GATE Biomedical Engineering Sensors and Bioinstrumentation Syllabus

What is the photoelectric energy condition for a photodetector, and how does it set the cutoff wavelength?

A photon is absorbed (generating carriers) only if its energy exceeds the band gap: $E_{ph} = h\nu = \dfrac{hc}{\lambda} \geq E_g$. The cutoff wavelength is $\lambda_{max} = \dfrac{hc}{E_g}$; longer wavelengths are not detected.

What is the function of a Wheatstone bridge in resistive sensor signal conditioning?

It converts a small resistance change into a proportional voltage. For a balanced bridge ($R_1 R_4 = R_2 R_3$) the output is zero; a small change $\Delta R$ in one arm gives $V_{out} \approx \dfrac{V_{ex}}{4}\cdot\dfrac{\Delta R}{R}$ (quarter bridge). Half- and full-bridge configurations increase sensitivity and provide temperature compensation.

Why is an instrumentation amplifier used in biopotential and sensor signal conditioning?

It provides high differential gain with very high input impedance, low output impedance, and high common-mode rejection ratio (CMRR), rejecting common-mode interference (e.g., 50/60 Hz power-line noise) while amplifying the small differential signal. The classic 3-op-amp design has gain $A = \left(1 + \dfrac{2R}{R_{gain}}\right)\dfrac{R_2}{R_1}$.

Define CMRR and why it is critical for biopotential amplifiers.

Common-Mode Rejection Ratio is the ratio of differential gain to common-mode gain: $\mathrm{CMRR} = \dfrac{A_d}{A_{cm}}$, often in dB: $\mathrm{CMRR_{dB}} = 20\log_{10}\!\left(\dfrac{A_d}{A_{cm}}\right)$. High CMRR (typically $>80\text{–}100$ dB) is essential to reject large common-mode interference (mainly power-line) so the tiny differential biopotential can be measured.

List the typical stages of a sensor signal-conditioning chain.

Excitation (if passive sensor) → bridge/transduction → instrumentation amplifier (differential gain) → filtering (low-pass anti-aliasing, high-pass for DC removal, notch for 50/60 Hz) → level shifting/linearization → isolation (for patient safety) → analog-to-digital conversion (ADC).

What is a notch filter's role in biopotential signal conditioning, and at what frequency?

A notch (band-stop) filter removes power-line interference at $50\ \mathrm{Hz}$ (India/Europe) or $60\ \mathrm{Hz}$ (US) while passing the biosignal band. It has a narrow stop-band centered at the mains frequency, characterized by a high quality factor $Q = \dfrac{f_0}{\Delta f}$.

What does the acronym LASER stand for, and what properties make laser light useful in sensing and therapy?

LASER = Light Amplification by Stimulated Emission of Radiation. Laser light is monochromatic, coherent, highly directional (collimated), and has high intensity. These allow precise focusing, accurate interferometry, and controlled energy delivery for sensing and tissue therapy.

Give two sensing applications and two therapeutic applications of lasers in biomedical engineering.

Sensing: laser Doppler flowmetry (blood flow velocity via Doppler shift), pulse oximetry/optical sensing, laser interferometry, and LIDAR/distance measurement. Therapy: laser surgery and tissue ablation, photocoagulation (ophthalmology, retinal repair), lithotripsy, and low-level laser (photobiomodulation) therapy. Lasers are also used in LASIK and dermatology.

What is the principle of laser Doppler flowmetry?

A laser beam illuminates tissue; light scattered by moving red blood cells is frequency-shifted by the Doppler effect, $\Delta f = \dfrac{2 v \cos\theta}{\lambda}$, where $v$ is cell velocity, $\theta$ the angle and $\lambda$ the wavelength. The frequency shift of the backscattered light gives a measure of microvascular blood flow (perfusion).

Explain the laser–tissue interaction mechanisms used in therapy.

Photothermal (heating → coagulation, vaporization, ablation), photochemical (e.g., photodynamic therapy activating photosensitizers), photomechanical/photoablation (breaking molecular bonds, e.g., excimer laser in LASIK), and photobiomodulation (low-level stimulation of healing). The effect depends on wavelength, power density, exposure time and tissue absorption.

What is a biopotential and what causes it at the cellular level?

A biopotential is the electrical potential difference measured across membranes of excitable cells (nerve, muscle, cardiac), arising from ionic concentration gradients (mainly $\ce{Na+}$, $\ce{K+}$, $\ce{Cl-}$, $\ce{Ca^2+}$) maintained by selective membrane permeability and the $\ce{Na+/K+}$ pump. Examples include ECG, EEG, EMG, EOG, ERG, PCG-related signals.

State the resting membrane potential and how the action potential is generated.

The resting membrane potential is typically about $-70\ \mathrm{mV}$ (inside negative). An action potential occurs when a stimulus depolarizes the membrane past threshold: voltage-gated $\ce{Na+}$ channels open (rapid depolarization toward $+30\ \mathrm{mV}$), then $\ce{Na+}$ channels inactivate and $\ce{K+}$ channels open (repolarization), followed by hyperpolarization and return to rest.

What is the Nernst equation for the equilibrium potential of a single ion, and give the $\ce{K+}$ value approximately?

$E_{ion} = \dfrac{RT}{zF}\ln\dfrac{[\text{ion}]_{out}}{[\text{ion}]_{in}}$, where $z$ is the ion's valence. For $\ce{K+}$ at body temperature this gives roughly $E_{K} \approx -90\ \mathrm{mV}$. The Goldman–Hodgkin–Katz equation extends this to multiple permeant ions.

What is a half-cell potential and why is the silver–silver chloride (Ag/AgCl) electrode preferred for biopotential recording?

A half-cell (electrode) potential is the potential developed at the electrode–electrolyte interface due to charge redistribution. Ag/AgCl is a non-polarizable electrode allowing free charge transfer, giving low and stable half-cell potential, low motion artifact, low noise and minimal drift—ideal for ECG/EEG/EMG.

What is the ECG and the standard amplitude/frequency range of the signal?

The electrocardiogram (ECG/EKG) records the electrical activity of the heart from skin-surface electrodes. Typical amplitude is about $0.5\text{–}5\ \mathrm{mV}$ (commonly $\sim 1\ \mathrm{mV}$ for the R wave) with a frequency content of roughly $0.05\text{–}150\ \mathrm{Hz}$ (diagnostic band $0.05\text{–}100\ \mathrm{Hz}$).

Name the ECG waves/intervals and what each represents.

P wave = atrial depolarization; QRS complex = ventricular depolarization (masks atrial repolarization); T wave = ventricular repolarization. PR interval = AV conduction time; QT interval = total ventricular depolarization+repolarization; ST segment = plateau between depolarization and repolarization.

What is Einthoven's triangle and the relationship among the limb leads?

Einthoven's triangle is the equilateral triangle formed by the bipolar limb leads I, II, III placed at the right arm, left arm and left leg. The leads satisfy Einthoven's law: $\text{Lead II} = \text{Lead I} + \text{Lead III}$ (i.e., $V_{II} = V_{I} + V_{III}$).

How many electrodes and leads are in a standard 12-lead ECG?

It uses 10 electrodes (4 limb + 6 chest) to produce 12 leads: 3 bipolar limb leads (I, II, III), 3 augmented unipolar limb leads (aVR, aVL, aVF), and 6 unipolar precordial/chest leads (V1–V6). The right-leg electrode serves as the driven/reference (not a recording lead).

What is the heart rate formula from the ECG R–R interval?

$\text{HR (bpm)} = \dfrac{60}{\text{R--R interval in seconds}}$. For paper at $25\ \mathrm{mm/s}$, $\text{HR} = \dfrac{1500}{\text{R--R in small squares}}$. Normal resting heart rate is about $60\text{–}100\ \mathrm{bpm}$.

What is the EEG and the amplitude/frequency range of brain waves?

The electroencephalogram records the brain's electrical activity from scalp electrodes. It is a very small signal, typically $10\text{–}100\ \mu\mathrm{V}$, with frequency content about $0.5\text{–}100\ \mathrm{Hz}$ (clinically usually $0.5\text{–}40\ \mathrm{Hz}$). Electrodes are placed by the international 10–20 system.

List the EEG frequency bands with their ranges.

Delta ($\delta$): $0.5\text{–}4\ \mathrm{Hz}$ (deep sleep); Theta ($\theta$): $4\text{–}8\ \mathrm{Hz}$ (drowsiness); Alpha ($\alpha$): $8\text{–}13\ \mathrm{Hz}$ (relaxed, eyes closed); Beta ($\beta$): $13\text{–}30\ \mathrm{Hz}$ (alert/active); Gamma ($\gamma$): $>30\ \mathrm{Hz}$ (high-level cognition).

What is the EMG and its typical amplitude and frequency range?

The electromyogram records the electrical activity of skeletal muscle (motor-unit action potentials). Surface EMG amplitude is roughly $0.1\text{–}5\ \mathrm{mV}$ with a frequency range of about $10\text{–}500\ \mathrm{Hz}$ (intramuscular/needle EMG extends higher, up to $\sim 2\text{–}10\ \mathrm{kHz}$).

What is a motor unit and a motor unit action potential (MUAP)?

A motor unit is a single motor neuron together with all the muscle fibers it innervates. The MUAP is the summated electrical signal from these fibers when the motor neuron fires; EMG is the spatial-temporal summation of many MUAPs. Increasing force recruits more motor units and raises firing rates.

What is the ERG and what does it record?

The electroretinogram records the electrical response of the retina to a light stimulus, measured between a corneal (contact-lens) electrode and a reference. It reflects activity of photoreceptors and inner retinal cells. Typical amplitude is a few hundred microvolts (up to $\sim 0.5\ \mathrm{mV}$).

Name the main components of the ERG waveform and their origins.

The a-wave is an initial negative deflection originating from the photoreceptors; the b-wave is a larger positive deflection originating mainly from bipolar/Müller cells. Slower components include the c-wave (retinal pigment epithelium) and oscillatory potentials superimposed on the rising b-wave.

What is the EOG and the physical basis of the signal?

The electrooculogram records eye movement by measuring the standing corneo-retinal potential: the cornea is electrically positive relative to the retina, forming a dipole. As the eye rotates, the dipole orientation changes, altering the voltage at periorbital electrodes. Amplitude is roughly $10\ \mu\mathrm{V}$ to $5\ \mathrm{mV}$, frequency band about $0\text{–}100\ \mathrm{Hz}$.

How is eye-rotation angle related to the EOG voltage?

Over the central range (about $\pm 30^{\circ}$), the EOG voltage is approximately linear with eye-rotation angle, typically about $15\text{–}20\ \mu\mathrm{V}$ per degree of rotation. This linearity allows EOG to track gaze direction and is used in sleep studies and eye-movement interfaces.

What is the GSR (galvanic skin response) and what does it measure?

GSR, also called electrodermal activity (EDA) or skin conductance, measures changes in the electrical conductance of the skin caused by sweat-gland (eccrine) activity, which is controlled by the sympathetic nervous system. It indexes emotional/psychological arousal and is used in lie detectors (polygraphs) and biofeedback.

What are the two components of GSR/EDA and how is conductance related to resistance?

GSR has a tonic component (slowly varying skin conductance level, SCL) and a phasic component (skin conductance responses, SCRs—rapid stimulus-evoked changes). Conductance is the reciprocal of resistance, $G = \dfrac{1}{R}$, measured in siemens; increased sweating lowers skin resistance and raises conductance.

What is the PCG (phonocardiogram) and what does it record?

The phonocardiogram records the acoustic vibrations (heart sounds and murmurs) of the heart using a microphone/transducer on the chest. It captures the first heart sound S1 (closure of mitral and tricuspid valves, marking systole onset) and the second heart sound S2 (closure of aortic and pulmonary valves, marking diastole onset), plus S3, S4 and murmurs. Audible band is roughly $20\text{–}1000\ \mathrm{Hz}$.

Compare ECG, EEG, EMG and EOG by typical amplitude and frequency range.

ECG: $\sim 1\text{–}5\ \mathrm{mV}$, $0.05\text{–}150\ \mathrm{Hz}$. EEG: $10\text{–}100\ \mu\mathrm{V}$, $0.5\text{–}100\ \mathrm{Hz}$ (smallest amplitude). EMG: $0.1\text{–}5\ \mathrm{mV}$, $10\text{–}500\ \mathrm{Hz}$ (widest band). EOG: $10\ \mu\mathrm{V}\text{–}5\ \mathrm{mV}$, near-DC to $\sim 100\ \mathrm{Hz}$ (slowest signal).