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In this paper, different techniques are combined in a unique characterization system dedicated to power transistor linearity improvement. Successive optimizations are performed using source-pull/load-pull techniques at fundamental, harmonic, and base-band frequencies associated with an instantaneous memoryless base-band predistortion procedure. Measurement results performed at 1.575 GHz on a metal semiconductor field effect transistor (MESFET) power device biased in class-AB under a quadrature phase shift keying (QPSK)-modulated signal excitation show that fundamental frequency load-pull increases the output power and power-added efficiency (PAE) by 2 dB and 16 percentage points, respectively. The fundamental frequency source-pull measurements lead to an adjacent channel power ratio (ACPR) variation equal to 3 dB. For harmonic load-pull (at 2f0), it has been found that some impedances greatly improve linearity but output power and PAE decrease. Source-pull characterization at the second harmonic frequency allows one to find a trade-off between output power, efficiency, and linearity. Moreover, the influence of base-band impedance on ACPR and error vector magnitude (EVM) is found to be 15 dB and 5.5 percentage points, respectively. Finally, instantaneous memoryless base-band predistortion improves ACPR and EVM values by 5 dB and 1 point, respectively.