This study investigates the closed-loop performance of the basic current feedback operational amplifier (CFOA), with particular emphasis on its dynamic response. It also focuses on the design, performance and advantages of the CFOA in its ability to provide a substantially constant closed-loop bandwidth for closed-loop voltage gain. Furthermore, an improved CFOA with wide bandwidth and common-mode-rejection ratio (CMRR) performance is also presented. The design presented in this article uses a bootstrapping technique with Quasi-Darlington in the input stage to reduce the influence of the Early effect which results in improved performance. Another advantage of this design is that the inverting input impedance is reduced significantly, which leads to further improvement in bandwidth and CMRR.

An exemplary design demonstrates how to extend the common-mode rejection ratio (CMRR) bandwidth of a CMOS differential amplifier. The design presented uses MOSFETs with a channel length of 180nm. A novel circuit technique is employed that partially compensates for the output capacitance of the tail current sink, thereby more than quadrupling the CMRR bandwidth in the example considered.

In this paper the authors analyse the conventional current-feedback operational amplifier (CFOA) in terms of commonmode-rejection ratio (CMRR) performance, and having identified the mechanism primarily responsible for the CMRR, they propose two new architecture CFOAs. These new CFOAs are further developed, and modified to provide improved bandwidth, AC gain accuracy and high CMRR performance. The key features of the two proposed new CFOAs are the designs of the internal voltage followers which have two separate biasing currents with a similar dynamic architecture to that of the conventional CFOA. The magnitude of one bias current determines the value of the maximum CMRR, and the second can be used to maximise bandwidth.

Despite excellent high frequency and high speed performance, current-feedback operational amplifiers (CFOAs) generally exhibit poor common-mode rejection (CMRR) properties, which limit their utility [Analogue IC design: The current–mode approach, IEE Circuits and Systems Series, Peter peregrinus, 1990]. A novel current feedback operational amplifier (CFOA) with improved performance is presented. The proposed CFOA has a new current-cell [Novel current-feedback operational amplifier Design Based on a floating circuit technique, IEE Colloquium on Analogue Signal Processing, 1998], to bias the entire circuit, which achieves an incremental output resistance twice that of the well-known "Wilson" circuit. Simulation results of this new CFOA architecture indicate that the amplifier exhibits performance characteristics superior to those obtained with an established input architecture: in particular, the CMRR (common-mode rejection ratio) is 91 dB, and the d.c. offset voltage less than 26 μV.

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