GATE BM - Biomedical Engineering: Physiological Signals, Instrumentation and Devices
GATE BM (Biomedical Engineering) tests undergraduate understanding of how physiological systems generate measurable signals, how transducers and instrumentation acquire them, and how those signals are conditioned and interpreted. Expect numerical items on electrode behaviour, biopotential amplifier design, biomechanics and biomaterials, plus conceptual items on medical imaging modalities.
Physiological sources and electrodes
Resting membrane potential follows the Nernst and Goldman–Hodgkin–Katz relations; the action potential is a transient change in Na+ and K+ conductance. Surface biopotentials reach electrodes by volume conduction, so amplitude falls with distance and tissue anisotropy distorts the field. Ag/AgCl electrodes dominate ECG and EEG because their half-cell potential is stable, yet the residual offset (up to ±300 mV) still swamps the 1 mV ECG, forcing AC coupling with a high-pass corner near 0.05 Hz to preserve diagnostic ST segments.
Amplifier and signal chain
An instrumentation amplifier supplies high input impedance, single-resistor gain and CMRR above 80 dB; the driven-right-leg circuit feeds common-mode back to the body to cancel 50 Hz mains pickup. Sample above the Nyquist rate with an anti-aliasing filter ahead of the ADC; an N-bit converter gives SQNR near 6.02N dB.
Mechanics, materials, imaging
Bone and tendon obey Hooke's law within the elastic region; soft tissue is viscoelastic, showing creep under constant stress and relaxation under constant strain. Biomaterial choice turns on biocompatibility per ISO 10993, corrosion resistance and controlled degradation of polyesters such as PLA. CT encodes linear attenuation as Hounsfield units (water 0, air −1000); MRI contrast derives from T1 and T2 relaxation; ultrasound needs an acoustic-impedance mismatch to reflect.
Worked example
A 1 mV ECG rides on 100 mV of 50 Hz common-mode interference, and output interference must stay under 1% of the signal. Input ratio is 100; the amplifier divides common-mode content by CMRR, so CMRR ≥ 100/0.01 = 10,000, i.e. 20 log10(10,000) = 80 dB.
Traps
Confusing linear CMRR with its decibel value; forgetting that electrode offset, not gain, saturates DC-coupled front ends; reading Hounsfield units as attenuation coefficients; swapping creep and stress relaxation; sampling at exactly 2f while ignoring filter roll-off.