A controller for an RF amplifier, in particular for a RF amplifier of an MR tomography apparatus, has an IQ control element for adjusting the magnitude and phase an RF signal that is be fed to the RF amplifier. The IQ control element has a signal splitter that splits the RF signal into two partial signals having a 90.degree. phase offset in an I path and a Q path, each having a multiplier for multiplying the partial signal by an I factor in the I path and a Q factor in the Q path. A summing unit recombines the partial signals. A detector determines the actual phase difference and actual amplification between the RF signal fed to the IQ control element and the RF signal amplified by the RF amplifier. An IQ controller determines the I factor and the Q factor from the actual difference and a desired phase difference and the actual amplifier and a desired amplification. The IQ controller has an operating point at which the I factor and the Q factor are the same magnitude if the actual and desired phase differences and the actual and desired amplifications are the same.