Abstract
The main emphasis in modern RF power amplifier (PA) research is on
improving
linearity while at the same time maintaining reasonably good efficiency, for
which purpose external linearization in the form of feedforward or predistortion
is often used. Linearity and linearization can be considered from both a
fundamental signal (amplitude and phase conversions, AM-AM & AM-PM) and an
intermodulation distortion (IMD) regeneration point of view, and since a study of
intermodulation gives more information on the behaviour of an amplifier,
linearity is studied in this thesis by analysing the amplitude and phase of IM
components under varying signal conditions, i.e. as functions of temperature,
modulation bandwidth and amplitude.
To study the behaviour of IM components analytically, a Volterra model including
electro-thermal distortion mechanisms is developed and a simulation technique is
introduced to determine how easily the amplifier can be linearized. An
S-parameter characterization method for extracting the Volterra model and the
simulation model is developed, and the amplitude and phase dependences of the IM
components are shown by means of measurements performed by a novel technique
developed here. The results show that the behaviour of IM components is more
complicated than had commonly been expected.
Three techniques are developed for eliminating the frequency dependence of IM
components, impedance optimization, envelope filtering and envelope injection. In
the envelope injection technique, a low frequency envelope signal is added to the
input of the amplifier in order to improve both the bandwidth and amplitude range
of the memoryless predistortion. The functionality of envelope injection is
demonstrated by Volterra calculations, simulations and measurements, and the
technique is applied to 1W, 1.8 GHz common-emitter BJT and common-source MESFET
amplifiers. IM cancellation better than 20 dB is achieved over a wide range of
bandwidths and amplitudes.
It is concluded that an inherently linear amplifier is not necessarily easy to
linearize any further using external techniques, but that the part of the
distortion that varies with bandwidth and amplitude can be cancelled out using
envelope injection and the remaining memoryless distortion by means of a simple
polynomial RF predistorter. This results in good cancellation of distortion, and
since both envelope injection and RF predistortion consume little power, both
good efficiency and linearity can be achieved.
| Identifer | oai:union.ndltd.org:oulo.fi/oai:oulu.fi:isbn951-42-6514-9 |
| Date | 05 October 2001 |
| Creators | Vuolevi, J. (Joel) |
| Publisher | University of Oulu |
| Source Sets | University of Oulu |
| Language | English |
| Detected Language | English |
| Type | info:eu-repo/semantics/doctoralThesis, info:eu-repo/semantics/publishedVersion |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess, © University of Oulu, 2001 |
| Relation | info:eu-repo/semantics/altIdentifier/pissn/0355-3213, info:eu-repo/semantics/altIdentifier/eissn/1796-2226 |
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