Open Access Te Herenga Waka-Victoria University of Wellington
thesis_access.pdf (17.17 MB)

High Impedance Amplifiers for Non-Contact Bio-Potential Sensing

Download (17.17 MB)
posted on 2021-11-15, 01:09 authored by Ryan, Brett

This research develops a non-contact bio-potential sensor which can quickly respond to input transient events, is insensitive to mechanical disturbances, and operates with a bandwidth from 0.04Hz – 20kHz, with input voltage noise spectral density of 200nV / √Hz at 1kHz.  Initial investigations focused on the development of an active biasing scheme to control the sensors input impedance in response to input transient events. This scheme was found to significantly reduce the settling time of the sensor; however the input impedance was degraded, and the device was sensitive to distance fluctuations. Further research was undertaken, and a circuit developed to preserve fast settling times, whilst decreasing the sensitivity to distance fluctuations.  A novel amplifier biasing network was developed using a pair of junction field effect transistors (JFETs), which actively compensates for DC and low frequency interference, whilst maintaining high impedance at signal frequencies. This biasing network significantly reduces the settling time, allowing bio-potentials to be measured quickly after sensor application, and speeding up recovery when the sensor is in saturation.  Further work focused on reducing the sensitivity to mechanical disturbances even further. A positive feedback path with low phase error was introduced to reduce the effective input capacitance of the sensor. Tuning of the positive feedback loop gain was achieved with coarse and fine control potentiometers, allowing very precise gains to be achieved. The sensor was found to be insensitive to distance fluctuations of up to 0.5mm at 1Hz, and up to 2mm at 5kHz.  As a complement to the non-contact sensor, an amplifier to measure differential bio-potentials was developed. This differential amplifier achieved a CMRR of greater than 100dB up to 10kHz. Precise fixed gains of 20±0:02dB, 40±0:01dB, 60±0:03dB, and 80±0:3dB were achieved, with input voltage noise density of 15nV / √Hz at 1kHz.


Copyright Date


Date of Award



Te Herenga Waka—Victoria University of Wellington

Rights License

Author Retains Copyright

Degree Discipline

Electronic and Computer System Engineering

Degree Grantor

Te Herenga Waka—Victoria University of Wellington

Degree Level


Degree Name

Master of Engineering

ANZSRC Type Of Activity code

970109 Expanding Knowledge in Engineering

Victoria University of Wellington Item Type

Awarded Research Masters Thesis



Victoria University of Wellington School

School of Engineering and Computer Science


Teal, Paul