|dc.description.abstract||Multipath amplifier theory:
In this thesis it is proven that the theoretical minimum noise measure of a multipath amplifier (an
amplifier which has multiple parallel amplifiers) is achieved by using the optimum source impedance for
the amplifier and the optimum gain for each amplification path. This optimum source impedance and
gain can be calculated by using the optimum–loaded input network, i.e. by replacing each amplifier with
its optimum source impedance. The resulting noise measure is the same as the minimum noise measure of
the amplifiers used in the amplification paths. Whereas single–path amplifiers can achieve this minimum
noise measure over narrow bandwidths, multipath amplifiers are theoretically able to achieve the minimum
noise measure over very broad bandwidths.
The theory is demonstrated by applying it to distributed amplifiers. In an ideal distributed amplifier,
the magnitude of the optimum gain of the amplification paths decreases and the phase delay increases
the farther the stage is from the input, with the decrease in gain being faster for higher frequencies. The
challenge in designing broadband, low–noise, distributed amplifiers is to achieve optimum gain matching
over broad bandwidths.
Multipath amplifier design procedure:
Based on the theory, a three step design and optimisation procedure is introduced. Firstly, unconditionally
stable amplification paths are designed to have small minimum noise measures, then an input network
is designed for optimum source impedance matching and lastly an output network is designed for gain
Multipath amplifier prototype:
The theory and design procedure is demonstrated by optimising a 0.5–2 GHz distributed amplifier. An
average noise measure of 0.3 dB is achieved, which is only 0.1 dB higher than the minimum noise measure
of the amplification stages used. This increase is mainly due to transmission line loss and gain mismatch.
Radio telescope feasibility:
Multipath amplifiers break the trade–off between noise temperature, bandwidth and source termination
that a single–path amplifier has, because they have much more design freedom when designing the input
network. In general, the more paths, the larger the low–noise bandwidth, but the larger and more complex
the amplifier. Roughly two to three amplification paths are required per octave of bandwidth for LNAs
around 1 GHz. When the bandwidth is very narrow, a single path is sufficient.
Multipath amplifiers have similar trade–offs between linearity and power consumption, between noise
temperature and noise resistance, and between noise temperature and size to a single–path amplifier.
Multipath amplifiers are therefore a feasible alternative for use in radio telescopes.||en_US