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Title: Design of low power electronic circuits for bio-medical applications
Author: Hasan, Saad Ahmed
ISNI:       0000 0004 2737 7238
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2011
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The operational transconductance amplifier, OTA is one of the basic building blocks in many analogue circuit applications. The low power consumption is an essential parameter in modem electronic designs for many areas particularly for portable devices and biomedical applications. For biomedical applications, the low- power low-voltage OTA-C filters operating at low-frequency ranges are desired. The low-power, low-voltage operation of electronic devices is very important for applications such as hearing aids, pacemakers, and EEG. The importance of such operation is due to the need to implant these electronic circuits inside the body of the patient for long times before re-charging or replacing the batteries as for pacemakers and future hearing aids. The small size lightweight wearable EEG systems are preferable for applications ranging from epilepsy diagnosis to brain-computer interfaces. The low power consumption is achieved by operation at very small levels of current. So, in such applications the operation in the nano-ampere current range is essential to ensure power consumption of nW or few uW. Such very small currents are obtained through the operation of MOS transistors in their sub-threshold regime. The design space in such applications is restricted by their specifications which in turn based on the nature of the application. In this work, the design and implementation of OTA-C filter topologies for two bio-medical applications are made and discussed. Those applications are represented by hearing aids and EEG applications. In hearing aids, the work focused on cochlear implant and specifically on its most important stage represented by the filter. Four OTA-C filter topologies are proposed and two of them are tested experimentally. For the filter in a hearing aid system, besides its low power operation, it is required to operate with a relatively high dynamic range of 60dB and above. The dynamic range is the operation space of the filter that specified by the range of signals which can process properly. It is bounded by the maximum power signal less than its distortion overhead level to the minimum power signal more than its noise floor. The maximum signal level the filter can perform properly represents its input linear range. The challenge in CMOS OTA sub-threshold operation is the very small input linear range which makes it extremely difficult to build low-power consumed OTA-C filters with a wide dynamic range, DR. In this work, an OTA with an input linear range of ±900mV for total harmonic distortion, THD<5% is proposed using MOSFET bumping and capacitor attenuation techniques, combined for the first time. The minimum signal level the filter can distinguish from noise is still relatively small with the use of appropriate OTA architecture and using the gm/ID methodology for MOSFET sizing. So, programmable CMOS OTA-C band-pass filter topologies operating in sub-threshold region with a dynamic range of 65dB for use in bionic ears were proposed. The power consumption for the proposed filters is in nano- Watt range for their frequency range of (lOO-I Ok) Hz. Also, a 4-channel OTA-C filter bank is designed and tested. The EEG signals have small amplitudes and frequency bands ranges of uV'S and (l-40) Hz respectively. The important issue is to design filters with small noise floor with white dominant. This is achieved with the proposed OTA which is of relatively simple architecture and with operation in the deep weak-inversion region using ±1.5V supply rails. The OTA-C filter has power consumption in the pico-Watt range for 0, e, and a signals and less than 3nW for B signals. Another topology is suggested for future work.
Supervisor: Hall, Stephen Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral