Enhancing an OTA Linearity Through Bulk Degeneration: a Comparative Analysis

Operational Transconductance Amplifiers (OTAs) are fundamental building blocks in analog and mixed-signal microelectronic circuits. A high linear input range and minimum harmonic distortion while preserving a reduced input offset, is of particular importance in the case of low transconductance OTAs as employed in biomedical and instrumentation circuits among others. Traditionally, source-degeneration (SD) was employed to enhance the linear range of differential pairs, and more recently SD was also combined with bulk-degeneration (BD). Both techniques make use of a resistor within the differential pair, but in the case of low-transconductance OTAs a several MΩ resistor is required occupying a large die area. In this work, firstly an overview of bulk linearization techniques aimed at low-transconductance OTAs is presented, followed by the introduction of a novel OTA substituting the linearizing resistor with an also bulk-linearized MOS resistor. The new circuit allows to fine tune the OTA, and in some cases may allow to reduce the total die area. Four different OTAs fabricated in a 180nm CMOS technology are presented, including measurement results to compare their performance. Particularly, in the case of (SD+BD), a unique operational sweet-spot with a significant third harmonic (HD <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> ) distortion reduction was observed.