Abstract.
For
modeling of microwave transistor amplifiers we propose to use the
electrodynamic method, where the transistor is represented by four-port
scheme. Design of Microwave Transistor Amplifiers (MTA) in last
decades was based on matrix calculations and methods of the theory of circuits
which was based on the theory of Kurokawa. According to this theory the
stability coefficient and optimum reflection coefficients are defined through
scattering matrix of the transistor. The
paper contains study
of MTA using the Method
of Moments. In the calculation in FEKO transistor is considered with 4 ports. A
transistor created in Russia was used in MTA. The model of the transistor,
given in table of S-parameters, is included in the structure of the microwave
amplifier. This model, implemented as a four port structure, is called
Non-radiating Networks and
it is connected to the ports specified in the all structure. We considered two
approaches to the modeling of microwave structures and have chosen the use of
FEKO, in which the structure is divided into the radiating and Non-radiating Networks (transistor).
Radiating structure with arbitrary shape retains all of its electrodynamic features.
In this case, the relationship between the input and output matching structure
is taken into account automatically, and it is possible to calculate the
characteristics of the near and far fields of the structure that includes
active elements. By using the four-port representation of a transistor, it is
possible to perform the feedback in MTA as part of complex matching structure.
Modeling and optimization of the microwave devices show that the inclusion of a
microwave transistor by the schemes of Common Emitter, Common Collector and
Common Base in microwave amplifier makes quasi-equivalence. Calculation
study shows that the effect of the electrodynamic connection between the input
and output matching structure is manifested not only by gain reduction but made
Gain=0 on 32 GHz due to the resonance properties of the electrodynamic
structure. Electrodynamic approach allows us to perform and optimize MTA with
arbitrary matching structures.
Each of the elements, including
shapes superimposed on each other, as well as with an arbitrary configuration,
can be parameterized, which leads to a matching structure of a complex, unpredictable
shape.
Modeling MTA with the help of electrodynamic
method allows us to consider MTA as the active antenna. We show how a current
in input, output and feed-back matching structure contributes to the far-field.
Directivity, efficiency and loss characteristics are also affecting gain and stability
of MTA. Comparison of calculated frequency response shows that using feedback makes
possible to receive the same
features
as in the case of tuning input and output matching circuits. It is seen that in
the region of 25 – 35 GHz gain of MTA has the same values. Synthesis of MTA by
using feedback or by using input and output matching circuit leads to the same
result. It can be shown that the maximum gain is achieved by any method and
takes the same value at the selected frequency. It should be noted that at low
frequencies it is considered that the scheme with Common Collector is the same
pattern with Common Emitter, but with 100% feedback. So we believe that
optimization of MTA on very high frequencies with Common Collector, Common
Emitter and Common Base leads to the same result. It should also be noted that
the polarization begins to play an important role in the construction of the
microwave amplifier, which affects the gain and stability.
With the introduction of modern
microwave CAD in design process, the theory of Kurokawa is transferred to the
new electrodynamic level. Electrodynamic design of microwave structures, in
which active elements are included, opens up new possibilities both for
improving the performance of traditional structures and for creating such
unique structures as waveguide multi-transistor microwave amplifiers.
(Kurushin A., Podkovyrin S. Synthesis of Microwave Structure. IEEE
Microwave Conference, Taipei, 3-6 Dec. 2001, DOI: 10.1109/APMC.2001.985650).
Key Words:
microwave transistor amplifier; stability; matching; electrodynamic
calculation; FEKO.
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