According to the evolution of existing and new standards of mobile communication systems and wireless multimedia services, the high quality of the signals to be transmitted from a base station is required. The RF(radio frequency) power amplifiers which are used in such systems, have to amplify all types of signals without significant distortion and keep operation capability at potentially high levels of output power.
For this purpose predistortion method was applied. It can reduce the 3rd order intermodulation signal by inserting the predistorter that has opposite characteristics of amplifier in front of the amplifier. The predistortion linearizer consisted of 3dB branch-line coupler, MESFET, attenuator, delayline, 20 dB directional coupler, and Wilkinson power combiner. The phase difference of two input signals into the Wilkinson power combiner was adjusted to maintain out-of phase by monitoring with 20 dB couplers, respectively, and controlling the length of delayline. Thus, the difference of two signals only can be emerged to the output port of the Wilkinson power combiner, and then it is possible to cancel the signals of two path. It is possible to have 180°phase difference between the fundamental signal and the 3rd order intermodulation signal. In addition, the passive attenuator and the phase shifter used for predistorter could minimize phase variation compared with the case of active ones.
The proposed predistorter was designed in the frequency band of 1,885 MHz ∼ 2,025 MHz, and cascaded to 4 W power amplifier, and tested with two-tone spreaded 2.5 MHz at the center frequency of 1.96 GHz. As an experimental result, it was clearly shown that the 3rd order intermodulation distortion characteristics have been improved as much as 17 dB when the output of the power amplifier is 30.67 dBm.