| In fourth generation (4G) communication system, the signal modulated by the OrthogonalFrequency Division Multiplexing (OFDM) technique with high spectral efficiency has high peak toaverage power ratio. Therefore, to achieve acceptable linearity, power amplifier (PA) has to beoperated with its average output power backed off from the peaking power, resulting in lowefficiency of the power amplifiers and base stations. For improving the efficiency of the basestations, research in this paper has been focused on the theory and efficiency characteristics oftraditional Doherty, asymmetrical Doherty, three-stage Doherty and multistage Doherty poweramplifier. Taking the characteristics of modulated signal in the4G communication system intoaccounting, the incomplete load impedance modulation of Doherty power amplifier, the reducingof the maximum output power and the small range of high efficiency back-off have been pointedout in practical design. Doherty power amplifier employing envelope tracking technique has beenproposed for improving that fault，in addition, three-stage Doherty structure has been adopted todeal with high PAPR signal in a4G communication system.Lump-components impedance match, bias network optimized with impedance match networkand three equivalent structures for traditional quarter-wave transmission line have been introducedto shrink the circuit size for the application of Doherty power amplifier in450-470MHz frequencyband. Equivalence property of the presented structures has been analyzed by ABCD matrixparameter and demonstrated by Advanced Design System (ADS) simulation. Finally, the proposedstructures have been applied in branch-line coupler, two-way Doherty and three-stage Dohertypower amplifier.Theory of envelope tracking power amplifier, drain modulation and grid bias with envelopetracking of Doherty power amplifier have been elaborated and a novel varying-grid voltage ofpeaking amplifier according to the envelope of signal was adopted in the design of Doherty poweramplifier. With the way of input signal power and grid voltage sweeping,450-470MHz two-wayDoherty power amplifier has been simulated and demonstrates the improvement of load impedancemodulation and maximum output power. Moreover, by utilizing the same devices, three-stageDoherty power amplifier has been simulated. The simulated results have shown good efficiencyperformance while output power backing9dB off from the peaking power. Finally, two-way and three-stage Doherty power amplifiers as well as envelope peak andaverage power detection module have been implemented. Taking1-tone signal and20MHz LTEsignal as input signal, the data obtained has demonstrated that two-way Doherty power amplifierwith envelope average power detection has a better enhancement in efficiency, three-stage Dohertypower amplifier shows high efficiency characteristic for dealing with8dB PAPR signal of LTE. |
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