The Design Of Predistortion Doherty Power Amplifier

The3G mobile communication system, for example, CDMA2000、WCDMA、OFDM, always uses some complicated modulation technique to process signal. With the4G wireless communication system, to transport amounts of data in time, thesemodulated signal changes rapidly and have high peak-to-average power ratio. Thecommunication system will produce nonlinearity distortion to those signal with largedynamic range, for example, harmonic distortion and intermodulation distortion. Poweramplifier is very important in communication base station. It should have good linearityto amplify those signal with high peak-to-average power ratio. Otherwise, thetransmission of the signal would be meaningless. It’s urgent to improve poweramplifier’s nonlinearity distortion.If power amolifier in back-off power work with class AB or class A status to makeless distortion, it means that lower out power for good linearity. Then power amplifier’sefficiency is inevitably lower. It will lead to more power consumption, thermalproblems, high cost and poor stability. Because of Doherty technique’s special structure,it’s main power amplifier and auxiliary power amplifier work with different status, so itcould make high efficiency with some back-off power. Many researchers focus onDoherty power amplifier with it’s simple architecture.Doherty technique and linearty technique is combined to ensure high efficiencyand good linearity. Analog predistortion technique is simple, large frequency rangeapplication, excellent stability and low cost,so it has a high value of application.This paper with the Agilent ADS software platform, choose LDMOS transistorMW6S010N to design DPA, and this chip is made by Freescale. Import the model ofthis transistor to ADS through DesignKit, and then choose quiescent operating point tomake main amplifier in class AB and auxiliary in class C. Design bias circuit and get theoptimum load impedance and the optimum source impedance to design matching circuit through load-pull. The simulation shows that the DPA gain is12.8dB, gain fluctuationsis±0.15dB, P1dBoutput power is43.3dBm, efficiency is56.3%in43.3dBm outputpower, efficiency is46.8%in39.3dBm output power. Then design analog predistorterwith MAXIM chip MAX2010. It consists of one MAX2010, two AH125gaincompensation amplifier and one AH202driver amplifier. Make MAX2010’s phase andgain curve inflection point be the same with amplifier. Make MAX2010’s phase andgain curve slope just invert with amplifier. Then predistorter could compensateamplifier’s compression characteristics.When make DPA actual object, hybrid electric bridge is instead of power divider.Main amplifier and auxiliary amplifier’s output and input matching circuits before andafter could adjust compensation line. So improve main amplifier with auxiliaryamplifier’s phase difference and avoid main amplifier’s power leakage to auxiliaryamplifier. The actual test results are basically the same with simulation data.