Two input signals are amplified and combined into an output signal by first and second amplification stages and a combiner element. The first amplification stage includes a first signal modification unit and a first amplifier which receives a first input signal and provides a first intermediate signal. The second amplification stage includes at least a second amplifier which receives a second input signal and provides a second intermediate signal. The combiner element combines the first and the second intermediate signals and produces the output signal. In addition, a sensor senses a reflected signal on a signal line connecting the first amplifier and the combiner element and a feedback loop extends between the sensor and the signal modification unit. The signal modification unit is adapted to modify the first input signal depending on a measurement signal output by the first sensor.