Current Feedback Operational Amplifiers, CFOA, and Applications

Abstract

The conventional operational amplifiers were successfully used over the years for the design of analog signal processing circuits. The maximum operation frequency of operational amplifier-based circuits is determined primarily by the limited gain-bandwidth product and by the slew rate of the operational amplifier. Since their introduction in 1985, the popularity of current feedback amplifiers has increased considerably as they were found to be able to overcome the limitations arising from conventional operational amplifiers. A current feedback amplifier is equivalent to a plus-type second-generation current conveyor with a voltage buffer. The term current feedback is used because the error signal entering at the feedback node of the op-amp is in the form of a current and this gives to the amplifier a constant closed-loop bandwidth capability. Ideally, the bandwidth of the current feedback op-amp is independent of the closed-loop gain. Therefore, the closed-loop gain-bandwidth product increases linearly with the closed-loop gain. This is a major advantage over the voltage-feedback op-amp architecture, which exhibits a constant GBW. Since the current feedback operational amplifier (CFOA) has a larger bandwidth and a higher slew rate than the conventional operational amplifier, analog signal processing circuits built around the CFOA are expected to operate at higher frequencies than the op-amp-based circuits. The current feedback amplifiers are gaining popularity as alternative building blocks for analog signal processing.In the following part, after giving the definition of the active element, using a systematic circuit generation method (Cicekoglu and Kuntman, Circuit design techniques: AD-HOC methods or systematic generation methods. In: Proceedings of the tenth international conference on microelectronics (Cat. No. 98EX186). IEEE, pp. 187–190, 1998), new single CFOA-based oscillator circuits are derived. In the frame of this study, we propose two topologies with six nodes (five nodes plus ground), starting from six nodes plus ground configuration which is more general than the previously reported general configurations. From these topologies five SRCOs are derived. Furthermore, the frequency stability of these oscillators is discussed. Moreover, an overview to the single-CFOA-based oscillators is also given.KeywordsAnalog circuitsCurrent feedback operational amplifiersCFOAOscillators