A signal processing apparatus for processing an input signal (x) has an adaptive predistorter, an amplifier and a down-converter. The amplifier is configured to amplify a processed signal (y′) to obtain an amplified signal (y″). The down-converter is configured to multiply a version of the processed signal (y′) with a version of the amplified signal (y″), one of the signal versions being phase shifted, to obtain a first down-converted signal (z1, z1′), and to multiply the processed signal (y′) with the amplified signal (y″) to obtain a second down-converted signal (z2, z2′). The predistorter is configured to predistort the input signal (x) according to a predistortion characteristic to obtain the processed signal (y′), the predistorter being further configured to adapt the predistortion characteristic based on the first down-converted signal (z1, z1′) and the second down-converted signal (z2, z2′).