Research On High Efficiency Modes Of Operation And Key Techniques For Broadband Power Amplifiers

Spectrum coverage solution in an efficient,flexible and cost-effective way has become the main trend in wireless communications.Due to the increasing demand for capacity expansion in mobile and wireless networks,the current modern wireless devices and subsystems are increasingly required to accommodate the co-existence of multiple telecommunication protocols and operations.Since the move to current 4G and beyond is of significant importance,mobile operators are continuing to upgrade their current wireless networks and terminals to yield operations at bands that are widely separated in frequency.Considering that it is important to ensure backward compatibility and suport new spectrum licenses,mobile operators prefer the solutions that support multi-band multi-model operations.Besides,according to the latest 3 GPP release,the most common configuration is the inter-band non-contiguous carrier aggregation,which complicates the spectrum coverage problem in practice to some extend.To fulfill the complex requirements of spectrum coverage,wideband systems have recently received a great deal of research interest.The main focus of this thesis includes the analysis of high efficiency modes of operation and the research of novel network synthesis mothed in broadband power amplifier(PA)designs,attempting to implement adequate broadband impedance transformations for optimum broadband operation of the PA.The promised high performances can be achieved by the precise broadband impedance matching ahieved at the input and output of the PA simultaneously.These figures of merit include high efficiency at rated power,sufficient power with minimal non-linear distortion,reliable gain versus frequency,good input and output return loss,etc.The contributions are as follows:(1)A new synthesis method for the design of broadband impedance matching network has been introduced to match the transistor input.The proposed matching network would be a realistic alternative to those that are frequently designed in low-pass structure.Easy-to-use design equations are given for this filter-based matching network.The required impedance of the stubs and the connecting lines can be directly calculated from the equations.This synthesis method simplifies the implementation of network in distributed form as compared with LC low-pass matching network where equivalences are necessarily required in the rough conversion from lumped to distributed elements.High efficiency mode of operation is guaranteed by the constructed output low-pass matching network,since the LC low-pass network can provide the prescribed load terminations at fundamental frequencies and at harmonics.A commercially available 10W GaN HEMT device(CGH40010)was used in this investigation.The proposed broadband PA exhibits band-pass Chebyshev response in S-parameter measurements,and yields good gain flatness(±0.5 dB)and input return loss(>13 dB).PAE of 58%-72%has been measured in large signal measurements.After digital predistortion,measured adjacent channel leakage power ratio is below-50 dBc at ±5 and ±10 MHz offset,when driven by 5 MHz and 10 MHz WCDMA signals at 7 dB power back-off.(2)The conventional broadband impedance matching network in low-pass structure cannot present adequate broadband impedance rotation for optimum broadband operation of the power amplifier,resulting in performance variations and degradations.The underlying constraint is the phenomenon of opposite impedance rotation.Against such a backdrop,this paper presents a methodology using the band-pass topology to compensate the frequency-variable response of the PA,attempting to investigate the potentials for improvement in adequate broadband impedance rotation.The CGH40010F from Cree is used to verify the effectiveness of this compensation method.Experimental results are in good agreement with the simulation.The proposed PA yields good gain flatness(±0.5 dB),input/output return loss(>13 dB)and high power added efficiency(>60%)over the band from 1.7 GHz to 2.7 GHz.After digital predistortion,the measured adjacent channel power ratio is about-55 dBc and-50 dBc at ±5 and ±10 MHz offset,when the PA is driven by 5-MHz and 10-MHz WCDMA signals at 6.5?7 dB power back-off.(3)This thesis reports a novel network synthesis method to design an extended dual-band power amplifier.The proposed matching networks exhibit dual-band impedance rotation,leading the transistor to yield flat gain at the two designed passbands and suppressed gain responses at frequencies outsides the passbands.The fabricated PA represents a competitive solution in multi-band and multi-mode applications as compared with the conventional dual-band PA based on the multi-frequency techniques.The commercially available CGH40010F from Cree is employed for verification.At 1.4 and 2.4 GHz,the bandwidth has been extended to over 150 MHz at each individual passband.The implemented PA can deliver the saturated output power of 10 W minimum,and power added efficiency of 65%minimum has been measured.(4)To overcome the weakness in harmonic impedance transformation,two modified network synthesis motheds are proposed.In first synthesis scenario,the straight stubs are individually replaced by the step stubs,attempting to generate the attenuation pole at the prescribed harmonic frequencies.The core of the second synthesis method is the cascade of the band-pass and low-pass broadband impedance matching networks,forming a hybrid structure.The low-pass networs are introduce to generate broadband real impedance transformation at the fundamentals and purely reactive terminations at the harmonics,which is the key strength of the low-pass network.The required optimum broadband impedance transformation at the transistor drain lead is achieved by the modified band-pass broadband matching networks.Hardward test verifies the feasibility of the two synthesis methods.Good gain flatness(±1 dB)and high efficiency(>70%)has been measured over the band from 1.7 GHz to 2.7 GHz.