The behavior of the convective growth rate of the electromagnetic proton-cyclotron instability is investigated in detail in H+, He+ plasma and when a third (minority) cold ion component such as O+ is taken into account. It is shown that amplification is the result of an interplay between the cold species and the thermal anisotropy of the energetic protons. The results seem to be in good agreement with the observations performed on board GEOS 1 and 2 concerning the generation of ULF waves, below the proton gyrofrequency. It is also shown that a small amount of O+ can strongly affect the behavior of the convective growth rate of the instability below the He+ gyrofrequency, even far away from the O+ stop band.