CSIR NET Physical Sciences Electronics and Experimental Methods Flashcards

24 sample cards from the Electronics and Experimental Methods deck

Sampled from the end of the deck, so these are different cards from the ones shown on the syllabus page.

1. How does a photomultiplier tube (PMT) detect light, and what is its key advantage?

   A photon strikes the photocathode releasing a photoelectron, which is accelerated through a chain of dynodes producing secondary-electron multiplication (cascade) before collection at the anode. Its advantage is extremely high gain and single-photon sensitivity with low noise.

2. How does a gas-filled ionization-based particle detector work?

   Ionizing radiation creates electron–ion pairs in a gas; an applied electric field collects these charges at electrodes, producing a measurable current/pulse. Depending on the applied voltage region it operates as an ionization chamber, proportional counter, or Geiger–Müller counter.

3. Distinguish a proportional counter from a Geiger–Müller counter.

   In a proportional counter the output pulse is proportional to the deposited energy (gas multiplication/avalanche but localized), allowing energy spectroscopy. In a Geiger–Müller counter the avalanche spreads through the whole tube, giving a large saturated pulse independent of particle energy — it only counts events, not energy.

4. How does a scintillation detector work?

   Ionizing radiation excites a scintillator material that emits flashes of visible/UV light; these photons are detected and amplified by a photomultiplier tube (or photodiode), producing an electrical pulse whose height is proportional to the deposited energy.

5. What is the purpose of signal conditioning in a measurement system?

   Signal conditioning processes a raw transducer signal to make it suitable for further processing or digitization — including amplification, filtering, linearization, level shifting, isolation, and impedance matching — improving SNR and compatibility with downstream electronics (e.g. ADC).

6. What is signal recovery and name a key technique used for it.

   Signal recovery is the extraction of a small signal buried in noise. A key technique is the lock-in amplifier (phase-sensitive detection), which multiplies the input by a reference at the signal frequency and low-pass filters, recovering signals far below the noise floor. Boxcar averaging and correlation are also used.

7. How does a lock-in amplifier recover a weak signal?

   It modulates the measurement at a known reference frequency, then multiplies (mixes) the noisy input with that reference and applies a narrow low-pass filter. Only components coherent with the reference frequency and phase survive, giving extremely narrow effective bandwidth and very high SNR (phase-sensitive detection).

8. What is impedance matching and why is it important?

   Impedance matching is setting the source, load (and line) impedances appropriately to optimize signal/power transfer and minimize reflections. It maximizes power transfer (when matched) or, in measurement, preserves voltage signals and prevents loading errors and standing waves in transmission lines.

9. State the maximum power transfer theorem condition for AC circuits.

   Maximum power is delivered to the load when the load impedance equals the complex conjugate of the source impedance: $Z_L = Z_S^{*}$, i.e. $R_L = R_S$ and $X_L = -X_S$. For purely resistive circuits, $R_L = R_S$.

10. For voltage signal transfer (not power), how should input and output impedances be set?

    For faithful voltage transfer, the load (input) impedance should be much larger than the source (output) impedance ($Z_{in} \gg Z_{out}$). This minimizes loading and voltage division, which is why instrumentation inputs have high impedance and outputs low impedance.

11. What is the role of amplification in instrumentation?

    Amplification increases the magnitude (voltage, current, or power) of a weak transducer signal so it can drive subsequent stages, overcome noise/quantization in an ADC, and use the full measurement range — ideally with high linearity, low noise, and appropriate bandwidth.

12. Give the closed-loop gain of an ideal non-inverting op-amp amplifier.

    $$A_v = 1 + \frac{R_f}{R_1}$$ where $R_f$ is the feedback resistor and $R_1$ the resistor from the inverting input to ground. Its input impedance is very high and output very low.

13. Give the closed-loop gain of an ideal inverting op-amp amplifier.

    $$A_v = -\frac{R_f}{R_{in}}$$ The negative sign denotes a $180^{\circ}$ phase inversion; the input impedance equals $R_{in}$ and the inverting node is a virtual ground.

14. State the two 'golden rules' (ideal conditions) for an op-amp with negative feedback.

    (1) No current flows into the input terminals (infinite input impedance). (2) The op-amp adjusts its output so that the voltage difference between the two inputs is zero ($V_+ = V_-$, the virtual short), valid when negative feedback and infinite open-loop gain are assumed.

15. What is the gain–bandwidth product (GBW) of an op-amp?

    GBW is the constant product of closed-loop gain and bandwidth: $A_v \times f_{BW} = \text{constant} = f_T$ (unity-gain frequency). Increasing the closed-loop gain proportionally reduces the available bandwidth for a single-pole compensated op-amp.

16. What is an instrumentation amplifier and what are its key features?

    An instrumentation amplifier is a precision differential amplifier (typically three op-amps) that amplifies the difference between two inputs while rejecting common-mode signals. Key features: very high input impedance on both inputs, high CMRR, low offset/drift, and gain set by a single resistor.

17. How is the gain of a standard 3-op-amp instrumentation amplifier set?

    Gain is set by one external resistor $R_G$: $$A_v = \left( 1 + \frac{2R_1}{R_G} \right)\frac{R_3}{R_2}$$ where $R_1$ are the input-stage feedback resistors and $R_2, R_3$ the difference-amplifier resistors. A single $R_G$ adjusts gain without disturbing CMRR.

18. Define Common-Mode Rejection Ratio (CMRR) and give its formula.

    CMRR quantifies an amplifier's ability to reject signals common to both inputs relative to differential signals: $$\text{CMRR} = \frac{A_d}{A_{cm}}, \quad \text{CMRR (dB)} = 20\log_{10}\left|\frac{A_d}{A_{cm}}\right|$$ where $A_d$ is differential gain and $A_{cm}$ common-mode gain. High CMRR is essential for instrumentation amplifiers.

19. What is negative feedback in an amplifier and what is the closed-loop gain expression?

    Negative feedback feeds a fraction $\beta$ of the output back in opposition to the input. The closed-loop gain is $$A_f = \frac{A}{1 + A\beta}$$ where $A$ is the open-loop gain. The loop gain is $A\beta$ and $(1+A\beta)$ is the feedback factor.

20. List four benefits that negative feedback provides to an amplifier.

    (1) Gain stability/desensitivity to parameter variations; (2) increased bandwidth (gain–bandwidth tradeoff); (3) reduced nonlinear distortion and noise (generated within the loop); (4) controllable input/output impedance (raised or lowered depending on topology).

21. How does negative feedback affect bandwidth and distortion quantitatively?

    Both improve by the factor $(1 + A\beta)$: bandwidth increases as $f_{BW,closed} = f_{BW,open}(1 + A\beta)$, and distortion/noise generated inside the loop is reduced by $\frac{1}{1 + A\beta}$, while gain is reduced by the same factor.

22. Contrast a low-pass filter and a high-pass filter, including the cutoff frequency formula for an RC filter.

    A low-pass filter passes frequencies below the cutoff and attenuates above it; a high-pass filter does the opposite. For a first-order RC filter the $-3\,\text{dB}$ cutoff is $$f_c = \frac{1}{2\pi R C}$$

23. What is a band-pass filter, and define its quality factor $Q$.

    A band-pass filter passes a band of frequencies between a lower and upper cutoff and attenuates outside it. Its quality factor is $Q = \frac{f_0}{\Delta f} = \frac{f_0}{f_H - f_L}$, where $f_0$ is the center frequency and $\Delta f$ the bandwidth; high $Q$ means a narrow, selective band.

24. List common noise types in electronic measurements and their frequency dependence.

    Thermal (Johnson–Nyquist) noise — white, frequency-independent; Shot noise — white, from discrete charge carriers; Flicker (1/f) noise — dominant at low frequencies; and Burst/popcorn noise. Johnson noise voltage: $V_n = \sqrt{4 k_B T R \, \Delta f}$, and shot noise current: $i_n = \sqrt{2 q I \, \Delta f}$.

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