THE DESIGN OF FULLY DIFFERENTIAL CMOS OPERATIONAL-AMPLIFIERS WITHOUT EXTRA COMMON-MODE FEEDBACK-CIRCUITS

Abstract

A new fully differential CMOS operational amplifier (op amp) without extra common-mode feedback (CMFB) circuit is proposed and analyzed. In this op amp, simple inversely connected current-mirror pairs are used as active loads. From the theoretical analysis, it is shown that the common-mode signal can be efficiently suppressed by the reduced effective common-mode resistance of the active load. The proposed op amp with 2 pF capacitance loadings has an open-loop unity-gain bandwidth of 63 MHz, a phase margin 47-degrees, and a dc gain of 67 dB in 3.5 mum p-well CMOS technology. The common-mode gain at a single output node can be as low as -38 dB without extra CMFB circuit. Experimental results have successfully confirmed the capability of the efficient common-mode rejection.