1. A digital predistortion device for broadband power amplifier, comprising: Digital to Analog Converters (DACs) for converting N input signals generated by a random signal generator into analog signals; first mixers for upconverting the N input signals converted by the DACs; real operators for selecting real number signals from the N input signals upconverted by the first mixer; a nonlinear power amplifier for amplifying N input signals and then generating N output signals; second mixers for downconverting the N output signals of the nonlinear power amplifier; Analog to Digital Converters (ADCs) for converting the N output signals downconverted by the second mixers into digital signals; low pass filters for eliminating image signals from the N output signals converted by the ADCs; an equivalent amplifier model estimator for receiving the N input signals and the N output signals, and estimating characteristics of the nonlinear power amplifier; N predistorters for eliminating or compensating for all or part of the N output signals; and a coefficient extractor for extracting coefficients of the N predistorters using the characteristics of the nonlinear power amplifier estimated by the equivalent amplifier model estimator.