A Wide Linear Output Range Biopotential Amplifier for Physiological Measurement Frontend

This paper presents a wide linear output range biopotential amplifier for physiological measurement frontend. The proposed amplifier consists of an open-loop rail-to-rail differential pair in the first stage and a closed-loop common source amplifier in the second stage, which extends the linear output range by maintaining a constant voltage gain with amplifier output level variation. The nMOS and pMOS gain enhancement switches provide additional compensation for the voltage gain degradation as the amplifier output level moves toward the supply rails. In addition, voltage buffers are used after the first and second stages to alleviate the loading effect. This enables using small-sized feedback resistors, where the amplifier performance is relatively insensitive to resistor mismatch. Aside from the wide linear output range, this approach leads to small silicon area, low power, and relatively high common mode rejection ratio and power supply rejection ratio, which are highly demanding for portable and implantable biomedical instrumentation frontend. The proposed amplifier is implemented using CMOS 0.35μm technology (3.3 V supply) with core area of 0.063 mm2. Measurement results show a stable voltage gain of 46.3 dB for amplifier output range from 0.15 to 3.12 V, and a 0.04% total harmonic distortion with input amplitude of 15 mVpp. Furthermore, the amplifier operation is demonstrated with an actual ECG measurement setup.