Analog & Op-Amp Interview Questions

Op-amp questions test whether you can derive, not recite. Start every answer from the three rules of negative feedback, then add the non-ideal effects that matter for the case in front of you.

Before you start

What you should already be comfortable with:

• KCL, KVL and Thevenin/Norton equivalents
• Impedance of R, L and C (jωL, 1/jωC) and reading a Bode plot
• How a BJT and a MOSFET behave in their active regions
• RC time constants and first-order filters

What interviewers are really testing

• Whether you can derive an op-amp circuit from the three rules instead of reciting a formula
• Where a real op-amp stops being ideal: gain-bandwidth, noise gain, offset, bias current, output swing, slew rate
• Whether you can keep a loop stable with a capacitive load or a photodiode at the input
• Putting numbers on noise — √(4kTR), voltage noise vs current noise
• Turning a sensor or ADC problem into the few specs that actually matter

Start with these two — free full answers

Study order

Work through the stages in order. ● screening · ●● onsite technical · ●●● design / senior. Pages marked "Going deeper" add explanation that is not in the book; the two marked "Free answer" show the full answer; every page tells you where the full answer is in the book.

1. Foundations
   • AN-01 — The three op-amp rules ● Free answer
   • AN-13 — Series RLC driven by a current ramp ●
   • AN-15 — Frequency response of L, C and LC tanks ●
   • AN-14 — Thevenin and a black box ●●
2. Op-amp stages
   • AN-02 — Inverting gain of −10, non-ideal ● Free answer
   • AN-03 — Inverting vs non-inverting ● Going deeper
   • AN-05 — Op-amp circuit gamut ●● Going deeper
   • AN-04 — Current source and current sink ●● Going deeper
3. Real-world limits
   • AN-10 — Op-amp noise sources ●
   • AN-09 — Op-amp driving an ADC oscillates ●●
   • AN-08 — Photodiode TIA with a 10 MΩ feedback resistor ●●
4. Design problems
   • AN-06 — AC-coupled BJT amplifier ●●
   • AN-11 — NTC front end for a battery pack ●●●
   • AN-07 — Amplify 1 µV at 10 MHz to 1 V ●●●
   • AN-12 — PID control ●●
   • AN-16 — Three ways to dim an LED ●

Related questions in other chapters

• DC-01 — Square wave into RC filters ●
• DC-02 — Anti-aliasing filter ●●
• DC-07 — Thermal and Johnson noise in numbers ●
• PC-06 — Zener in a divider ●

The book

The book has all 16 questions in this chapter with the full answer: how to explain it on a whiteboard, the numbers worth quoting, and the follow-ups that come next.

More topics

PCB Design Interview Questions · Power Electronics Interview Questions · Signal Integrity Interview Questions · Embedded Hardware Interview Questions · Power Integrity Interview Questions