The performance of high-level modulation schemes in non-linear broadband digital communication systems is investigated. The tolerance of 16-, 64-, 256-QAM and 49-, 225-QPRS schemes to these non-linearities is given in terms of their phase margins. Our research results show that high level QAM schemes are less sensitive to non-linearities when operated in offset mode. Comparison of non-linearly amplified broadband QAM and QPRS schemes reveals that QPRS schemes need less carrier-to-noise ratio (CNR) than corresponding QAM schemes for identical operating conditions. Also, when the non-linearities are not severe, the degradation in performance is controlled by AM/PM characteristic of the broadband amplifier.