Top 20 Analog Design Interview Questions (With Answers)

Analog design interviews are known for testing both fundamentals and practical problem-solving. Whether you’re preparing for placements or industry roles, these 20 questions will give you a strong starting point.

MOSFET & Transistor Basics

1. What is the difference between saturation and linear region in MOSFETs?
   In the linear region, the MOSFET behaves like a resistor. In saturation, it acts as a current source controlled by Vgs.
2. What happens to MOSFET current when Vgs < Vth?
   The MOSFET stays OFF, and only leakage current flows.
3. Why is channel length modulation important?
   It causes the drain current to slightly increase with Vds even in saturation, reducing output resistance.

Amplifiers & Op-Amps

4. What is the difference between open-loop and closed-loop gain?
   Open-loop gain is the op-amp’s raw gain (very high). Closed-loop gain is controlled using external feedback resistors.
5. Explain CMRR and its importance.
   Common-Mode Rejection Ratio measures how well an op-amp rejects signals common to both inputs. High CMRR is critical for differential measurements.
6. How does slew rate limit op-amp performance?
   Slew rate defines how fast output can change (V/μs). It limits response to fast signals.
7. What is the trade-off between gain and bandwidth?
   Higher gain reduces bandwidth due to the constant gain-bandwidth product (GBW).

Biasing & Current Mirrors

8. Why do we use current mirrors?
   To replicate and bias circuits with stable currents, improving performance and saving area.
9. What is the Early effect?
   In BJTs/MOSFETs, as Vds increases, the effective channel length reduces, increasing current slightly.
10. How does degeneration improve linearity?
    Adding source/emitter resistance stabilizes gain, improves linearity, and reduces sensitivity to parameter variations.

Frequency Response & Stability

11. How do you find poles and zeros of a circuit?
    By analyzing transfer function or small-signal equivalent. Poles indicate roll-off, zeros can boost or cancel response.
12. Why do we use Miller compensation?
    To stabilize multi-stage amplifiers by pushing dominant pole lower and separating poles.
13. What is phase margin and why does it matter?
    Phase margin indicates how far system is from oscillation. A 60° margin is considered stable.

Data Converters & Practical Circuits

14. What is quantization error in ADCs?
    The error due to finite resolution (±0.5 LSB).
15. Explain DNL and INL.
    DNL = step size error between adjacent codes. INL = deviation of actual transfer curve from ideal.
16. How do you calculate ENOB from SINAD?
    ENOB = (SINAD – 1.76)/6.02
17. What is jitter and how does it affect ADC performance?
    Timing uncertainty in clock edges reduces SNR, especially at high frequencies.

Practical Design & Testing

18. How do you measure offset voltage of an op-amp?
    Short inputs to ground and measure output. Divide by closed-loop gain for input offset.
19. Why is layout critical in analog design?
    Parasitics, mismatches, and coupling can dominate performance if layout isn’t optimized.
20. What is the difference between corner and Monte Carlo simulations?
    Corner → checks worst-case process corners. Monte Carlo → runs many random variations to check yield and mismatch.

Conclusion

These 20 questions cover the core areas most analog interviews test: MOSFETs, op-amps, current mirrors, frequency response, and data converters. Mastering these fundamentals will give you confidence in placements and job interviews.