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Author

Thomas Lehmann

Bio: Thomas Lehmann is an academic researcher from University of Kiel. The author has contributed to research in topics: Power dividers and directional couplers & Amplifier. The author has an hindex of 7, co-authored 18 publications receiving 161 citations.

Papers
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Journal ArticleDOI
TL;DR: In this paper, a reconfigurable Wilkinson power dividers with a minimum number of switches is presented. But the performance of the dividers depends on the operating modes of the switches.
Abstract: Novel reconfigurable Wilkinson power dividers with a minimum number of switches are introduced. In the first design, re-routing of output power between one and two ports is achieved with only three switches. A second design employs a switched coupled line transformer, and requires four switches. Both operating modes of the power dividers are matched and achieve a relative bandwidth of approximately 10%. The construction of the dividers and experimental results are presented at 2.14 GHz.

27 citations

Proceedings ArticleDOI
15 Jun 2008
TL;DR: In this article, an analysis of a sequential power amplifier (SPA) in a transmitter with an FPGA-circuit at the input and a directional coupler at the output is presented.
Abstract: This paper presents an analysis of a sequential power amplifier (SPA) in a transmitter with an FPGA-circuit at the input and a directional coupler at the output. The nonlinear combining characteristics of the output coupler are investigated. The coupling strength and the ratio of maximum power of peaking and main amplifier are discussed as design parameters. A switched coupler is examined as the combiner. Due to the sequential transmitter concept the back-off efficiency can be significantly increased. E.g. using conventional class-B amplifiers a 10 dB back-off efficiency of 38% can be achieved. The average power added efficiency of the whole transmitter may be improved by 30% compared to conventional amplifiers. Design tradeoffs, theoretical performance and measurement results of the proposed transmitters are presented.

26 citations

Proceedings ArticleDOI
17 Dec 2007
TL;DR: In this paper, a power amplifier is built up with several individual smaller stages connected in parallel using the balanced amplifier scheme, and some of the stages are switched off in order to decrease the DC power consumption and increase the drive of the remaining stages, thus increasing efficiency.
Abstract: Reconfigurable power amplifiers are presented, which utilize switched directional couplers. A power amplifier is built up with several individual smaller stages connected in parallel using the "balanced" amplifier scheme. If the required output power decreases, some of the stages are switched off in order to decrease the DC power consumption and increase the drive of the remaining stages, thus increasing efficiency. The idea is to use switched branch line (right handed and left handed) couplers with switches to ground at their transversal planes as rf power switch in static and dynamic "hot" switching reconfigurable PA networks. The paper presents theoretical considerations and experimental results for built networks.

21 citations

Proceedings ArticleDOI
07 Jun 2009
TL;DR: In this paper, the authors present an approach for the systematic design of reconfigurable quadrature directional couplers using even-odd analysis of four-ports with two-fold symmetry four eigenreflections are derived.
Abstract: This paper presents an approach for the systematic design of reconfigurable quadrature directional couplers. From even-odd-analysis of four-ports with two-fold symmetry four eigenreflections are derived. The scattering-parameters are linear combinations of them. The eigenreflections can be positioned deliberately with tuning reactances to create directional couplers which are reconfigurable in coupling ratio, operating frequency or the arrangement of the output ports. Two exemplary couplers, a LC-bridge and a modified branchline-coupler, are presented and measurement results are shown. Directional couplers with adjustable coupling ratio can also be created with cascaded couplers and additional phase shifters or discontinuities. A compact directional coupler using this principle is presented together with measurement results.

17 citations

Proceedings ArticleDOI
07 Jun 2009
TL;DR: In this paper, the design aspects of sequential power amplifiers (SPAs) were discussed and an optimum practical realisation was suggested, using an FPGA-circuit at the input and an adaptable directional coupler as the signal combiner at the output.
Abstract: This paper discusses design aspects of sequential power amplifiers (SPAs). After derivation of theoretical performance limits, an optimum practical realisation is suggested, using an FPGA-circuit at the input and an adaptable directional coupler as the signal combiner at the output. Design trade-offs between the optimum coupling and the output power capability of the involved amplifiers are outlined. Using SPAs efficiencies of more than 50% can be achieved under up 10dB back-off from maximum power. The combining characteristics of the adaptable combiner are investigated. Off-the-shelf power amplifiers may be employed, which can be linearized in a conventional manner. The paper closes with measurement results.

16 citations


Cited by
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Journal ArticleDOI
TL;DR: A highly reconfigurable radio frequency patch hybrid coupler with a simple topology is presented for the first time and tunable coupling coefficient can be implemented by changing the capacitance.
Abstract: A highly reconfigurable radio frequency patch hybrid coupler with a simple topology is presented for the first time. Due to the introduction of lumped equivalent circuit and flexibility of varactor loaded patterned ground plane, the complex reconfigurability in port arrangement and coupling coefficient can be implemented. Varying the capacitance across different planes allows the electronic switching between different operating modes of the patch hybrid coupler. When the auxiliary patch and patterned ground plane is placed closer to the two output ports, tunable coupling coefficient can also be implemented by changing the capacitance. This results in wide band reconfigurability for the coupling coefficient range. For demonstration purposes, a patch hybrid coupler operating at 2.4 GHz is designed and measured to validate the port reconfigurable concept. Also, a patch hybrid coupler with tunable coupling coefficient operating at 3.5 GHz is designed, fabricated, and measured. This circuit gives a reconfigurable coupling coefficient ranging from -4.2 to -10 dB while maintaining good performance.

61 citations

Journal ArticleDOI
TL;DR: It is illustrated that the sequential operation greatly extends the high-efficiency power range and enables the proposed SLMBA to achieve high back-off efficiency across a wide bandwidth.
Abstract: The analysis and design of an RF-input sequential load modulated balanced power amplifier (SLMBA) are presented in this article. Unlike the existing LMBAs, in this new configuration, an over-driven class-B amplifier is used as the carrier amplifier while the balanced PA pair is biased in class-C mode to serve as the peaking amplifier. It is illustrated that the sequential operation greatly extends the high-efficiency power range and enables the proposed SLMBA to achieve high back-off efficiency across a wide bandwidth. An RF-input SLMBA at 3.05–3.55-GHz band using commercial GaN transistors is designed and implemented to validate the proposed architecture. The fabricated SLMBA attains a measured 9.5–10.3-dB gain and 42.3–43.7-dBm saturated power. Drain efficiency of 50.9–64.9/46.8–60.7/43.2–51.4% is achieved at 6-/8-/10-dB output power back-off within the designed bandwidth. By changing the bias condition of the carrier device, higher than 49.1% drain efficiency can be obtained within the 12.8-dB output power range at 3.3 GHz. When driven by a 40-MHz orthogonal frequency-division multiplexing (OFDM) signal with 8-dB peak-to-average power ratio (PAPR), the proposed SLMBA achieves adjacent channel leakage ratio (ACLR) better than −25 dBc with an average efficiency of 63.2% without digital predistortion (DPD). When excited by a ten-carrier 200-MHz OFDM signal with 10-dB PAPR, the average efficiency can reach 48.2% and −43.9-dBc ACLR can be obtained after DPD.

61 citations

Journal ArticleDOI
TL;DR: In this article, a reconfigurable quadrature hybrid coupler and a tunable low-loss reflective-type phase shifter built using the Cavendish Kinetics RF microelectromechanical systems switched capacitor devices are presented.
Abstract: This paper presents the design, fabrication, and measurement of a reconfigurable quadrature hybrid coupler and a tunable low-loss reflective-type phase shifter built using the Cavendish Kinetics RF microelectromechanical systems switched capacitor devices. The quadrature coupler tunes from 1.5 to 2.2 GHz with 20–50-MHz steps, and $> {\hbox {20-dB}}$ reflection and isolation. The phase shifter covers 360 $^{\circ } $ at 2.0 GHz with an insertion loss of ${\hbox {2.0}} \pm {\hbox {0.5}}~{\hbox {dB}}$ at different phase states. The linearity measurements show that these devices can handle up to 5 W of input power at 1.8–2.2 GHz with an input third-order intercept point $( {\rm IIP}_{3}) > 44~{{\hbox {dBm}}}$ over all phase states. The measured second and third harmonic components are $ at 30 dBm of RF input power. The application areas are in-phase shifters for micro and pico base-stations in the 1.7–2.3-GHz range.

57 citations

Journal ArticleDOI
TL;DR: By introducing a Doherty-type output combiner into the conventional sequential power amplifier (PA), highly efficient amplification over broadband and wide dynamic range for the reconfigurable output power back-off can be achieved.
Abstract: This paper presents the concept and the detailed analysis of a sequential power amplifier with Doherty-type active load modulation (SPA-D). By introducing a Doherty-type output combiner into the conventional sequential power amplifier (PA), highly efficient amplification over broadband and wide dynamic range for the reconfigurable output power back-off can be achieved. The proposed amplifier mimics both the sequential and the Doherty PAs by exploiting the active load modulation region that has not been considered in the design of Doherty amplifiers. A broadband SPA-D prototype having 8-dB output back-off power (OBO) is designed and implemented. The experimental results exhibit drain efficiencies of 50%–75% at 7–10-dB OBO and 57%–70% at peak power with Doherty-like behavior over 700-MHz bandwidth centered at 2.35 GHz. The ability to easily reconfigure the breakpoint is experimentally verified, providing a convenient means for optimizing the amplifier efficiency according to various wireless standards. In addition, by using a memoryless digital predistortion, the spectrum emission mask required for long-term evolution down link can be fulfilled, making it well suited for applications in modern wireless transmitters.

52 citations

Journal ArticleDOI
TL;DR: In this paper, a highly reconfigurable, low power, and compact directional coupler is presented, which uses varactors and novel active inductors to achieve wide tuning ranges of operating frequencies and coupling coefficients.
Abstract: This paper presents a highly reconfigurable, low-power, and compact directional coupler. The coupler uses varactors and novel active inductors to achieve wide tuning ranges of operating frequencies and coupling coefficients. The use of a low-pass circuit architecture with only two inductors minimizes chip area, power consumption, and noise. The coupler is implemented in a 0.13-μm CMOS process. It occupies an area of 350 μm× 340 μm and consumes 40 mW or less power. The obtained 1-dB compression point is -3.2 dBm, and the measured noise figure is ~ 23 dB. These parameters compare favorably with previously published reconfigurable couplers. The measured coupling coefficient can be tuned from 1.3 to 9.0 dB at 4 GHz with 32 dB or better isolation and 15 dB or better return loss. The operating center frequency can be tuned from 2.0 to 6.0 GHz for a nominal 3-dB operation. These results agree with theoretical predictions and simulations reasonably well.

51 citations