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

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 10 8 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.