|dc.description.abstract||The process of designing high power RF amplifiers has in the past relied heavily on
measurements, in conjunction with simple linear theory. With the advent of the
harmonic balance method and increasingly faster computers, CAD techniques can be
of great value in designing these nonlinear circuits.
Relatively little work has been done in modelling RF power MOSFETs. The methods
described in numerous papers for the nonlinear modelling of microwave GaAsFETs
cannot be applied easily to these high power devices. This thesis describes a
modelling procedure applicable to RF MOSFETs rated at over 100 W. This is
achieved by the use of cold S parameters and pulsed drain current measurements
taken at controlled temperatures. A method of determining the required device
thermal impedance is given.
A complete nonlinear equivalent circuit model is extracted for an MRF136
MOSFET, a 28 V, 15 W device. This includes two nonlinear capacitors. An
equation is developed to describe accurately the drain current as a function of the
internal gate and drain voltages. The model parameters are found by computer
optimisation with measured data. Techniques for modelling the passive components
in RF power amplifiers are given. These include resistors, inductors, capacitors, and
ferrite transformers. Although linear ferrite transformer models are used, nonlinear
forms are also investigated.
The accuracy of the MOSFET model is verified by comparison to large signal
measurements in a 50 0 system. A complete power amplifier using the MRF136,
operating from 118 MHz to 175 MHz is built and analysed. The accuracy of
predictions is generally within 10 % for output power and DC supply current, and
around 30 % for input impedance. An amplifier is designed using the CAD package,
and then built, requiring only a small final adjustment of the input matching circuit.
The computer based methods described lead quickly to a near-optimal design and
reduce the need for extensive high power measurements. The use of nonlinear
analysis programs is thus established as a valuable design tool for engineers working
with RF power amplifiers.||en