Basic Electronics interview questions

150 questions with answers (ELE-001 to ELE-150). Try each one aloud before reading the answer.

Semiconductor Physics

ELE-001 · What is a semiconductor?

A material whose conductivity lies between conductors and insulators and can be controlled by doping, temperature or light. Common examples are Si, Ge and GaAs.

ELE-002 · What is the difference between intrinsic and extrinsic semiconductors?

An intrinsic semiconductor is pure, with equal electron and hole concentrations. An extrinsic semiconductor is doped: N-type with pentavalent impurities (e.g. P, As) and P-type with trivalent impurities (e.g. B).

ELE-003 · What is the band gap, and what are its values for Si and Ge?

The energy gap between the top of the valence band and the bottom of the conduction band. At 300 K it is about 1.12 eV for Si, 0.66 eV for Ge and 1.42 eV for GaAs.

ELE-004 · What are majority and minority carriers?

Majority carriers are the more numerous charge carriers (electrons in N-type, holes in P-type). Minority carriers are the less numerous ones, generated thermally (holes in N-type, electrons in P-type).

ELE-005 · What is a hole?

The vacancy left when an electron leaves a covalent bond. It behaves like a positive charge carrier because neighbouring valence electrons move into it, so the vacancy appears to move.

ELE-006 · What is doping, and what are donors and acceptors?

Doping is the controlled addition of impurities to change conductivity. Donors (group V, e.g. phosphorus) give free electrons. Acceptors (group III, e.g. boron) accept electrons and create holes.

ELE-007 · What is the Fermi level?

The energy level at which the probability of electron occupancy is 50%. It sits near mid-gap in an intrinsic semiconductor, moves toward the conduction band in N-type and toward the valence band in P-type.

ELE-008 · What is the difference between drift and diffusion current?

Drift current is carrier flow caused by an electric field. Diffusion current is carrier flow from regions of high to low concentration, caused by a concentration gradient.

ELE-009 · What is carrier mobility?

The ratio of drift velocity to electric field, μ = v_d/E, in cm²/V·s. In lightly doped Si at 300 K, electron mobility is about 1350 and hole mobility about 480 cm²/V·s.

ELE-010 · What is the intrinsic carrier concentration of silicon at room temperature?

About 10¹⁰ cm⁻³ at 300 K; textbooks commonly use nᵢ ≈ 1.5 × 10¹⁰ cm⁻³. It rises steeply with temperature.

ELE-011 · What is the mass-action law?

In thermal equilibrium, n·p = nᵢ². Increasing one carrier type by doping proportionally reduces the other.

ELE-012 · What is the depletion region?

The zone around a PN junction that is emptied of mobile carriers, leaving fixed ionized donor and acceptor charges. Its electric field creates the built-in potential that opposes further diffusion.

ELE-013 · Why is silicon preferred over germanium for most devices?

Silicon's larger band gap gives much lower leakage current and higher operating temperature. It also forms a stable native oxide (SiO₂) and is abundant and cheap.

Diodes & Rectifiers

ELE-014 · What is a PN junction diode, and what is its cut-in voltage?

A two-terminal device formed by joining P- and N-type material that conducts mainly in one direction. The cut-in voltage is about 0.7 V for Si and 0.3 V for Ge.

ELE-015 · What is the difference between forward and reverse bias?

Forward bias (P to +, N to −) narrows the depletion region and allows large current. Reverse bias widens the depletion region and allows only a tiny leakage current until breakdown.

ELE-016 · What is reverse saturation current?

The small current (Iₛ) that flows under reverse bias due to minority carriers. It is in the nA range or lower for Si and roughly doubles for every 10 °C rise.

ELE-017 · What is the Shockley diode equation?

I = Iₛ(e^(V/(nV_T)) − 1), where n is the ideality factor (1–2) and V_T = kT/q ≈ 26 mV at 300 K.

ELE-018 · What is a Zener diode used for?

Voltage regulation and reference. It is designed to operate in reverse breakdown, where its voltage stays nearly constant over a wide current range.

ELE-019 · What is the difference between Zener and avalanche breakdown?

Zener breakdown occurs in heavily doped junctions by quantum tunnelling, typically below about 5 V, with a negative temperature coefficient. Avalanche breakdown occurs by impact ionization at higher voltages, with a positive temperature coefficient.

ELE-020 · What is a rectifier?

A circuit that converts AC to pulsating DC using diodes. Common types are half-wave, center-tapped full-wave and bridge rectifiers.

ELE-021 · What is peak inverse voltage (PIV)?

The maximum reverse voltage a diode sees in a circuit. The diode's rated reverse voltage must exceed it with margin.

ELE-022 · What is a Schottky diode?

A metal-semiconductor junction diode with a low forward drop (about 0.2–0.4 V) and almost no reverse-recovery charge, making it fast. Its drawbacks are higher leakage and lower reverse voltage ratings.

ELE-023 · What is an LED, and what decides its colour?

A light-emitting diode emits photons when electrons and holes recombine under forward bias. The emitted wavelength is set by the band gap of the semiconductor (e.g. GaN for blue, AlGaInP for red).

ELE-024 · What is a photodiode?

A diode operated in reverse bias (or zero bias) in which incident light generates electron-hole pairs, producing a current proportional to light intensity.

ELE-025 · What is a clipper circuit?

A diode circuit that removes the part of a waveform above or below a set reference level, used for waveform shaping and input protection.

ELE-026 · What is a clamper circuit?

A diode-capacitor circuit that shifts a waveform's DC level without changing its shape, so that its positive or negative peak sits at a reference level.

BJT

ELE-027 · What is a bipolar junction transistor (BJT)?

A three-layer, three-terminal device (emitter, base, collector) of NPN or PNP type. A small base current controls a much larger collector current, and both electrons and holes take part in conduction.

ELE-028 · What are the regions of operation of a BJT?

Cutoff (both junctions reverse biased), active (BE forward, BC reverse; used for amplification), saturation (both forward; used as a closed switch) and reverse active (BE reverse, BC forward; rarely used).

ELE-029 · Define α and β of a BJT and their relation.

α = I_C/I_E (common-base gain, slightly below 1) and β = I_C/I_B (common-emitter gain, typically 50–300). They are related by β = α/(1 − α).

ELE-030 · Why is the emitter heavily doped and the base thin and lightly doped?

A heavily doped emitter injects many carriers into the base. A thin, lightly doped base minimises recombination so that most carriers reach the collector, giving high α and β.

ELE-031 · What are the three BJT configurations?

Common emitter (voltage and current gain, 180° phase shift), common base (voltage gain, low input impedance, good high-frequency response) and common collector or emitter follower (current gain, voltage gain ≈ 1, buffer).

ELE-032 · Why is the common-emitter configuration the most widely used?

It provides both voltage and current gain, hence the highest power gain, with moderate input and output impedances suitable for cascading.

ELE-033 · What is an emitter follower used for?

As a buffer. Its voltage gain is close to 1, but it has high input impedance and low output impedance, so it can drive low-impedance loads from a weak source.

ELE-034 · What is the relation between BJT terminal currents?

I_E = I_B + I_C, with I_C = βI_B in the active region.

ELE-035 · What is transistor biasing, and what is the Q-point?

Biasing sets fixed DC voltages and currents so the transistor operates in the desired region. The Q-point (I_C, V_CE) is the resulting quiescent operating point on the output characteristics.

ELE-036 · What are typical V_BE and V_CE(sat) values for a silicon BJT?

V_BE ≈ 0.6–0.7 V in the active region, and V_CE(sat) ≈ 0.1–0.3 V (commonly taken as 0.2 V) in saturation.

ELE-037 · What is the Early effect?

Base-width modulation: increasing V_CE widens the collector-base depletion region, narrowing the effective base and raising I_C slightly. It gives the BJT a finite output resistance r_o = V_A/I_C.

ELE-038 · How does a BJT work as a switch?

It is driven between cutoff (off, no collector current) and saturation (on, V_CE ≈ 0.2 V). Enough base current is supplied to ensure saturation at the load current.

ELE-039 · What is the difference between NPN and PNP transistors?

In NPN, electrons are the main carriers and current flows into the collector. In PNP, holes are the main carriers and polarities reverse. NPN is generally faster because electron mobility is higher.

FET & MOSFET

ELE-040 · What is a field-effect transistor (FET)?

A voltage-controlled, unipolar transistor in which the gate voltage controls the conductivity of a channel between source and drain. Only majority carriers conduct.

ELE-041 · What is the difference between a JFET and a MOSFET?

A JFET controls the channel with a reverse-biased PN junction gate. A MOSFET has a gate insulated by an oxide layer, so its input impedance is far higher and it can be enhancement or depletion mode.

ELE-042 · What is the difference between enhancement and depletion MOSFETs?

An enhancement MOSFET has no channel at V_GS = 0 and needs V_GS beyond the threshold to conduct (normally off). A depletion MOSFET has a built-in channel and conducts at V_GS = 0 (normally on).

ELE-043 · What is threshold voltage in a MOSFET?

The gate-source voltage V_th at which an inversion channel forms and the device begins to conduct appreciably.

ELE-044 · What are the operating regions of a MOSFET?

Cutoff (V_GS < V_th), triode or linear (V_DS < V_GS − V_th; behaves like a voltage-controlled resistor) and saturation (V_DS ≥ V_GS − V_th; drain current nearly constant, used for amplification).

ELE-045 · What is pinch-off?

The condition where the channel narrows to zero at the drain end, at V_DS = V_GS − V_th in a MOSFET (or V_GS = V_P in a JFET). Beyond it, drain current stays roughly constant.

ELE-046 · Why does a MOSFET have very high input impedance?

The gate is insulated from the channel by a thin oxide layer, so DC gate current is essentially zero (pA range). The gate behaves like a capacitor.

ELE-047 · What is transconductance (g_m)?

The ratio of change in output current to change in input voltage, g_m = ΔI_D/ΔV_GS, measured in siemens. It indicates how effectively the gate controls the drain current.

ELE-048 · What is CMOS?

Complementary MOS logic uses an NMOS pull-down network and a PMOS pull-up network. Only one network conducts in steady state, so static power is very low.

ELE-049 · What is the body diode of a power MOSFET?

A parasitic PN diode between source and drain formed by the device structure. It conducts when the drain is pulled below the source (for NMOS), which is useful in bridges but limits reverse blocking.

ELE-050 · Why are N-channel MOSFETs usually preferred over P-channel?

Electron mobility is about 2–3 times hole mobility, so an N-channel device has lower R_DS(on) and gate charge for the same die area and cost.

ELE-051 · What is R_DS(on)?

The drain-source resistance of a MOSFET when fully turned on at a specified V_GS. It sets conduction loss, P = I²·R_DS(on).

ELE-052 · Why are MOSFETs sensitive to electrostatic discharge (ESD)?

The gate oxide is very thin and breaks down at modest voltages, while the high gate impedance lets static charge build up. Protection diodes and handling precautions are needed.

Amplifiers & Feedback

ELE-053 · What is an amplifier?

A circuit that increases the voltage, current or power of a signal using energy from a DC supply. Gain is the output-to-input ratio.

ELE-054 · How is gain expressed in decibels?

Voltage or current gain in dB = 20 log₁₀(ratio); power gain in dB = 10 log₁₀(ratio). A voltage gain of 100 is 40 dB.

ELE-055 · What is bandwidth of an amplifier?

The frequency range between the lower and upper −3 dB points, where gain falls to 1/√2 (70.7%) of midband gain, i.e. half power.

ELE-056 · What is gain-bandwidth product?

For a single-pole amplifier, the product of gain and bandwidth stays constant. Raising the gain lowers the bandwidth proportionally.

ELE-057 · What is the difference between negative and positive feedback?

Negative feedback returns part of the output out of phase with the input, reducing gain but improving stability and linearity. Positive feedback returns it in phase, increasing gain and used in oscillators and Schmitt triggers.

ELE-058 · What are the four feedback topologies?

Voltage-series (series-shunt), voltage-shunt (shunt-shunt), current-series (series-series) and current-shunt (shunt-series). They differ in whether output voltage or current is sampled and whether feedback is mixed in series or shunt at the input.

ELE-059 · What are coupling and bypass capacitors?

A coupling capacitor passes AC between stages while blocking DC bias. A bypass capacitor shorts an emitter or source resistor to ground at signal frequencies, raising AC gain without disturbing DC bias.

ELE-060 · What is a Darlington pair?

Two BJTs connected so the first's emitter drives the second's base, giving overall β ≈ β₁β₂. The drawback is a V_BE of about 1.4 V and slower switching.

ELE-061 · What is a differential amplifier?

An amplifier that amplifies the difference between two inputs and rejects signals common to both. It is the input stage of almost every op-amp.

ELE-062 · What is a multistage (cascaded) amplifier?

Several amplifier stages connected in series to obtain higher gain. Overall gain is the product of stage gains (sum in dB), while overall bandwidth shrinks.

ELE-063 · What is the difference between a voltage amplifier and a power amplifier?

A voltage amplifier raises signal voltage with small currents and focuses on gain. A power amplifier delivers large current and power to a load (speaker, motor) and focuses on efficiency and heat handling.

ELE-064 · What types of distortion occur in amplifiers?

Harmonic distortion (new multiples of the input frequency), intermodulation distortion (sum and difference products), crossover distortion in class B stages and clipping when the output hits the supply limits.

Op-Amps

ELE-065 · What is an operational amplifier?

A high-gain, DC-coupled differential amplifier with two inputs (inverting and non-inverting) and one output. Its behaviour is set by external feedback components.

ELE-066 · What is the gain of an inverting amplifier?

A_v = −R_f/R_in. The output is inverted, and the input impedance equals R_in.

ELE-067 · What is the gain of a non-inverting amplifier?

A_v = 1 + R_f/R₁. The output is in phase with the input, and the input impedance is very high.

ELE-068 · What is a voltage follower?

A non-inverting amplifier with output tied directly to the inverting input, giving a gain of 1. It buffers a high-impedance source to drive a low-impedance load.

ELE-069 · What is CMRR?

Common-mode rejection ratio, CMRR = A_d/A_cm, usually in dB. It measures how well the op-amp rejects signals common to both inputs; general-purpose op-amps typically exceed 70–90 dB.

ELE-070 · What is slew rate?

The maximum rate of change of the output voltage, in V/µs. The classic 741 has about 0.5 V/µs; exceeding it distorts large fast signals.

ELE-071 · What is input offset voltage?

The small differential DC voltage that must be applied between the inputs to make the output zero. It comes from mismatch in the input transistors.

ELE-072 · What is a summing amplifier?

An inverting amplifier with several input resistors. V_out = −R_f(V₁/R₁ + V₂/R₂ + …); with equal resistors it adds the inputs.

ELE-073 · What is a comparator?

A circuit that compares two voltages and drives its output high or low depending on which is larger. It operates open-loop, without negative feedback.

ELE-074 · What is an op-amp integrator?

An inverting amplifier with a capacitor in the feedback path. V_out = −(1/RC)∫V_in dt; a square-wave input gives a triangular output.

ELE-075 · What is an op-amp differentiator?

An inverting amplifier with a capacitor at the input and resistor in feedback. V_out = −RC·dV_in/dt; it amplifies high-frequency noise, so practical versions add a series resistor.

ELE-076 · What is input bias current?

The average DC current flowing into the two op-amp inputs, needed to bias the input transistors. It is nA for BJT inputs and pA or less for FET inputs.

ELE-077 · What is the typical open-loop gain of an op-amp?

Roughly 10⁵ to 10⁶ (100–120 dB) at DC; the 741 is about 2 × 10⁵. It falls at 20 dB/decade above a low dominant-pole frequency.

Oscillators & Waveform Generators

ELE-078 · What is an oscillator?

A circuit that produces a periodic output (sine, square, triangle) from a DC supply without any external input signal, using positive feedback or a relaxation mechanism.

ELE-079 · How does an oscillator differ from an amplifier?

An amplifier needs an input signal and uses negative feedback for stability. An oscillator generates its own output using positive feedback through a frequency-selective network.

ELE-080 · What is an RC phase-shift oscillator?

An inverting amplifier with three RC sections that together add 180° of phase shift. For equal sections, f = 1/(2πRC√6), and the amplifier gain must be at least 29.

ELE-081 · What is a Wien bridge oscillator?

A sine oscillator using a series-RC and parallel-RC network in positive feedback around a non-inverting amplifier. For equal R and C, f = 1/(2πRC), and the amplifier gain must be 3.

ELE-082 · What are Colpitts and Hartley oscillators?

LC oscillators for RF. Colpitts uses a capacitive divider (two capacitors and one inductor) for feedback; Hartley uses an inductive divider (tapped or two inductors with one capacitor).

ELE-083 · Why are crystal oscillators used?

A quartz crystal has a very high Q (often 10⁴–10⁶) and a stable resonant frequency, giving far better frequency accuracy and stability than RC or LC oscillators.

ELE-084 · What is a 555 timer?

An IC with two comparators, a flip-flop and a discharge transistor, using thresholds at 1/3 and 2/3 of V_CC. It works as an astable, monostable or bistable circuit.

ELE-085 · What is the difference between astable and monostable multivibrators?

An astable multivibrator has no stable state and oscillates continuously. A monostable has one stable state and produces a single pulse of fixed width when triggered.

ELE-086 · What is a Schmitt trigger?

A comparator with positive feedback that has two different switching thresholds (hysteresis). It converts slow or noisy signals into clean digital edges.

ELE-087 · What is a relaxation oscillator?

An oscillator that charges a capacitor until a threshold is reached and then rapidly discharges or reverses it, producing square, sawtooth or triangular waves. The 555 astable is an example.

ELE-088 · What is a voltage-controlled oscillator (VCO)?

An oscillator whose output frequency varies with an applied control voltage. It is used in PLLs, frequency modulation and function generators.

ELE-089 · What is duty cycle?

The fraction of a period for which a pulse waveform is high, D = t_on/T, usually expressed as a percentage. A symmetric square wave has a 50% duty cycle.

Combinational Logic

ELE-090 · What is the difference between combinational and sequential logic?

Combinational logic output depends only on current inputs (adders, multiplexers). Sequential logic output depends on current inputs and stored past state, so it needs memory elements (flip-flops, counters).

ELE-091 · What are universal gates?

NAND and NOR. Any Boolean function can be built using only one of them.

ELE-092 · What does an XOR gate do?

It outputs 1 when its inputs differ, Y = A⊕B = A'B + AB'. It is used in adders, parity circuits and comparators.

ELE-093 · State De Morgan's theorems.

(A·B)' = A' + B' and (A + B)' = A'·B'. A NAND equals an OR with inverted inputs, and a NOR equals an AND with inverted inputs.

ELE-094 · What is a half adder?

A circuit that adds two bits. Sum S = A⊕B and carry C = A·B. It has no carry input.

ELE-095 · What is a full adder?

A circuit that adds three bits (A, B, C_in). S = A⊕B⊕C_in and C_out = AB + C_in(A⊕B).

ELE-096 · What is a multiplexer?

A data selector that routes one of 2ⁿ inputs to a single output using n select lines. It can also implement any Boolean function.

ELE-097 · What is a decoder?

A circuit that converts an n-bit binary input into one active output among up to 2ⁿ lines, used for address decoding and seven-segment drivers.

ELE-098 · What is a priority encoder?

An encoder that outputs the binary code of the highest-priority active input when several inputs are active at once, usually with a "valid" output flag.

ELE-099 · What is a Karnaugh map?

A grid representation of a truth table in Gray-code order, where adjacent cells differ by one variable. Grouping adjacent 1s (or 0s) in powers of two gives a minimal SOP (or POS) expression.

ELE-100 · What is the difference between SOP and POS forms?

Sum of products ORs together AND terms (minterms), e.g. AB + A'C. Product of sums ANDs together OR terms (maxterms), e.g. (A + B)(A' + C).

ELE-101 · What is two's complement?

A signed binary representation where a negative number is formed by inverting all bits and adding 1. An n-bit number ranges from −2ⁿ⁻¹ to 2ⁿ⁻¹ − 1, and addition and subtraction use the same hardware.

ELE-102 · What is Gray code?

A binary code in which successive values differ in only one bit. It avoids multi-bit transition errors in encoders and clock-domain crossings.

Sequential Logic

ELE-103 · What is the difference between a latch and a flip-flop?

A latch is level-sensitive: its output follows the input while enabled. A flip-flop is edge-triggered: it samples the input only at a clock edge.

ELE-104 · What is an SR flip-flop, and what is its invalid state?

A flip-flop with Set and Reset inputs. For a NOR-based SR latch, S = R = 1 is invalid because both outputs go low, and the final state is unpredictable when both inputs are released.

ELE-105 · What is a JK flip-flop?

An improved SR flip-flop where J = K = 1 toggles the output instead of being invalid. J = 1, K = 0 sets; J = 0, K = 1 resets; J = K = 0 holds.

ELE-106 · What is a D flip-flop?

A flip-flop whose output takes the value of the D input at the active clock edge (Q⁺ = D). It is the basic storage element in registers.

ELE-107 · What is a T flip-flop?

A toggle flip-flop: T = 1 inverts the output at each clock edge, T = 0 holds it. It divides the clock frequency by two and is used in counters.

ELE-108 · What are setup time and hold time?

Setup time is the minimum time data must be stable before the clock edge. Hold time is the minimum time it must remain stable after the edge. Violating either can cause metastability.

ELE-109 · What is a shift register?

A chain of flip-flops sharing a clock, where data moves one position per clock. Types include SISO, SIPO, PISO and PIPO, used for serial-parallel conversion and delays.

ELE-110 · What is the difference between synchronous and asynchronous counters?

In a synchronous counter, all flip-flops share the same clock and change together. In an asynchronous (ripple) counter, each flip-flop is clocked by the previous one's output, so delays accumulate.

ELE-111 · How many flip-flops does a mod-N counter need?

At least ⌈log₂N⌉. A mod-10 (decade) counter needs 4 flip-flops.

ELE-112 · What is the difference between ring and Johnson counters?

A ring counter circulates a single 1 and has n states with n flip-flops. A Johnson (twisted-ring) counter feeds back the inverted output and has 2n states.

ELE-113 · What is the difference between Moore and Mealy machines?

In a Moore machine, outputs depend only on the current state. In a Mealy machine, outputs depend on the current state and inputs, so it often needs fewer states and responds a cycle sooner.

ELE-114 · What is the difference between level and edge triggering?

Level triggering responds while the clock (enable) is at a given level, as in latches. Edge triggering responds only at the rising or falling transition, as in flip-flops.

Power Supplies & Regulators

ELE-115 · What are the blocks of a linear DC power supply?

Transformer (step-down and isolation), rectifier, filter capacitor and voltage regulator.

ELE-116 · What is ripple factor?

The ratio of the RMS value of the AC component to the DC value of the output. Without a filter, it is 1.21 for a half-wave and 0.482 for a full-wave rectifier.

ELE-117 · What is voltage regulation?

The ability of a supply to keep output voltage constant. Load regulation (%) = (V_NL − V_FL)/V_FL × 100; lower is better.

ELE-118 · What is the difference between line and load regulation?

Line regulation is the output change for a given input voltage change. Load regulation is the output change for a given load current change.

ELE-119 · What is a linear voltage regulator?

A regulator that controls a series pass transistor in its linear region to drop excess voltage, e.g. the 78xx series. It is simple and quiet but dissipates (V_in − V_out)·I as heat.

ELE-120 · What is dropout voltage, and what is an LDO?

Dropout voltage is the minimum V_in − V_out needed to stay in regulation, about 2 V for a 7805. A low-dropout (LDO) regulator uses a PNP or MOSFET pass element to work with a dropout of a few hundred mV or less.

ELE-121 · What is a switch-mode power supply (SMPS)?

A supply that rapidly switches a transistor on and off and uses inductors, capacitors or transformers to store and transfer energy. It achieves high efficiency at the cost of switching noise.

ELE-122 · What are buck, boost and buck-boost converters?

A buck converter steps voltage down, a boost converter steps it up, and a buck-boost converter can do either (the basic inverting type gives an output of opposite polarity).

ELE-123 · What does the filter capacitor do in a rectifier circuit?

It charges to the peak voltage and supplies load current between peaks, smoothing the pulsating DC and reducing ripple.

ELE-124 · What is a voltage reference?

A circuit that provides a precise, stable voltage independent of supply and temperature, such as a Zener or a bandgap reference (about 1.2 V). It sets accuracy for regulators and ADCs.

ELE-125 · What is the maximum efficiency of a linear regulator?

Approximately V_out/V_in, since the same current flows at input and output. A 12 V to 5 V linear regulator is at best about 42% efficient.

ELE-126 · What is current limiting in a power supply?

Protection that restricts output current during overload or short circuit. Foldback limiting reduces the current further as output voltage falls, which cuts pass-transistor dissipation.

Filters & Passive Networks

ELE-127 · What is the difference between active and passive components?

Active components can control current or provide power gain (transistors, ICs). Passive components cannot (resistors, inductors, capacitors).

ELE-128 · What are the basic types of filters?

Low-pass (passes below cutoff), high-pass (passes above cutoff), band-pass (passes a band) and band-stop or notch (rejects a band).

ELE-129 · What is the cutoff frequency of an RC filter?

f_c = 1/(2πRC). At this frequency, output is 3 dB below the passband and the phase shift is 45°.

ELE-130 · What is the time constant of an RC circuit?

τ = RC, in seconds. A capacitor charges to about 63.2% of its final value in one τ and is considered fully charged after about 5τ.

ELE-131 · What are capacitive and inductive reactance?

X_C = 1/(2πfC) falls with frequency, and X_L = 2πfL rises with frequency. Both are measured in ohms.

ELE-132 · What is the resonant frequency of an LC circuit?

f₀ = 1/(2π√(LC)), the frequency where X_L = X_C.

ELE-133 · What is the quality factor (Q)?

A measure of sharpness of resonance or of energy stored versus energy lost per cycle. For a resonant circuit, Q = f₀/BW.

ELE-134 · How do series and parallel resonance differ?

At series resonance, impedance is minimum (equal to R) and current is maximum. At parallel resonance, impedance is maximum and line current is minimum.

ELE-135 · What is the difference between passive and active filters?

Passive filters use only R, L and C and cannot provide gain. Active filters add op-amps or transistors to provide gain, buffering and inductor-free designs, but need a power supply and have limited bandwidth.

ELE-136 · State Thevenin's theorem.

Any linear two-terminal network can be replaced by a voltage source V_th (the open-circuit voltage) in series with a resistance R_th (seen from the terminals with independent sources set to zero).

ELE-137 · State Norton's theorem.

Any linear two-terminal network can be replaced by a current source I_N (the short-circuit current) in parallel with R_N, where R_N = R_th and I_N = V_th/R_th.

ELE-138 · State the maximum power transfer theorem.

Maximum power reaches a resistive load when R_L equals the source's Thevenin resistance. Efficiency at that point is only 50%. For AC, the load should be the complex conjugate of the source impedance.

Practical Electronics, PCB & Measurement

ELE-139 · How do you read a four-band resistor colour code?

The first two bands are digits, the third is the multiplier and the fourth is tolerance (gold ±5%, silver ±10%). Brown-black-red-gold is 10 × 10² = 1 kΩ ±5%.

ELE-140 · What are E-series resistor values?

Standard preferred value sets per decade, such as E12 (12 values, ±10%), E24 (±5%) and E96 (±1%). Values are spaced logarithmically so tolerance bands roughly overlap.

ELE-141 · What is a printed circuit board (PCB)?

An insulating substrate (commonly FR-4) with copper traces, pads and planes that mechanically supports and electrically connects components. Boards may have one, two or many layers.

ELE-142 · What is a via?

A plated hole that connects copper on different PCB layers. Types include through-hole, blind and buried vias.

ELE-143 · What is a decoupling capacitor?

A capacitor (often 100 nF ceramic) placed close to an IC's supply pins that supplies fast transient current and shunts high-frequency noise to ground.

ELE-144 · What is an oscilloscope used for?

To display voltage versus time, so you can measure amplitude, frequency, rise time, noise and timing relationships. Key specifications are bandwidth, sample rate and memory depth.

ELE-145 · How do you connect a multimeter to measure voltage and current?

Voltage is measured in parallel across the component. Current is measured in series by breaking the circuit and inserting the meter.

ELE-146 · What is the difference between through-hole and SMD components?

Through-hole parts have leads inserted through drilled holes, which makes them robust and easy to hand solder. Surface-mount devices sit on pads, are smaller, suit automated assembly and have lower parasitics.

ELE-147 · What is a ground plane?

A large continuous copper area connected to ground. It provides a low-impedance return path, reduces loop area and EMI, and helps heat spreading.

ELE-148 · What is a cold (dry) solder joint?

A joint where solder did not properly wet the pad or lead, usually due to insufficient heat or contamination. It looks dull or grainy and causes intermittent or high-resistance connections.

ELE-149 · What are pull-up and pull-down resistors?

Resistors that tie a signal line weakly to V_CC (pull-up) or ground (pull-down), so the input has a defined logic level when nothing actively drives it.

ELE-150 · What is a 10× oscilloscope probe?

A passive probe that attenuates the signal 10:1 and presents about 10 MΩ with low capacitance to the circuit. Loading is lower and bandwidth higher than with a 1× probe.

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