GATE Biomedical Engineering Sensors and Bioinstrumentation Syllabus

Sensors and Bioinstrumentation syllabus — full chapter and topic list

Expand any chapter to see its topics and sub-topics. This is the whole examinable outline for Sensors and Bioinstrumentation in GATE Biomedical Engineering, not a summary of it.

1. 1

   Sensors

   3 topics

   • Types of Sensors
     • Resistive
     • Capacitive
     • Inductive
     • Piezoelectric
     • Hall Effect
     • Electrochemical
     • Optical
   • Sensor Signal Conditioning Circuits
   • Application of LASER in Sensing and Therapy
2. 2

   Origin of Bio Potentials and Measurement Techniques

   2 topics

   • Bio Potentials
     • ECG
     • EEG
     • EMG
     • ERG
     • EOG
     • GSR
     • PCG
   • Measurement Techniques
     • Measuring Blood Pressure
     • Measuring Body Temperature
     • Measuring Volume and Flow in Arteries, Veins, and Tissues
     • Respiratory Measurements
     • Cardiac Output Measurement
3. 3

   Operating Principle of Medical Equipment

   1 topic

   • Medical Equipment
     • Sphygmomanometer
     • Ventilator
     • Cardiac Pacemaker
     • Defibrillator
     • Pulse Oximeter
     • Hemodialyzer
4. 4

   Electrical Isolation and Safety of Biomedical Instruments

   2 topics

   • Electrical Isolation
     • Optical Isolation
     • Electrical Isolation
   • Safety of Biomedical Instruments

Sensors and Bioinstrumentation flashcards for GATE Biomedical Engineering

21 of 51 cards from the Sensors and Bioinstrumentation deck — real questions with worked answers.

1. What is a sensor (transducer) in bioinstrumentation, and how do active and passive sensors differ?

   A sensor converts a physical/biological quantity (e.g., pressure, temperature, biopotential) into a measurable electrical signal. An active (self-generating) sensor produces its own electrical output without external excitation (e.g., piezoelectric, thermocouple), while a passive (modulating) sensor requires external excitation and modulates a parameter such as resistance, capacitance or inductance (e.g., strain gauge, thermistor).

2. List the major classifications of sensors based on transduction principle.

   Resistive, capacitive, inductive, piezoelectric, Hall effect (magnetic), electrochemical, optical (photoelectric), and thermoelectric. They may also be classified as active vs passive, contact vs non-contact, and analog vs digital.

3. For a resistive strain gauge, write the relation between fractional resistance change and strain using the gauge factor.

   The gauge factor is $GF = \dfrac{\Delta R / R}{\Delta L / L} = \dfrac{\Delta R/R}{\varepsilon}$, so $\dfrac{\Delta R}{R} = GF \cdot \varepsilon$. Typical metallic gauges have $GF \approx 2$; semiconductor gauges have $GF \approx 100\text{–}200$.

4. How does the resistance of a wire depend on its physical parameters, and how does this underlie resistive sensing?

   $R = \dfrac{\rho L}{A}$, where $\rho$ is resistivity, $L$ length and $A$ cross-sectional area. Resistive sensors work by changing $\rho$ (piezoresistivity), $L$ or $A$ (strain), giving a measurable $\Delta R$.

5. What is a thermistor and how does an NTC thermistor's resistance vary with temperature?

   A thermistor is a resistive temperature sensor made from semiconductor material. For an NTC (negative temperature coefficient) thermistor, resistance decreases as temperature rises, following $R_T = R_0 \exp\!\left[\beta\left(\dfrac{1}{T} - \dfrac{1}{T_0}\right)\right]$, where $\beta$ is the material constant and $T$ is in kelvin.

6. Give the resistance–temperature relation for a metallic RTD (e.g., Pt100).

   $R_T = R_0\,(1 + \alpha\,\Delta T)$ for a linear approximation, where $\alpha$ is the temperature coefficient of resistance (for platinum $\alpha \approx 0.00385\ ^{\circ}\mathrm{C}^{-1}$) and $R_0$ is resistance at the reference temperature. A Pt100 reads $100\ \Omega$ at $0\ ^{\circ}\mathrm{C}$.

7. What is the capacitance formula for a parallel-plate capacitive sensor, and which parameters are varied for sensing?

   $C = \dfrac{\varepsilon_0 \varepsilon_r A}{d}$, where $A$ is plate area, $d$ the separation and $\varepsilon_r$ the relative permittivity. Capacitive sensors transduce by changing $d$ (displacement/pressure), $A$ (overlap area), or $\varepsilon_r$ (humidity, level).

8. For a capacitive displacement sensor, why is varying the gap $d$ nonlinear and varying the overlap area $A$ linear?

   Since $C = \varepsilon_0\varepsilon_r A / d$, varying gap gives $C \propto 1/d$ (hyperbolic, nonlinear), whereas varying overlap area gives $C \propto A$ (linear). Hence area-variation designs are preferred when linearity is required.

9. What is the operating principle of an LVDT (Linear Variable Differential Transformer)?

   An LVDT is an inductive displacement sensor with one primary and two secondary coils wound symmetrically, and a movable ferromagnetic core. AC excitation on the primary induces voltages in the secondaries; the differential output $V_{out} = V_{s1} - V_{s2}$ is proportional to core displacement, with phase indicating direction and zero output at the null position.

10. State the key characteristics of an LVDT output.

    The output is linear over its range, has infinite mechanical resolution, is frictionless/contactless (high reliability), gives zero output at the null point, and uses phase of the AC output to indicate direction of displacement. Output must be demodulated (phase-sensitive detection) to recover a DC proportional signal.

11. What is the self-inductance of a coil, and how do inductive sensors transduce displacement?

    $L = \dfrac{N^2 \mu A}{l}$, where $N$ is number of turns, $\mu$ permeability of the core, $A$ cross-sectional area and $l$ magnetic path length. Inductive sensors change $L$ by moving a ferromagnetic core (changing $\mu$/reluctance) or varying air-gap, altering reluctance $\mathcal{R} = l/(\mu A)$.

12. Explain the piezoelectric effect and its inverse.

    The direct piezoelectric effect is the generation of an electric charge/voltage when mechanical stress is applied to certain crystals (e.g., quartz, PZT, barium titanate). The inverse (converse) piezoelectric effect is mechanical deformation produced when a voltage is applied. Direct effect is used for sensing (force, pressure, acceleration); inverse for actuation/ultrasound generation.

13. Write the charge and voltage relations for a piezoelectric sensor under applied force.

    Charge generated: $Q = d \cdot F$, where $d$ is the piezoelectric charge constant (C/N). Output voltage: $V = \dfrac{Q}{C} = \dfrac{d\,F}{C}$, where $C$ is the sensor capacitance. Equivalently $V = g\,t\,P$, with $g$ the voltage sensitivity, $t$ thickness and $P$ pressure.

14. Why can't a piezoelectric sensor measure a static (DC) force, and what does this imply about its frequency response?

    The generated charge leaks away through the finite insulation/input resistance, so a constant force gives a decaying output. Piezoelectric sensors are therefore inherently high-pass/AC devices, suited to dynamic measurements (vibration, ultrasound, transient pressure) and unable to measure steady-state quantities.

15. State the Hall effect and the expression for the Hall voltage.

    When a current-carrying conductor/semiconductor is placed in a magnetic field perpendicular to the current, a transverse voltage develops. $V_H = \dfrac{I\,B}{n\,q\,t}$, where $I$ is current, $B$ magnetic flux density, $n$ charge carrier density, $q$ carrier charge and $t$ thickness. It is used for magnetic field, current, position and proximity sensing.

16. Why are semiconductors preferred over metals for Hall effect sensors?

    The Hall voltage $V_H = IB/(nqt)$ is inversely proportional to carrier density $n$. Semiconductors have much lower $n$ than metals, producing a much larger, more easily measured Hall voltage for a given current and field.

17. What is the basic principle of an electrochemical sensor?

    An electrochemical sensor converts the concentration of a chemical species into an electrical signal via a redox reaction at electrodes in contact with an electrolyte. Main types: potentiometric (measure voltage at zero current, e.g., pH, ion-selective electrodes), amperometric (measure current at fixed potential, e.g., Clark $\ce{O2}$ electrode, glucose sensor), and conductometric (measure conductivity).

18. What does the Nernst equation give for a potentiometric electrochemical sensor?

    It relates electrode potential to ion activity: $E = E^{0} - \dfrac{RT}{nF}\ln Q$, where $E^0$ is the standard potential, $R$ the gas constant, $T$ temperature, $n$ electrons transferred, $F$ Faraday's constant and $Q$ the reaction quotient. For a 10-fold change in concentration of a monovalent ion at $25\ ^{\circ}\mathrm{C}$, the potential shifts by about $59\ \mathrm{mV}$.

19. Describe the Clark electrode and the glucose biosensor.

    The Clark electrode is an amperometric sensor measuring dissolved oxygen via $\ce{O2}$ reduction at a platinum cathode under fixed polarizing voltage; the current is proportional to $\ce{O2}$ concentration. The enzymatic glucose biosensor uses glucose oxidase: $\ce{glucose + O2 -> gluconic\ acid + H2O2}$, and the consumed $\ce{O2}$ (or produced $\ce{H2O2}$) is measured amperometrically, giving a current proportional to glucose concentration.

20. Name the main types of optical sensors and the effects they exploit.

    Photoconductive (LDR – resistance falls with light), photovoltaic (solar cell/photodiode generates voltage), photodiode (reverse-biased, photocurrent $\propto$ light), phototransistor (amplified photocurrent), and CCD/CMOS image sensors. They exploit the photoelectric/photoconductive effect where photons generate electron–hole pairs.

21. 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.

See more Sensors and Bioinstrumentation flashcards →

Planning Sensors and Bioinstrumentation for GATE Biomedical Engineering

Sensors and Bioinstrumentation (GATE Biomedical Engineering) FAQ