Electronics Hardware Engineer - Interview Questions - 25

 

537. Why does a capacitor block DC but allow AC to pass?

 • A capacitor consists of two plates separated by a dielectric.

 • For DC (constant voltage):

 • When DC is applied, the capacitor charges up to the supply voltage.

 • Once fully charged, no current flows (except a tiny leakage), so it blocks DC.

 • For AC (changing voltage):

 • The voltage across the capacitor is continuously changing.

 • The capacitor charges and discharges in step with the AC signal.

 • This creates a displacement current, so the capacitor effectively allows AC to pass.

Mathematically:

X_C = {1}/(2 pi f C)

 • At f = 0 (DC) → X_C = infinity → acts like an open circuit.

 • At high f (AC) → X_C = 0 → acts like a short circuit.

538. Why is source degeneration used in analog amplifiers?

• Adding a resistor in series with the source of a MOSFET (or emitter of a BJT) provides negative feedback.

 • This stabilizes gain, improves linearity, and reduces sensitivity to process variations.

 • The trade-off: reduced overall voltage gain.

539. In high-speed digital design, why is controlled impedance important for PCB traces?

 • At high frequencies, traces behave like transmission lines.

 • If impedance is not matched, signals reflect, causing ringing, overshoot, and timing errors.

 • Controlled impedance (e.g., 50 Ω or 100 Ω differential) ensures signal integrity.

540. Why do we prefer differential signaling (like LVDS, USB, PCIe) instead of single-ended at high speed?

• Differential pairs reject common-mode noise, reduce EMI, and allow higher speeds with lower voltage swings.

 • They provide better signal integrity over long interconnects compared to single-ended signals.

541. What happens if you don’t meet the Nyquist sampling criterion in ADCs?

 • If the sampling rate is less than twice the maximum frequency of the signal, aliasing occurs.

 • Higher frequency components fold back into lower frequencies, corrupting the original signal.

 • Solution: Use an anti-aliasing filter before the ADC.

542. Why does a capacitor in a DC power supply circuit reduce voltage ripple?

 • In a rectifier circuit, the output after diodes is pulsating DC (not smooth).

 • When a filter capacitor is placed across the output:

 • During the peaks of the rectified waveform, the capacitor charges up.

 • During the valleys, when the input voltage drops, the capacitor discharges slowly, supplying current to the load.

 • This fills in the gaps between pulses, reducing ripple and producing smoother DC.

The effectiveness depends on:

V_{ripple}= {I}/(fC)

 • I = load current

 • f = frequency of rectified signal

 • C = capacitance

So, a larger capacitor or higher frequency → less ripple.

Credits to Shivraj dharne for these questions:
https://www.linkedin.com/in/shivraj-dharne-345398b/