efficiency larger than 90% and between 0.34 and 1.18 with efficiency about 80%. It is also found that a tunable capacitor in parallel or series with the output port can be used to regulate the voltage gain with efficiency higher than 90%.