Systems based on Orthogonal Frequency Division Multiplexing (OFDM) take advantages of multicarrier modulations. Properties in term of dynamic spectrum adjustment make OFDM a promising technology for Cognitive Radio systems. An OFDM modulation suffers from high power peaks, which significantly degrade the power efficiency and the linearity of the transmitter. In this paper, the Tone Reservation method is used to reduce the Peak-To-Average Power-Ratio (PAPR) in OFDM systems. We intend to consider an adaptive algorithm to improve speed convergence and PAPR reduction gain, taking into account the complexity of the algorithm. The proposed approach is based on the Conjugate-gradient method which is a powerful gradient descent algorithm, reliable and efficient on a wide range of optimization problems. The radio-frequency Power Amplifier (PA) being the most sensitive element to envelope fluctuations, a class AB 2.4 GHz-2 W InGaP PA is used to evaluate the performance of the proposed method in terms of Bit Error Rate and Error Vector Magnitude.