Design Of L-Band High-Efficiency Doherty Amplifier

As modern wireless communication technology develops, RF power amplifiers are becoming the attractive study, because they are the key component of wireless mobile communication transmitter and communication quality depends on high performance of RF power amplifiers. Modern wireless communication systems such as CDMA2000, WCDMA, OFDM, and so on, are intended to maximize the data rate in a fast moving environment. The modulated signals of these systems vary rapidly and have high peak-to-average power ratios resulting in high linearity for power amplifiers. The improvement of linearity of power amplifier is mainly achieved by backing off power, but the efficiency of power amplifier will quickly decrease. So it is very important to improve the efficiency of power amplifiers above the demanding linearity. Doherty technique has both of high efficiency and high linearity and can achieve high efficiency at low output power as same as at high output power and it has advantages of easy implement and low cost. Moreover, the combination of Doherty technique and digital pre-distortion technique will achieve higher linearity and more importance for applications.Based on conventional Doherty technique, a L-band 40W high-efficiency Doherty power amplifier with microstrip offset lines was designed in this thesis. The main performances of RF power amplifiers is introduced and the problems related to power amplifier design such as stability, matching methods, Load-Pull theory, Doherty theory, effect of peaking amplifier stage bias on Doherty PA ,and so on , are analyzed in detailed. The emphasis is put on using Freescale'LDMOS transistor MRF6S19060N as the power amplifier model and Angilent's simulation software ADS, adding microstrip lines to the conventional Doherty PA to make output signals phase difference constant between carrier amplifier and peak amplifier and to prevent power leakage from peak amplifier because of high impedance of microstrip. In this case, the ACLR less than -45dBc,the power added efficiency(PAE) high to 42.2% and the third-order intermodulation distortion(IMD3) less than -26dBc are achieved with 36W output power 6dB backed off from the compression point at frequency of 1960MHz. Compared with the Doherty amplifier without microstrip offset lines and balanced class AB amplifier, the PAE of the Doherty amplifier with offset lines is improved 17.3% higher than the former one and 25% higher than the balanced class AB amplifier. Moreover, the IMD3 of the Doherty amplifier with offset lines is improved 3dB less than the Doherty amplifier without microstrip offset lines. At last, the PCB of the Doherty power amplifier has been fabricated.