Device level characterization of outphasing amplifiers

Bogusz, Aleksander 2022. Device level characterization of outphasing amplifiers. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

The outphasing technique proposed by Chireix in 1935 is one of the classical methods of addressing power amplifier (PA) efficiency degradation caused by operating in output back-off (OBO) conditions, where PA efficiency is typically low. Essentially, the envelope from the input signal is eliminated, and two CW signals are constructed; these have constant amplitude, while their relative phase offset holds the original information contained by amplitude modulation. Consequently, efficiency improvements are achieved by amplifying signals with constant amplitude using PAs operating in saturation, where efficiency typically peaks. The envelope is restored at the output by means of a vector summation of both signals, using a non-isolating combiner at the output stage The main focus of the work described in this thesis was placed on extending bandwidth of the inherently narrowband technique of outphasing and then adopting this method to modern telecommunication standards. Two prototype PAs were designed to investigate whether bandwidth improvements can be achieved by adopting a broadband balun as a combining structure in the outphasing PA. Two baluns were designed and fabricated to be used in the demonstrator circuits; one using a section of semirigid coaxial cable and the other, a planar balun realized on 10 mil thick Alumina substrate. A novel method of fabrication was proposed for the former structure, which achieved more than double octave bandwidth, from 1.25 GHz to 4.7 GHz with losses lower than 1dB, an amplitude imbalance (trace separation) below 0.75 dB and phase imbalance within ±5 degrees. The measured CW performance of the prototype circuits produced results comparable with the state-of-the-art solutions available in literature. Moreover, this work demonstrated that a balun with sufficient bandwidth allows load modulation to be prescribed at fundamental and second harmonic frequencies, opening the possibility of waveform engineering to implement continuous PA modes such as class J in outphasing PAs. The desired harmonic load termination was achieved without any specialized matching networks, and solely by means of load modulation provided through active device interaction. The thesis concludes with the formulation, analysis and description of the novel concept derived from Chireix outphasing. Several outdated assumptions still prevalent in outphasing analysis included in literature today are challenged and reformulated for modern semiconductor devices such as GaN HEMTs. Through this process, a new concept of Current Mode Outphasing (CMOP), is proposed and described in detail. One of the significant advantages of the proposed approach is it allows the elimination of the combiner structure, which typically dominates the size of the final outphasing circuit, due to the presence of λ/4 transmission lines. Consequently, the demonstrator MMIC circuit, containing DC bias, stability elements and pre-matched to 50 Ω on input and output, has been deployed on an area of 2.3 mm x 2.8 mm. The CMOP circuit was fabricated using 0.25 µm GaN technology and achieved a bandwidth of 1.6 GHz centered at 3.35 GHz, whereas the maximum CW output power remains within 43 dBm ± 0.5 dB. A total gain of more than 12 dB is reported from 2.95 to 3.95 GHz, while a maximum Power Added Efficiency was measured as 68.5% at 3.25 GHz and remains greater than 60% from 2.85 to 3.8 GHz, and above 50% for almost the entire frequency range. The output back-off (OBO) efficiency peaks at 3.25 GHz with 53.5% and 45.6% for 6 dB and 8 dB back-off, respectively, and remains above 30% and 23.7% for the entire frequency range. To the best of the authors’ knowledge, this is the largest fractional bandwidth achieved in an outphasing PA, that has been reported in literature.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Engineering
Uncontrolled Keywords:	Outphasing, Chireix, Power Amplifier, Efficiency Enhancement, Balun, Gallium Nitride
Date of First Compliant Deposit:	8 September 2022
Last Modified:	08 Sep 2022 11:11
URI:	https://orca.cardiff.ac.uk/id/eprint/152414

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