The invention relates to apparatus for treating biological tissue (G) using a low-pressure plasma, comprising a transformer (1) for generating a high frequency electromagnetic field, a probe which can be coupled electrically to the transformer (1), and 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) arranged coaxially with respect thereto, the interspace between primary coil (4) and secondary coil (5) in the overlapping area (8) of the two coils (4, 5) increasing from a first distance (d1) to a second, greater distance (d2) in the direction of a coupling (7) for the probe (2).
