A model is presented for the core emission from radio pulsars. Energetic positrons produced from the intense electric field gap of a fast rotating neutron star cascade and move outward along the magnetic field lines in the region of the polar cap. On the other hand, it is possible to form a low-density plasma envelope around the neutron star which is accreting the stellar wind. In this case, the positron beam interacts with the background plasma envelope under the influence of the intense magnetic field. The Cerenkov and maser-like instabilities in the Z-mode branch are excited, and then coherent radiation near the ion cyclotron frequency can be amplified.