16.1 Introduction

The term "current feedback" when applied to voltage amplifiers is a source of considerable confusion and misunderstanding. When applied to operational amplifiers, the term is commonly used to describe a method of feedback relying on an inherently low impedance inverting input [1] When applied to instrumentation amplifiers, the term has been utilised (by myself, unfortunately) to denote a differential feedback system relying upon the use of a linearised voltage to current converter as the feedback element [2]. Furthermore, the basic techniques described in reference [1] have their origins in instrumentation amplifier design ^[3, 4, 9 ^]. The term "active feedback" has also been carelessly applied, and has frequently been perpetuated in the description of any feedback system using other than passive elements in a feedback loop. However, the term "active feedback" seems to have been initially used to describe the use of a voltage to current converter as an overall feedback element [5]. I have to admit that the latter usage is a useful and (potentially) non-ambiguous description for such a type of feedback, and from here on the term "current feedback" will be applied only in the foremost context.

Current and active feedback techniques are applicable to a wide range of analog functions, including nonlinear components ( Chapter 2, 12) and digital-to-analog converters ( Chapter 13): however, for the purposes of definition and illustration, I believe it is sufficient to restrict the discussion to operational and instrumentation amplifiers. Unfortunately, the term "instrumentation...