The aim of this research programme was to design and develop a novel bipolar
junction transistor Current Feedback Operational Amplifier (CFOA) with a good
Common-Mode Rejection Ratio (CMRR), suitable for radio frequency (RF)
applications. This research focused on investigation of the established CFOA with
the emphases of improving CMRR, bandwidth, Voltage-Offset and Slew-rate
performance. The majority of the results of this work have been reported by the
author in references [11 to [6].
Initially a thorough analysis of the conventional CFOA was undertaken to provide an
in depth understanding of the amplifier's operation, and this work revealed that the
main shortcomings of the CFOA are in the design of the input stage. This initial
study focussed on establishing reasons for the poor DC offset-voltage performance
and CMRR and confirmed that these designs have inherently poor performance in
these two elements. The analysis was carried out using both theoretical modelling
and computer simulation.
Using this analysis of the conventional CFOA as a benchmark, various novel circuit
techniques were investigated. Several new input circuits for the CFOA were
proposed with respect to improving the three previously mentioned key
characteristics, viz., CMRR, offset voltage, and slew-rate. The first technique
explored is based on floating the entire input stage of the CFOA which yielded
significant improvements in CMRR, Offset-Voltage and bandwidth, and the results of
this workwere published in [11, [2], and P). Based on these initial findings a second
major development was undertaken. This time a bootstrapping technique was
employed to key sections of the input stage, leading to new, simplified input circuit
topology. This development leads to low DC offset voltage, wide bandwidth and high
CNIRR, as well as improved gain accuracy, and was published by the author in [4,5].
A logical approach to the different input stage architectures examined by the author
resulted in identification of a hierarchy of 6 different input CFOA circuit designs and
a comparative study was undertaken showing their relative performance in respect of
CMRR, Offset-Voltage and Slew-rate. This work was presented by the author, [6]