Comparison between the noise performance of current-mode and voltage-mode amplifiers

Abstract

Abstract.A comparison is made between the noise performance of current mode-and voltage mode feedback amplifiers, in terms of their noise figure Fi and Fv, respectively. Whereas F~ of a well designed voltage amplifier is dominantly affected by the input stage only, all stages of a current amplifier including the feedback network may potentially contribute to F;. This paper investigates the transferfunction to the output of the various noise sources of current mode feedback amplifiers, whose full recognition is essential in order to design CMFA's of reasonable noise performance. Also discussed is the effect the dynamic range of a postamplifier may have on its noise figure. Although the bulk of this paper deals with postamplifiers, the important subject of low noise current amplification is covered as well.

Negative feedback revisited

“…It follows, that the output of a well designed current follower equals with high accuracy its input which can be feedback stabilized. Furthermore, feedback increases its dynamic output impedance by the factor LT due to the series feedback applied and reduces significantly its noise contribution [9]. The design of the output stage for a YT feedback amplifier by the same kind of BB is straightforward.…”

“…This point cannot be over emphasized since it excludes the multitude of amplifiers employing current sensing current mirrors as output stage, some of which are erroneously claimed to represent possible realizations of nullors. Non-inclusion of the output stage of any kind of basic amplifier in the feedback loop foregoes its stabilization, the increase in its output immittance and the reduction of its noise contribution by feedback [9].…”

“…True CMFA's and Yr amplifiers require a current follower output stage whose noise contribution is reduced by LT [9], whose accuracy does not depend upon transistor matching and whose dynamic resistance is included in the feedback path of the output current loop, thereby causing the feedback to increase the open loop output impedance of the amplifier by a factor (1 -L T). These requirements exclude translinear circuits such as CCII realizations employing current mirrors which are inherently noisy BB's, whose current follower accuracy does depend upon transistor matching and whose output resistance cannot be included within the amplifier's feedback loop.…”