The invention relates to apparatus for treating biological tissue (G) using a low-pressure plasma witha) a transformer (1) for generating a high-frequency electromagnetic field,b) a probe, which can be electrically coupled to the transformer (1) andc) a control device (3) for controlling the high-frequency electromagnetic field generated by the transformer (1),wherein the transformer (1) has a primary coil (4) and a secondary coil (5) disposed coaxially therewith and wherein the intermediate space between the primary coil (4) and the secondary coil (5) in the overlap region (B) of the two coils (4, 5) increases from a first spacing (d1) to a second, greater spacing (d2) in the direction of a coupling (7) for the probe (2).
