A.A. Tammam

High CMRR current-feedback operational amplifier

Despite excellent high frequency and high speed performance, current-feedback operational amplifiers (CFOAs) generally exhibit poor common-mode rejection properties, which limits their utility. In this paper the authors analyse the conventional (CFOA) in terms of common-mode rejection ratio (CMRR) performance, and, having identified the mechanism primarily responsible for the CMRR, they present a modified CFOA input stage circuit design by introducing a combination of a bootstrapping technique and folded cascode transistors. Simulation results of this new CFOA architecture indicate that the amplifier has significantly improved both CMRR and gain accuracy. Other key characteristics are also improved, with the notable exception of slew rate, which is reduced as a consequence of the new topology.

Novel high performance current-feedback op-amp

An improved current-feedback op-amp (CFOA) with good DC and CMRR performance is presented. Early-effects in the input stage of the conventional CFOA limit CMRR, PSRR and DC performance. The design presented here uses a bootstrapping technique with quasi-Darlingtons in the input stage to reduce the influence of the Early-effect resulting in improved performance. Another advantage of this design is that the inverting input impedance is reduced significantly which results in further improvements in the CMRR, the bandwidth and the input referred offset.

A novel current-feedback op-amp exploiting bootstrapping techniques

The operation of the conventional current feedback operational amplifier (CFOA) is reviewed and its performance parameters used as benchmarks in the development of a new input stage architecture that provides a common-mode rejection ratio (CMRR) improvement of some 45 dB and offset voltage less than 10 mV.

A hierarchy of input stages for current feedback operational amplifiers

This paper considers the trade-offs involved in the design of six new input stages intended to improve the performance of a current feedback operational amplifier (CFOA), over that possible using an established input circuit configuration, with respect to three major characteristics, viz, common mode rejection ratio (CMRR), offset voltage and skew rate.

High performance current-feedback op-amps

This paper presents the designs of the two new CFOAs (current feedback op-amps), one employing a bootstrapping technique, the other a cascoding technique, that provide both high CMRR (common-mode rejection ratio) and slew rate. Moreover, the new CFOA designs exhibit a low DC offset voltage, high bandwidth, and improved gain accuracy, enabling them to be used in applications requiring variable closed-loop gains with constant bandwidth, such as in automatic-gain-control, video, graphics and multimedia applications.

Improved designs for current feedback op-amps

The performance of the current feedback op-amps (CFOAs) is very much determined by the input stage of CFOAs, including common-mode rejection ratio (CMRR). Two new CFOAs topologies are presented in this article: one topology uses a cascoding technique, and the second one uses a bootstrapping technique, both of which provide a much better CMRR and lower DC offset voltage than the conventional CFOAs. Moreover, the new CFOAs design exhibits an extended high frequency bandwidth, with a gain accuracy improvement. Applications requiring constant bandwidth with variable (closed loop) gain will benefit from the proposed topologies.

Theoretical study of the circuit architecture of the basic CFOA and testing techniques

ABSTRACT This paper examines the closed-loop characteristics of the basic Current-Feedback Operational Amplifier (CFOA), and in particular, the dynamic response. Additionally, it also examines the design and advantages of the CFOA regarding its ability to provide a significantly constant closed-loop bandwidth for closed-loop voltage gain. Secondly, the almost limitless slew–rate provided by the class AB input stage that makes it superior to the voltage-mode operational amplifier (VOA) counterpart. Additionally, this paper also concerns the definitions and measurements of the terminal parameters of the CFOA, regarded as a ‘black box’. It does not deal with the way that these parameters are related to the properties of the active passive and active components of a particular circuit configuration. Simulation is used in terminal parameter determination: this brings with it the facility of using test conditions that would not normally prevail in a laboratory test on silicon implementations of the CFOAs. Thus, we can apply 1mA and 1mV test signals from, respectively, infinite and zero source impedances that range in frequency from d.c to some tens of GHz. Also, we assume the existence of resistors with identical Ohmic value and very high value ideal capacitors. Where appropriate, practical test methods are referred to physical laboratory prototypes.

ANALYSIS AND DESIGN OF A HIGH PRECISION-HIGH OUTPUT IMPEDANCE TISSUE CURRENT DRIVER FOR MEDICAL APPLICATIONS

This paper describes the design and operation of a high output impedance tissue current driver circuit, for use in medical electronics, such as electrical impedance tomography (EIT). This novel architecture was designed for implementation in bipolar technology, to meet the specifications for EIT, namely operating frequency range 10 kHz–1 MHz with a target output resistance of 16 MΩ. Simulation results are presented, showing that the current source more than met the minimum specification for EIT.