Tricky Electronics interview questions

75 questions with answers (ELE-426 to ELE-500). Try each one aloud before reading the answer.

Semiconductor Physics

ELE-426 · "Holes are positive particles that physically travel through the crystal." True?

Trap: Treating holes as real particles. A hole is a missing electron in a covalent bond; it is really valence electrons that move in the opposite direction. The hole is a useful quasi-particle with positive charge and its own effective mass.

ELE-427 · "N-type silicon carries a net negative charge because it has extra electrons." True?

Trap: Confusing carriers with net charge. Each free electron comes from a neutral donor atom that becomes a positive ion, so the material stays electrically neutral overall.

ELE-428 · "Heavier doping always gives better carriers, so mobility rises with doping." True?

Trap: Confusing conductivity with mobility. Heavier doping raises carrier concentration and conductivity, but ionized-impurity scattering lowers mobility.

ELE-429 · "A semiconductor's resistance always falls as temperature rises." True?

Trap: Applying intrinsic behaviour to doped material. In the extrinsic range, carrier density is fixed by doping while mobility falls, so the resistance of doped silicon rises with temperature. It falls only when intrinsic generation dominates at high temperature.

ELE-430 · "The depletion region contains no charge, so it is just an insulator." True?

Trap: Confusing "no mobile carriers" with "no charge". The depletion region holds fixed ionized donor and acceptor charges, which create the built-in field and the junction capacitance.

ELE-431 · "Germanium's lower cut-in voltage makes it better than silicon for modern ICs." True?

Trap: Looking only at forward drop. Germanium's small band gap gives high leakage and poor high-temperature behaviour, and it lacks a stable native oxide. Silicon dominates for these reasons.

Diodes & Rectifiers

ELE-432 · "Why does an LED need a series resistor if it only needs 2 V?"

Trap: Treating the LED like a fixed-voltage load. Its I–V curve is exponential, so a tiny voltage change causes a huge current change and V_F varies with temperature. A resistor or constant-current driver must set the current.

ELE-433 · "A silicon diode always drops exactly 0.7 V." True?

Trap: Treating a model value as a constant. V_F rises about 60 mV per decade of current, falls about 2 mV/°C and varies by type: around 0.5 V at microamps and over 1 V at high current.

ELE-434 · "A reverse-biased diode carries zero current." True?

Trap: Ignoring leakage. Reverse saturation and generation currents flow (nA for Si, much more for Schottky and at high temperature), and the diode conducts heavily once breakdown is reached.

ELE-435 · "A Zener is just an ordinary diode used backwards." True?

Trap: Ignoring construction. Zeners are doped and specified for controlled, repeatable breakdown at a set voltage and power. An ordinary rectifier's breakdown is high, poorly specified and not designed to be used continuously.

ELE-436 · "A full-wave rectifier gives pure DC." True?

Trap: Confusing unidirectional with constant. The output is pulsating DC with large ripple at twice the mains frequency (ripple factor 0.482) until it is filtered and regulated.

ELE-437 · "A bigger reservoir capacitor is always better." True?

Trap: Looking only at ripple. A larger capacitor shortens the conduction angle, so peak diode currents, inrush, transformer heating and harmonic currents rise and power factor falls. Size it for the required ripple, not as large as possible.

ELE-438 · "Paralleling two identical diodes doubles the current rating." True?

Trap: Ignoring thermal behaviour. V_F falls with temperature, so the hotter diode takes more current and heats further (current hogging). Use ballast resistors, a single larger diode or matched, thermally coupled parts.

BJT

ELE-439 · "Can you make a transistor from two diodes back-to-back?"

Trap: Assuming topology alone makes a transistor. Transistor action needs a thin, lightly doped shared base so carriers injected by the emitter cross it to the collector. Two discrete diodes have no such base, so they give no current gain.

ELE-440 · "β is a fixed number from the datasheet, so design around it." True?

Trap: Trusting β as constant. It varies widely between units, with collector current and with temperature. Good designs (emitter degeneration, feedback) make performance nearly independent of β.

ELE-441 · "A BJT is purely current-controlled by its base current." True?

Trap: Mistaking a model for the physics. Collector current is set by V_BE: I_C = Iₛ·e^(V_BE/V_T). Base current is a by-product, though the current-controlled model is a useful approximation.

ELE-442 · "Emitter and collector are interchangeable since both are the same type of material." True?

Trap: Ignoring doping and geometry. The emitter is heavily doped and the collector is lightly doped and larger. In reverse-active mode, β is very low and breakdown ratings differ.

ELE-443 · "In saturation, a BJT carries its maximum collector current." True?

Trap: Confusing saturation with current limit. Saturation means both junctions are forward-biased and V_CE is minimal; I_C is set by the external circuit and is below β·I_B.

ELE-444 · "A higher-β transistor is always the better choice." True?

Trap: Ignoring trade-offs. High-β parts often have lower breakdown voltage and Early voltage and more leakage, and β spread is large. Circuits should not depend on high β.

FET & MOSFET

ELE-445 · "A MOSFET gate draws no current, so it needs no real drive." True?

Trap: Confusing DC with transient behaviour. The gate is a capacitor; switching it fast takes amps of peak current (I ≈ Q_g/t). Weak drive means slow edges, high switching loss and overheating.

ELE-446 · "At V_GS = V_th the MOSFET is fully on." True?

Trap: Misreading the datasheet. V_th is where the device just begins conducting, often defined at about 250 µA. R_DS(on) is specified at much higher V_GS (e.g. 4.5 V or 10 V).

ELE-447 · "Saturation means the same for MOSFETs and BJTs." True?

Trap: Confusing terminology. MOSFET saturation is the constant-current region used for amplification. BJT saturation is the fully-on switch state, closer to the MOSFET triode region.

ELE-448 · "An off MOSFET blocks current in both directions." True?

Trap: Forgetting the body diode. A discrete power MOSFET blocks only one polarity. Reverse voltage forward-biases the intrinsic body diode, so bidirectional blocking needs back-to-back MOSFETs.

ELE-449 · "MOSFETs cannot have thermal runaway because R_DS(on) has a positive temperature coefficient." True?

Trap: Applying fully-on behaviour to linear mode. At low V_GS below the zero-temperature-coefficient point, drain current increases with temperature. Hot spots can form in linear operation (e.g. electronic loads, hot-swap), so check the DC SOA.

ELE-450 · "A floating MOSFET gate is fine because the gate is high impedance." True?

Trap: High impedance makes floating worse. Stray charge and noise can partly turn the device on, causing heating, oscillation or shoot-through in CMOS inputs. Always use a gate pull-down or a defined drive.

Amplifiers & Feedback

ELE-451 · "Negative feedback always makes an amplifier stable." True?

Trap: Ignoring phase. If loop phase shift reaches 180° while loop gain is still at least 1, negative feedback becomes positive and the amplifier oscillates. Compensation provides phase margin.

ELE-452 · "Positive feedback always produces oscillation." True?

Trap: Overgeneralising. Positive feedback is used for hysteresis in Schmitt triggers, latches and bootstrapping. Sustained sinusoidal oscillation needs the Barkhausen conditions at a frequency selected by the network.

ELE-453 · "A unity-gain buffer is useless because it has no gain." True?

Trap: Thinking only of voltage gain. A buffer gives large current and power gain and converts a high source impedance to a low output impedance, preventing loading.

ELE-454 · "A Class A amplifier is 50% efficient, so it wastes half its power." True?

Trap: Quoting the wrong maximum. With a resistive load, maximum efficiency is 25% (50% only when transformer- or inductor-coupled), and only at full output. At typical lower signal levels it is far less, since quiescent dissipation is constant.

ELE-455 · "Voltage gain in dB always equals power gain in dB." True?

Trap: Ignoring impedances. 20 log(V_out/V_in) equals 10 log(P_out/P_in) only when input and output impedances are equal.

ELE-456 · "Cascading more stages increases gain and bandwidth." True?

Trap: Assuming bandwidth adds. Gain multiplies, but the overall bandwidth shrinks; n identical stages give f_H·√(2^(1/n) − 1).

Op-Amps

ELE-457 · "If gain is infinite, why doesn't the op-amp output go to infinity?"

Trap: Forgetting the supply and the loop. The output cannot exceed the supply rails and saturates near them. With negative feedback, the loop drives the input difference to nearly zero, so the closed-loop gain is set by the feedback network.

ELE-458 · "A virtual short means the two inputs are physically connected." True?

Trap: Confusing equal voltage with a short. The inputs sit at nearly the same potential because of feedback, but almost no current flows between them.

ELE-459 · "V+ = V− also applies when an op-amp is used as a comparator." True?

Trap: Applying the virtual-short rule without negative feedback. In open-loop or positive-feedback circuits, the output saturates and the inputs can differ widely.

ELE-460 · "Op-amp inputs draw no current, so an AC-coupled input needs no resistor to ground." True?

Trap: Ignoring bias current. Bias current needs a DC return path. Without one, it charges the coupling capacitor until the output drifts to a rail.

ELE-461 · "A rail-to-rail output reaches exactly the supply rails." True?

Trap: Taking the label literally. The output gets within millivolts at light load but tens to hundreds of millivolts away under heavier load, as shown in the datasheet's output swing specification.

ELE-462 · "A unity-gain-stable op-amp is stable with any load." True?

Trap: Ignoring load poles. A capacitive load with the output resistance adds a pole that can remove phase margin, causing ringing or oscillation even at unity gain.

ELE-463 · "An inverting amplifier with R_f = R_in has gain 1, so its bandwidth equals GBW." True?

Trap: Using signal gain instead of noise gain. Bandwidth depends on noise gain 1 + R_f/R_in = 2, so bandwidth ≈ GBW/2.

Oscillators & Waveform Generators

ELE-464 · "Loop gain must be exactly 1 for an oscillator to start." True?

Trap: Confusing start-up and steady state. Start-up needs loop gain slightly above 1 so noise grows. Amplitude limiting then reduces it to exactly 1 in steady state; exactly 1 from the start would never reliably begin.

ELE-465 · "An oscillator creates a signal without consuming energy from its input." True?

Trap: Thinking "no input" means no energy. An oscillator converts DC supply power into an AC signal and replaces losses in the resonator each cycle.

ELE-466 · "Crystal oscillators never drift." True?

Trap: Assuming perfect stability. Crystals drift with temperature, ageing (ppm per year), load capacitance and drive level. TCXOs and OCXOs are used when tighter stability is needed.

ELE-467 · "Make R_A = R_B in a basic 555 astable to get 50% duty." True?

Trap: Misreading the formula. Duty = (R_A + R_B)/(R_A + 2R_B), so R_A = R_B gives about 67%. R_A cannot be zero, since it would short the supply through the discharge transistor. Use a diode across R_B or a different topology.

ELE-468 · "Any amplifier with positive feedback is an oscillator." True?

Trap: Ignoring frequency selection. Without a frequency-selective network, strong positive feedback simply latches the output to a rail, as in a bistable or Schmitt trigger.

ELE-469 · "A square wave from an oscillator contains only its fundamental frequency." True?

Trap: Ignoring harmonics. A symmetric square wave contains the fundamental plus all odd harmonics with amplitudes falling as 1/n. Fast edges carry high-frequency energy that matters for EMI.

Combinational Logic

ELE-470 · "NAND is AND plus an inverter, so it must be slower than AND." True?

Trap: Assuming the symbol shows the structure. In CMOS, NAND and NOR are the native single-stage gates; an AND gate is a NAND followed by an inverter and is slower.

ELE-471 · "XOR is a universal gate." True?

Trap: Confusing usefulness with completeness. XOR gates alone (even with constant 1 to make NOT) cannot produce AND or OR, so XOR is not functionally complete. Only NAND and NOR are universal on their own.

ELE-472 · "Combinational circuits have no timing problems because they have no clock." True?

Trap: Ignoring propagation delay. Unequal path delays cause glitches and hazards, and the longest path limits system clock speed when outputs are registered.

ELE-473 · "Unused CMOS inputs can be left floating." True?

Trap: Treating CMOS like TTL. Floating CMOS inputs drift to mid-rail, turning on both transistors and causing excess current, oscillation and erratic logic. Tie them to V_DD or ground.

ELE-474 · "An 8-bit two's complement number ranges from −127 to +127." True?

Trap: Assuming symmetry. The range is −128 to +127, because zero has a single representation. Negating −128 overflows.

ELE-475 · "A decoder and a demultiplexer are completely different circuits." True?

Trap: Missing the equivalence. A decoder with an enable input acts as a demultiplexer: the enable becomes the data input and the address lines select the output.

Sequential Logic

ELE-476 · "Latches and flip-flops can be swapped freely." True?

Trap: Ignoring transparency. A latch passes changes while enabled, so data can race through several stages in one phase. Timing analysis and design differ, and latch-based design needs careful clocking.

ELE-477 · "Slowing the clock fixes all timing violations." True?

Trap: Treating hold like setup. A longer clock period fixes setup violations only. Hold violations depend on minimum path delay and skew and must be fixed by adding delay.

ELE-478 · "A two-flip-flop synchronizer eliminates metastability." True?

Trap: Thinking a probabilistic problem can be removed. Synchronizers reduce the failure rate exponentially, raising MTBF, but never to zero; very fast clocks may need more stages.

ELE-479 · "Ripple counter outputs can be decoded directly for any purpose." True?

Trap: Ignoring ripple delay. Outputs change one after another, creating brief false states that produce glitches in decoding logic. Use a synchronous counter or register the decoded result.

ELE-480 · "An n-bit Johnson counter has 2ⁿ states." True?

Trap: Confusing it with a binary counter. A Johnson counter has 2n valid states; a 4-bit one counts 8 states, not 16, and the unused states must be prevented or self-corrected.

ELE-481 · "Gray code is used because it counts faster than binary." True?

Trap: Misunderstanding its purpose. Gray code is used because only one bit changes per step. This prevents multi-bit sampling errors in encoders and clock-domain crossings; it does not count faster.

Power Supplies & Regulators

ELE-482 · "A 7805 can produce 5 V from a 5.5 V input." True?

Trap: Ignoring dropout. The 7805 needs about 2 V headroom, so roughly 7 V or more in. For 5.5 V in, use an LDO with dropout below 0.5 V at the load current.

ELE-483 · "An SMPS rated 92% efficient is 92% efficient at every load." True?

Trap: Quoting peak efficiency. Fixed losses (gate drive, quiescent current, core loss) dominate at light load, so efficiency falls sharply. Light-load modes like burst or pulse skipping mitigate it.

ELE-484 · "A buck converter's input current equals its output current." True?

Trap: Thinking like a linear regulator. Power is conserved, so average input current ≈ I_out·V_out/(V_in·η), lower than output current. The input current is pulsed, though, so input capacitors see high RMS ripple.

ELE-485 · "Higher switching frequency always gives a better converter." True?

Trap: Looking only at size. Higher frequency shrinks magnetics but raises switching, gate-drive and core losses, worsens EMI and narrows the duty-cycle range because of minimum on-times.

ELE-486 · "For any LDO, the lower the output capacitor ESR the better." True?

Trap: Ignoring stability requirements. Many older LDOs need ESR within a window to create a stabilising zero, and very low-ESR ceramics can make them oscillate. Check the datasheet; modern parts are ceramic-stable.

ELE-487 · "Linear regulators reject all input ripple and noise." True?

Trap: Assuming PSRR is flat. PSRR is high at low frequency but falls at high frequency as loop gain drops, so SMPS switching noise can pass through. Add filtering or choose high-PSRR LDOs.

Filters & Passive Networks

ELE-488 · "Does a capacitor block DC?"

Trap: Ignoring transients and leakage. A capacitor blocks DC only in steady state; current flows while it charges or discharges, and real capacitors leak.

ELE-489 · "Ohm's law applies to every component, right?"

Trap: Treating V = IR as universal. It holds only for ohmic (linear) materials at constant temperature. Diodes, transistors, gas discharges and filament lamps are nonlinear; for them, R = V/I varies with operating point.

ELE-490 · "An inductor passes DC with no loss." True?

Trap: Assuming ideal behaviour. Real inductors have winding resistance (DCR) causing I²R loss, plus core loss with ripple. Their inductance collapses when the core saturates at high current.

ELE-491 · "At the cutoff frequency, output voltage is half the input." True?

Trap: Confusing half-power with half-voltage. At −3 dB, voltage is 1/√2 ≈ 70.7% of passband, and the power is half.

ELE-492 · "Cascading two identical RC low-pass sections gives a second-order Butterworth with the same f_c." True?

Trap: Ignoring loading and Q. Even buffered, two real poles give Q = 0.5 and −6 dB at the original f_c. Unbuffered, loading lowers Q to about 1/3. A Butterworth response needs Q = 0.707, which requires an active or LC design.

ELE-493 · "At resonance, a series RLC circuit has zero impedance." True?

Trap: Forgetting resistance. The reactances cancel, leaving impedance equal to R. The current is maximum but finite, and voltages across L and C can be Q times the source.

ELE-494 · "A 100 nF capacitor is 100 nF at all frequencies and voltages." True?

Trap: Ignoring parasitics and dielectrics. Above self-resonance, ESL makes it inductive. Class II ceramics (X7R, X5R) lose much of their capacitance under DC bias and change with temperature and age.

Practical Electronics, PCB & Measurement

ELE-495 · "Ground is zero volts everywhere on a board." True?

Trap: Treating ground as ideal. Ground conductors have resistance and inductance, so return currents create voltage differences (ground bounce, shifts). Layout must control return paths.

ELE-496 · "A 100 MHz oscilloscope accurately measures 100 MHz signals." True?

Trap: Misreading bandwidth. Bandwidth is the −3 dB point, so a 100 MHz sine reads about 30% low and edges are slowed. Use 3–5 times the signal frequency.

ELE-497 · "Any multimeter's AC reading is the true RMS value." True?

Trap: Assuming all meters are true-RMS. Average-responding meters are calibrated for sine waves and misread square waves, PWM or distorted mains. Use a true-RMS meter within its crest factor and bandwidth limits.

ELE-498 · "To measure current, place the multimeter across the component." True?

Trap: Confusing current and voltage measurement. An ammeter has very low resistance; across a component, it shorts the circuit and can blow its fuse. Current must be measured in series.

ELE-499 · "Clipping the scope ground lead onto any node of a mains circuit is safe." True?

Trap: Forgetting the scope ground is earthed. Connecting it to a non-earth-referenced node creates a short through the scope ground, which can destroy equipment and endanger the user. Use a differential probe or an isolation transformer on the device under test.

ELE-500 · "A wide, thick trace solves all signal-integrity problems." True?

Trap: Ignoring the return path. High-frequency current returns directly beneath the signal on the reference plane; gaps or splits in that plane create large loops, crosstalk and EMI regardless of trace width.

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