Transconductance enhancement in bulk-driven input stages and its applications

Two different circuit techniques to enhance the effective transconductance of a CMOS bulk-driven differential input stage are presented in this paper. Both approaches rely on a partial positive feedback, which leads to improved values for the DC gain and the gain-bandwidth product. The operation principle of the first solution is based on modifying the effective conductance of the active load of the input stage, while the second method acts directly on the input differential pair. The suitability of the presented techniques is demonstrated by the design of operational transconductance amplifiers operating at two different supply voltages, i.e., 2.4 and 1.0 V. Besides, the overall design of two applications, namely a 3 V input/output rail-to-rail operational amplifier with high linearity and a 1.2 V second-order OTA-C low-pass filter, is addressed. Simulated results obtained in standard 0.35 μm CMOS technology demonstrate the applicability of the solutions introduced.