Title:
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Design and development of low distortion bipolar voltage-to-current converters for RF applications
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The research work described in this thesis is concerned with the analysis and design of
low distortion voltage-to-current (V-I) converter bipolar junction transistor circuits. In
this thesis, various voltage-to-current converter circuits published in the past have been
reviewed by the author in order to understand the different techniques employed to
improve the linear operating range, total harmonic distortion and transconductance.
Throughout this research, the emphasis has been to improve the above mentioned
parameters. All the V-I converter circuits reported have been simulated using PSpice and
the results compared with the values obtained by theoretical analysis. The majority of the
results of this work have been reported by the author; see Chapter 10 at the end of this
thesis where all 5 publications by the author can be found in full.
It was necessary to obtain precise values for certain parameters, in particular, transition
frequency (fT), Early voltage (VA) and current gain (P) of the transistor to facilitate the
design process. This was done using an extensive set of simulations for the transistor
operating at different collector current levels. A commonly encountered requirement of
the V-I circuits is an accurate non-integer ratio of current biasing. Several published such
biasing schemes were studied and three new designs were conceived and evaluated.
In the next part of the work several V-I converter circuits were reviewed to understand
the various existing techniques and their limitations. These can be conveniently
classified into three main classes of technique, namely (i) boosted-gm (ii) cross-coupled,
and (iii) multi-tanh. Of these, the boosted-gm technique showed the most promise for
further exploitation and development.
New circuits were then developed using the boosted-gm technique with different types of
feedback, classified as collector-base feedback, collector-emitter feedback and global
feedback. In terms of the preferred circuit performance, these were assessed by an
arbitrary but convenient, figure-of-merit (FOM) which is defined as (THDRxBW)/PD
where THDR, in dB is the total harmonic distortion .reduction, BW is the 3dB bandwidth and PD is the power dissipation. On this basis, the best value of FOM (2421.24
dBMHz/mW) was achieved with the circuit called Type 2B in this thesis, which is based
on collector-emitter feedback.
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