12 research outputs found
The digital predistorter goes multi-dimensional: DPD for concurrent multi-band envelope tracking and outphasing power amplifiers
Over at least the last two decades, digital predistortion (DPD) has become the most common and widespread solution to cope with the power amplifier's (PA's) inherent linearity-versus-efficiency tradeoff. When compared with other linearization techniques, such as Cartesian feedback or feedforward, DPD has proven able to adapt to the always-growing demands of technology: wider bandwidths, stringent spectrum masks, and reconfigurability. The principles of predistortion linearization (in its analog or digital forms) are straightforward, and the linearization subsystem precedes the PA (a nonlinear function in a digital signal processor in the case of DPD or nonlinear device in the case of analog predistortion and counteracts the nonlinear characteristic of the PA. Some excellent overviews on DPD can be found in [1]-[4]. Let us now look at the challenges that DPD linearization has faced and will continue to face in the near future with 5G new radio (5G-NR).This work has been supported in part by the Spanish Government and FEDER under MICINN projects TEC2017-83343-C4-1-R and TEC2017-83343-C4-2-R and by the Generalitat de Catalunya under Grant 2017 SGR 813
High Efficiency Microwave Amplifiers and SiC Varactors Optimized for Dynamic Load Modulation
The increasing use of mobile networks as the main source of internet connectivity is creating challenges in the infrastructure. Customer demand is a moving target and continuous hardware developments are necessary to supply higher data rates in an environmentally sustainable and cost effective way. This thesis reviews and advances the status of realizing wideband and high efficiency power amplifiers, which will facilitate improvements in network capacity and energy efficiency.
Several demonstrator PAs are proposed, analyzed, designed, and characterized: First, resistive loading at higher harmonics in wideband power amplifier design suitable for envelope tracking (ET) is proposed. A 40 dBm decade bandwidth 0.4â4.1 GHz PA is designed, with 10â15 dB gain and 40â62% drain efficiency. Its versatility is demonstrated by digital pre-distortion (DPD) linearized measurements resulting in adjacent channel leakage ratios (ACLR) lower than â46 dBc for various downlink signals (WCDMA, LTE, WiMAX).
Second, a theory for class-J microwave frequency dynamic load modulation (DLM) PAs is derived. This connects transistor technology and load network requirements to enable power-scalable and bandwidth conscious designs. A 38 dBm PA is designed at 2.08 GHz, maintaining efficiencies >45% over 8 dB of output power back-off (OPBO) dynamic range. From this pre-study a fully packaged 86-W peak power version at 2.14 GHz is designed. ACLR after DPD is â46 dBc at a drain efficiency of 34%.
For DLM PAs there is a need for varactors with large effective tuning range and high breakdown voltage. For this purpose, SiC Schottky diode varactors are developed with an effective tuning range of 6:1 and supporting a 3:1 tuning ratio at 36 V of RF swing. Nonlinear characterization to enable Q-factor extraction in the presence of distortion is proposed and demonstrated by multi-harmonic active source- and load-pull, offering insights to tunable network design.
Third, a method to evaluate and optimize dual-RF input PAs, while catering to higher harmonic conditions and transistor parasitics, is proposed. The method is validated by a PA design having a peak power of 44 +/- 0.9 dBm and 6 dB OPBO PAE exceeding 45% over a 1â3 GHz bandwidth.
The results in this thesis contribute with a novel device and analysis of high efficiency and wideband PAs, aiding in the design of key components for future energy efficient and high capacity wireless systems
Supply modulated GaN HEMT power amplifiers - From transistor to system
Power amplifiers (PAs) for mobile communication applications are required to fulfil stringent requirements concerning linearity while keeping a high efficiency over a wide power range and bandwidth. To
achieve this, a number of advanced PA topologies have been developed, mostly based on either load modulation, such as Doherty PAs or load modulation balanced PAs, or on supply modulation such
as envelope tracking or envelope elimination and restoration. Supply modulation has an advantage
over other topologies as the power range of high efficiency can be realised over arbitrary bandwidths,
only limited by the bandwidth of the PA. This does, however, come at the cost of a significantly
more complicated voltage supply. Instead of a static supply voltage, the PA needs to be provided
with one which is rapidly changing, requiring a supply modulator capable of powering the PA while
modulating its supply voltage. This thesis investigates a number of challenges in supply modulated
power amplifiers, ranging from the transistor itself to circuit design and system level considerations
and focusses on power levels up to 10 W and frequencies between 1 GHz and 4 GHz.
Transistors, as the centre-piece of a PA, determine how well the PA reacts to a changing supply
voltage. In this work, the traits that make GaN HEMTs suitable for supply modulated PAs were
investigated, and gain variation with changing supply voltage was established as an important parameter. This gain variation is described in detail and its impacts on PA performance are discussed. By
comparing transistors in literature, gain variation has been demonstrated to be a prevalent characteristic in transistors with GaN HEMTs showing a very wide range of gain variation. Using a small-signal
model based on measurements, the voltage dependent behaviour of the feedback capacitance CGD is,
for the first time, identified as the origin of small-signal gain variation. This is traced down to the
gate field plate which is commonly used to combat surface trapping effects in GaN HEMTs. With
this in mind, two different ways of changing the transistor geometry to reduce the impact of gain
variation and thus optimise the transistor for operation in supply modulated PAs are discussed and
demonstrated using a 250 nm GaN HEMT.
As a result of the non-linearity of the feedback and gate-source capacitances, the input impedance
of GaN HEMTs changes with supply voltage and drive power. This prevents the transistor from being
matched at all supply voltages and input powers and introduces phase distortion. Using simulation and
measurement, the impact of input impedance on linearity and efficiency of supply modulated power
amplifiers is demonstrated on a 2.9 GHz 10 W PA. Careful selection of the input impedance allows
improvement of AM/PM distortion of a supply modulated PA with little cost in terms of AM/AM
and PAE.
I
Supply modulators have a significant impact on efficiency and linearity of the ET system. One
supply modulator topology with the potential to generate a supply voltage with a high modulation
bandwidth is the RF modulator in which the input DC voltage is turned into an RF signal and
rectified, resulting in an output voltage which depends on the excitation of the PA. While PAs are
well understood in every detail, there are gaps in the understanding of RF rectifiers. Using active
load-pull/source-pull measurements, intrinsic gate and drain waveforms of a GaN HEMT operated as
a rectifier are demonstrated for the first time. This allows in-detail evaluation of the impact of the
gate termination in self-synchronous rectifiers. It also allows detailed analysis of the loss mechanisms
in rectifiers and formulation of the required impedances to realise efficient self-synchronous operation,
resulting in efficiencies exceeding 90% over wide power ranges. Using waveform engineering, a new type
of RF modulator, with potentially very high bandwidths, based on even harmonic generation/injection
is proposed. The necessary operating conditions of the rectifier part of the modulator are emulated
using an active load-pull/source-pull system to successfully demonstrate that the rectifier behaves
as predicted. Using a simple demonstrator, preliminary measurements were conducted and the RF
modulator was shown to work, reaching efficiencies up to 78%.
As PA and supply modulator are combined, they present impedances to each other. These
impedances have a significant impact on the behaviour of both sub-systems. A simple way to characterise both the impedance presented to the PA by the modulator and the impedance presented to the
modulator by the PA is described. Using a state-of-the-art modulator, these impedances are measured,
the modulator impedance is demonstrated to be close to the simulated value. These measurements
also demonstrate that the impedances change significantly with the operating conditions