8 research outputs found
A design strategy for AM/PM compensation in GaN Doherty power amplifiers
This paper presents the theoretical analysis of phase distortion (AM/PM) mechanisms in Gallium Nitride (GaN) Doherty power amplifiers (DPAs) and a novel approach to optimize the tradeoff between linearity and efficiency. In particular, it is demonstrated how it is possible to mitigate the AM/PM by designing a suitable mismatch at the input of the active devices, based on the identification of constant AM/PM and gain contour circles. The proposed theory is experimentally confirmed by source- and load-pull measurements and further validated through the design and realization of a 7 GHz 10 W DPA based on GaN monolithic technology
3.1-3.6 GHz 22 W GaN Doherty Power Amplifier
This paper presents a Doherty power amplifier working from 3.1 GHz to 3.6 GHz. It adopts 10 W packaged GaN HEMTs from Cree/Wolfspeed and achieves a saturated output power in excess of 43.4 dBm. Saturated efficiency ranges from 57.7 % to 75.2 %, while efficiency at 6 dB back-off is between 44.2 % and 59.8 %. System-level simulations at 3.5 GHz adopting a 16QAM signal with 5 MHz bandwidth and 4 dB peak to average power ratio showed an adjacent channel power ratio of -28 dBc/Hz without pre-distortion, at an average output power of 43 dBm and with an average efficiency of 71 %
High efficiency power amplifiers for modern mobile communications: The load-modulation approach
Modern mobile communication signals require power amplifiers able to maintain very high efficiency in a wide range of output power levels, which is a major issue for classical power amplifier architectures. Following the load-modulation approach, efficiency enhancement is achieved by dynamically changing the amplifier load impedance as a function of the input power. In this paper, a review of the widely-adopted Doherty power amplifier and of the other load-modulation efficiency enhancement techniques is presented. The main theoretical aspects behind each method are introduced, and the most relevant practical implementations available in recent literature are reported and discussed
Advanced High Efficiency Architectures for Next Generation Wireless Communications
L'abstract è presente nell'allegato / the abstract is in the attachmen
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