Extending voltage-mode op amps to current-mode performance

This paper reviews techniques for extending the output cababilities of conventional voltage-mode operational amplifiers to achieve current-mode performance. Tracing the development over the past two decades, a number of elegant and useful circuit topologies for realising amplifiers with controlled current output properties are described. In particular, the operational amplifier supply current sensing technique and its application to current follower and current conveyor circuit implementations is examined. The authors then review their work, based on enhancing the performance of conventional voltage-mode amplifier designs by application of these currentmode alternatives.A high performance follower based amplifier, a biquadratic filter section, a precision full-wave rectifier and an instrumentation amplifier based upon enhancement to the voltage operational amplifier are shown to outperform their conventional counterparts. The integrated circuit realisation of a seven terminal operational amplifier that allows direct access to the operational amplifier output current via its push-pull output stage is then presented. The integrated seven terminal operational amplifier is realised in CMOS technology and experimental results presented. Finally a current feedback, high precision, current gain cell is described that overcomes several of the drawbacks that exist with the supply current sensing technique.

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