A Study Of Radio Frequency Amplifier

Wireless is one of the main signal carriers for current and next generation communication system.The performance of RF front-end amplification modules has a great role on communication system,from quality and capacity to communication distance.In Receiving link,as the first stage of the receiver,the performance of low noise amplifier(LNA) has a decisive effect on the whole receiver.Due to semiconductor process we must trade off between input voltage standing wave ratio(VSWR) and Noise Figure.In this paper,A balanced LNA is adopted.Input VSWR and Noise Figure can be improved dramatically at the same time.The balanced LNA is suitable for GPS frequency band(center frequency 1575.42MHz).For modern communication system,in order to save spectrum resources and boost spectrum efficiency,some kinds of complex digital modulation are used,for example QPSK,QAM,OFDM,which are not constant envelop modulation.Furthermore the peak to average power ratio(PAR) is very big.Taking WCDMA for example,PAR is 6～9dB at 0.01%probability on the CCDF.If power amplifier back-off from P₁dB is used for high linearity,the power added efficiency(PAE) is degraded greatly at the same time.In the paper Doherty technique is studied to solve the problem.So the high efficiency can be achieved with little linearity sacrifice.In the paper,design method of LNA is deeply studied in theory.By choosing low noise High Electron Mobility Transistors(HEMT) ATF55143 from Avago Technology, utilizing ADS2008 simulation software from Agilent Technology,a balanced LNA is designed.EM-circuit co-simulation is carried out on balanced LNA in order to verify the complete circuit design,and layout files for PCB fabrication are given in GDSⅡ. Lastly the fabricated and soldered balanced LNA is tested by Vector Network Analyzer. The scheme of Noise Figure measurement best matched for this balanced LNA is proposed.Matching technique,loadpull technique and Doherty theory are studied and investigated in detail.RF high power LDMOS MRF6S21050L from Freescale is chosen as core component.Combined with ADS2008 and Freescale LDMOS design kit,the WCDMA base station downlink(2110MHz～2170MHz) power amplifier is designed and simulated.At last,Doherty power amplifier is designed and simulated The simulation result proves that the Doherty power amplifier has a greater Power added efficiency(PAE) than common balanced power amplifier when output power backs off 6dB.