An RF power amplifier system for amplifying a first RF signal, comprising: an RF power amplifier being configured to amplify a second RF signal; a control loop for controlling a power level of the second RF signal, comprising: a RF output power determining unit: for determining a power level of the amplified second RF signal; a gain determining unit: for determining an actual large signal gain in dependence of the determined power level of the amplified second RF signal and a power level of the second RF signal; an attenuator for attenuating the first RF signal in dependence of a difference between the determined actual large signal gain and a desired large signal gain and for providing the attenuated first RF signal to the RF power amplifier as the second RF signal; characterised in that the RF output power determining unit comprises: a detecting system for detecting a DC power supplied to the RF power amplifier, and for estimating a power dissipated by the RF power amplifier, wherein the detecting system comprises a thermal unit for determining a temperature of the RF power amplifier and a dissipated power estimating unit for estimating the dissipated power in dependence of the determined temperature of the RF power amplifier; a power estimating unit for estimating the power level of the amplified second RF signal in dependence of the detected DC power and the estimated dissipated power. an RF power amplifier being configured to amplify a second RF signal; a control loop for controlling a power level of the second RF signal, comprising: a RF output power determining unit: for determining a power level of the amplified second RF signal; a gain determining unit: for determining an actual large signal gain in dependence of the determined power level of the amplified second RF signal and a power level of the second RF signal; an attenuator for attenuating the first RF signal in dependence of a difference between the determined actual large signal gain and a desired large signal gain and for providing the attenuated first RF signal to the RF power amplifier as the second RF signal; characterised in that the RF output power determining unit comprises: a detecting system for detecting a DC power supplied to the RF power amplifier, and for estimating a power dissipated by the RF power amplifier, wherein the detecting system comprises a thermal unit for determining a temperature of the RF power amplifier and a dissipated power estimating unit for estimating the dissipated power in dependence of the determined temperature of the RF power amplifier; a power estimating unit for estimating the power level of the amplified second RF signal in dependence of the detected DC power and the estimated dissipated power. a RF output power determining unit: for determining a power level of the amplified second RF signal; a gain determining unit: for determining an actual large signal gain in dependence of the determined power level of the amplified second RF signal and a power level of the second RF signal; an attenuator for attenuating the first RF signal in dependence of a difference between the determined actual large signal gain and a desired large signal gain and for providing the attenuated first RF signal to the RF power amplifier as the second RF signal; characterised in that the RF output power determining unit comprises: a detecting system for detecting a DC power supplied to the RF power amplifier, and for estimating a power dissipated by the RF power amplifier, wherein the detecting system comprises a thermal unit for determining a temperature of the RF power amplifier and a dissipated power estimating unit for estimating the dissipated power in dependence of the determined temperature of the RF power amplifier; a power estimating unit for estimating the power level of the amplified second RF signal in dependence of the detected DC power and the estimated dissipated power.