CMOS Active Inductor Linearity Improvement Using Feed-Forward Current Source Technique

MOSFET drain current second-order nonlinearity has a significant impact on the linearity of current regulated CMOS active inductors. It tends to compress MOSFET transconductance (g m) by generating excess dc current (I EX) in the channel, which is a function of incoming input signal amplitude. This generated excess dc current can change the original dc operating point of the current regulated CMOS active inductor, and thus, influence the inductance. Unfortunately, MOSFET drain current second-order nonlinearity contributes more to MOSFET g m compression than MOSFET drain current third-order nonlinearity. In this paper, a new technique known as feed-forward current source (FFCS) has been proposed to improve the linearity of the active inductor. The proposed FFCS technique makes use of the second-order nonlinear property of a MOSFET that generates I EX when an input ac signal is applied. The generated I EX is then fed-forward to the current source of the active inductor to drain out the I EX in the active inductor. This prevents the dc operating point from shifting and improves its inductance linearity. Single-ended and differential active inductors with the proposed FFCS circuit have been fabricated using Silterra's CMOS 0.18-mum technology to verify the proposed technique.