A linear operational transconductance amplifier with automatic offset cancellation and transconductance calibration

Operational Transconductance Amplifiers (OTA's) with linear input-output characteristics have become the focus of attention as a fundamental building block for analogue time-continuous circuits. Especially they are used in filters and instrumentation amplifiers with high cutoff- or corner frequencies. This paper describes a simple CMOS circuit technique for realizing a precision linear amplifier using the square-law function of MOS transistors. The linear input-output characteristic is not influenced by the MOS threshold voltage dependence on the source-bulk voltage or other effects. To eliminate the remaining imperfections the new proposed technique combines analogue time-continuous and time-discrete circuitry. By the use of two adjustment phases, the offset can be suppressed effectively and the transconductance can be put back or newly defined in a given range. In a third phase the OTA works along with further time-continuous circuits. The proposed circuitry can be implemented with a double or a single-ended output stage. Due to symmetrical internal signal paths, the best results in terms of linearity, PSRR and CMRR are obtained in combination with a double-ended output stage.