This paper presents a study on the main characteristics of an integrated transformer i.e. the low and high cut-off frequencies (FCL and FCH) and the voltage gain within the bandwidth (G0). After a description of the integrated transformer and its micro-fabrication, the measured transformer frequency response is given. An equivalent circuit is derived ant the method to determine each element is shortly described. Then, the paper focuses on the influence of geometrical dimensions and a few characteristics of materials on the low and high cut-off frequencies (FCL and FCH) and on the voltage gain (G0). An analytical expression is derived for FCL, FCH and G0. The aim of this article is to identify the main parameters that limit the bandwidth and the voltage gain.