F. J. Lidgey

Common-mode rejection ratio in current-mode instrumentation amplifiers

The current-mode instrumentation amplifier (CMIA) based on op-amp power-supply current sensing offers distinct advantages over conventional architecture designs. The CMIA is studied in detail and the CMRR expression is derived in terms of op-amp and transistor parameters. A good CMRR is shown to depend upon several factors, including the power-supply rejection ratio of the op-amps used in the CMIA. The results obtained are shown to compare well with SPICE simulation and provide valuable design insight for development of the next generation of CMIAs.

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.

Degradation mechanisms in operational amplifier precision rectifiers

Two principal degradation mechanisms that are generally present in op-amp precision rectifiers have been studied in detail. The process of rectification increases the spectral content, and subsequent band-limited signal processing causes a loss in depth at the zero-crossings. The second source of degradation is caused by diode feedback nonlinearity effects that cause the op-amp to become open loop, and this results in a missing segment in the final output. Total harmonic distortion is an inadequate figure of merit to quantify these two effects, and so an alternative is presented. >

High frequency performance of current-mode precision full wave rectifiers

The current-mode topology reported by Hayatleh et al. [1993] provides an improved high frequency performance over the traditional design, but the circuit response still begins to degrade significantly at-about GB/100, where GB is the gain-bandwidth of the op-amps being used. Detailed investigation of the principal degradation mechanisms has been undertaken and from the results of this study two main causes for the non-ideal performance have been identified. Even though the work reported here is specifically related to the PFWR, these two-mechanisms responsible for degrading the performance at high frequencies also apply in part to all PFWR and a wide number of circuits that employ opamp/diode feedback structures.<<ETX>>

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.

Using the ‘T’ feedback network with the current feedback operational amplifier

Selecting a suitable feedback resistor R F and gain resistor R G is critical to ensure amplifier gain and stability when using current-feedback operational amplifiers (CFOAs). The bandwidth is determined primarily by R F and the gain then set using R G , which also equals the input resistance for the inverting configuration. This presents the applications engineer with very little choice in terms of component selection. In this paper, the feedback resistor R F is replaced by a modified ‘T’ network, resulting in greater design flexibility and improved performance in terms of gain accuracy and bandwidth. The inverting and non-inverting configurations are analysed and the design equations developed for the CFOA with and without the ‘T’ network, which clearly show the performance improvements. These theoretical results are confirmed with SPICE simulations using a commercially available macromodel of an industry standard CFOA.

Frequency Performance Compensation of Operational Amplifiers with the ‘T’ Feedback Network

A simple ‘T’ network can be used in the feedback path of a conventional voltage feedback operational amplifier (VFOA) to avoid using a large feedback resistor. However detailed analysis shows that the bandwidth of amplifier will be degraded. In this paper, a capacitive compensation method for the ‘T’ network is described which yields a significant improvement in closed-loop bandwidth of the amplifier.

Tuneable linear transconductance cell for Gm-C filter applications

In this paper a novel transconductance Bi-CMOS stage is described. This design is based on a combined translinear current gain cell and a negative resistance cell, to generate a linear tuneable transconductor. The transconductance stage was designed specifically for an integrated Gm-C filter RF application, and is shown to perform well particularly at high frequencies.

Improved current-feedback op-amp with good DC and CMRR performance

A novel high DC accuracy and high CMRR performance current-feedback op amp (CFOA) is presented. The conventional architecture CFOA suffers from poor DC gain accuracy and CMRR due mainly to Early effects in the input stages. The new design is based on a bootstrapping technique applied to the input stage, and results in a CFOA with DC performance, CMRR and gain accuracy all substantially improved. All of these improvements are obtained without increase in circuit complexity and power consumption. Most of the other characteristics of the new CFOA are similar to those of a conventional design.