| Power amplifiers are key components in wireless communication systems because they have the function of amplifying the signal for transmission. The linearity and efficiency of PA are always the emphases of RF designers. The nonlinearity of PA will cause a lot of negative effect for the wireless communication system, such as the shift of the output signal constellation and the closure of the eye diagram which lead to the difficulty of decoding and increase of bit error rate, the spectrum growth and interference with adjacent channels. Though we can improve the linearity by simply backing off the input signal, the efficiency of PA will be very low. Therefore, more advanced linearization techniques are needed to improve the linearity and maintain a moderate efficiency.The design theory and the nonlinearity characteristics of power amplifers are introduced in the first place, and the principle of memory effect of power amplifier is also analyzed. Then, several linearization techniques are analyzed, and the merits of analog predistortion compared to other methods are presented.Second, the system of analog predistortion power amplifier are designed and simulated according to the given requirement. The design procedures of the drive stage PA and the main PA are introduced in detail. The modules of the analog predistorter are analyzed specially, and the functions of IM3 Generator, Phase Shifter, Attenuator and Delay Timer are introduced and verified by simulation.Finally, the 120 Watts analog predistortion power amplifier is manufactured and tested. The results shows the IMD3 has been suppressed to be less than -40dBc for CW two-tone signal with the tone spacing changing from 100KHz to 2.5MHz at 120W output power. When the tone spacing of two-tone input signal is 2.5MHz, the IMD3 has been improved by 10dB at the output power of 120W, and the power added efficiency remains above 30%. In a word, both the linearity and the efficiency of the power amplifier are improved by the adopted perdistorter. |
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