Very-high-output-impedance cascode current sources/current mirrors/transresistance stages and their applications

Abstract

A reconfigurable MOS/BiMOS cascode current source/transresistance stage with very high output impedance is presented in this paper. A cascode active feedback circuit has been employed to achieve such a high output impedance, which approaches the leakage resistance of a reverse-biased junction, assuming that the MOS devices have a large channel length and do not exhibit the short-channel effect. Through small-signal analysis and SPICE simulation it is shown that the proposed current source/transresistance stage provides an output open-circuit Thevinin voltage greater than 108 V. This is an increase of about four orders of magnitude as compared with the conventional cascode current source with the same output voltage range. For scaled technologies this current source/transresistance stage is shown to be ideal for practical high-performance analogue circuit design. We have incorporated the current sources with active feedback in the output stage of a folded cascode op amp and obtained a large DC gain improvement without significantly affecting its high-frequency/transient behaviour.