Interview questions & answers
Q1. What is an ideal op-amp and how does it differ from a real op-amp?
An ideal op-amp has infinite open-loop gain (AOL = ∞), infinite input impedance, zero output impedance, infinite bandwidth, and zero offset voltage, allowing the virtual short and virtual open approximations to be used exactly. A real LM741 has AOL = 200,000, input impedance = 2 MΩ, output impedance = 75 Ω, GBW = 1 MHz, and input offset voltage up to 6 mV. These non-idealities cause gain error in precision circuits, output loading when driving low-impedance loads, and frequency-dependent closed-loop behavior that must be accounted for in filter and control loop design.
Follow-up: What is the virtual short circuit approximation and when can it be applied?
Q2. Derive the gain formula for an inverting op-amp amplifier.
With virtual short: V- ≈ V+ = 0 (since V+ is grounded). By KCL at the inverting input node (which draws no current): (Vin - 0)/R1 = (0 - Vout)/Rf, giving Av = -Rf/R1. For a circuit with R1 = 10 kΩ and Rf = 100 kΩ using an LM358, Av = -10, so a 100 mV input gives -1 V output. The negative sign indicates phase inversion; the gain depends only on the resistor ratio as long as the op-amp maintains virtual short, which requires adequate open-loop gain at the operating frequency.
Follow-up: What limits the accuracy of the gain at high frequencies?
Q3. What is the gain-bandwidth product (GBW) of an op-amp and how do you use it?
GBW is the constant product of closed-loop gain and -3 dB bandwidth for a unity-gain compensated op-amp: GBW = Av × BW = constant. The LM741 has GBW = 1 MHz, so configured as a ×100 amplifier it has only 10 kHz bandwidth; the TL071 with GBW = 3 MHz gives 30 kHz at the same gain. When designing an audio preamplifier needing 40 dB gain (×100) and 20 kHz bandwidth, you need an op-amp with GBW ≥ 2 MHz minimum, explaining why the OP27 (GBW = 8 MHz) or NE5534 (GBW = 10 MHz) are used for quality audio front-ends.
Follow-up: Why does the closed-loop gain roll off at 20 dB/decade above the -3 dB frequency?
Q4. What is slew rate and how does it cause distortion?
Slew rate is the maximum rate of change of output voltage (dVout/dt, in V/µs) of an op-amp, limited by the internal compensation capacitor charging current. The LM741 has SR = 0.5 V/µs; driving a 10 Vpp sine wave at 10 kHz requires dV/dt_max = π × 10 V × 10 kHz = 314 kV/s = 0.314 V/µs, within the LM741's limit, but at 30 kHz (0.94 V/µs required) the output becomes a triangular wave rather than a sine. This slew-rate distortion is why audio op-amps like the NE5534 specify SR = 13 V/µs to handle 20 kHz full-scale signals without distortion.
Follow-up: What determines the slew rate of an op-amp internally?
Q5. Explain the working of an op-amp integrator and its practical limitations.
An op-amp integrator replaces the feedback resistor with a capacitor (C) and input resistor (R), giving Vout = -(1/RC) ∫Vin dt; with R = 10 kΩ and C = 100 nF (RC = 1 ms), a 1 V step input produces a ramp falling at 1 V/ms. In practice, even a few µV of op-amp input offset voltage is integrated over time, saturating the output to one of the supply rails within seconds; a 1 MΩ feedback resistor in parallel with the capacitor limits DC gain to -100 and prevents saturation, at the cost of reducing integration accuracy at low frequencies.
Follow-up: How does a practical integrator circuit prevent output saturation due to offset voltage?
Q6. What is the difference between an op-amp comparator and a dedicated comparator IC like the LM339?
A general-purpose op-amp used as a comparator has slow slew rate (LM741: 0.5 V/µs) and is not designed for output to rail-to-rail swings, causing slow switching and possible output not reaching logic levels. The LM339 dedicated comparator switches in 1.3 µs with an open-collector output that can interface directly to any logic supply voltage, unlike the LM741 output which is limited to the supply rails. Using an LM741 as a comparator driving 3.3 V logic from a ±15 V supply produces a ±13 V output that would destroy the logic IC without a voltage divider or level shifter.
Follow-up: Why does an op-amp with feedback become unstable when used as a comparator?
Q7. Explain the virtual ground concept in an inverting op-amp amplifier.
Virtual ground means the inverting input of an op-amp in a closed-loop inverting configuration is maintained at approximately 0 V (if non-inverting is at GND) by the negative feedback action, even though it is not physically connected to ground. The LM358 with Rf = 100 kΩ, R1 = 10 kΩ and Vin = 1 V maintains V- = 0.0001 V (essentially 0 V) because any deviation causes a large Vout change that corrects it. This virtual ground allows a summing amplifier to independently sum multiple input voltages without interaction — each input 'sees' ground at the summing node, so adding more inputs doesn't affect other channels.
Follow-up: What happens to the virtual ground accuracy if the op-amp's open-loop gain is finite?
Q8. What is common-mode rejection ratio (CMRR) and why is it important?
CMRR = 20 log(Adiff/Acm) is the ratio of differential gain to common-mode gain, measuring how well the op-amp rejects noise present on both inputs simultaneously. The OP07 has CMRR = 120 dB, meaning a 1 V common-mode noise signal (like 50 Hz hum on both sensor leads) appears as only 1 µV equivalent differential signal at the output. In an ECG amplifier where the patient's 50 Hz interference (1–2 V) vastly exceeds the mV-level cardiac signal, the INA128 instrumentation amplifier's 120 dB CMRR at 60 Hz is essential for usable measurements.
Follow-up: How does CMRR degrade with frequency?
Q9. What is the difference between a non-inverting amplifier and a voltage follower?
A non-inverting amplifier has Rf connected from output to inverting input and Rg from inverting input to ground, giving Av = 1 + Rf/Rg; a voltage follower is the special case where Rf = 0 and Rg = ∞ (output shorted to inverting input), giving Av = 1. The TL071 voltage follower has output impedance < 1 Ω and input impedance > 1 TΩ (FET input), making it ideal as a buffer between a high-impedance piezoelectric sensor and a low-impedance ADC input. Both configurations use the full open-loop gain to enforce the virtual short, but the follower has maximum feedback and hence maximum stability and bandwidth.
Follow-up: Why does a voltage follower have better stability than a high-gain non-inverting amplifier with the same op-amp?
Q10. What is an instrumentation amplifier and how does it differ from a simple differential amplifier?
An instrumentation amplifier (IA) uses three op-amps in a specific topology where the gain is set by a single resistor, input impedance is extremely high (no loading on source), and CMRR is maximized by the balanced buffered input stage. The INA128 has Rin > 10 GΩ differential input, CMRR > 120 dB, and gain set by a single RG: G = 1 + 50kΩ/RG from 1 to 10,000. A simple differential op-amp amplifier has finite input impedance (R1 typically kΩ range) that loads sensors and degrades CMRR if resistor matching is imperfect, making it unsuitable for strain gauge bridges and biosignal acquisition.
Follow-up: Why must the four resistors in a differential amplifier be precision matched?
Q11. What is input offset voltage in an op-amp and how do you minimize its effect?
Input offset voltage (Vos) is the small differential DC voltage that must be applied between the inputs to make the output zero, caused by transistor mismatches in the differential input stage. The LM741 has Vos up to 6 mV, so with gain = 100 the output has a 600 mV DC error even with no signal. The OP07 reduces Vos to 25 µV, the OPA2188 to 5 µV using auto-zeroing techniques, and the LM741 includes trimming pins to null Vos using a 10 kΩ potentiometer — essential in DC-coupled instrumentation chains where DC accuracy matters.
Follow-up: What is the difference between input offset voltage and input offset current?
Q12. Design a Sallen-Key low-pass filter with 1 kHz cutoff frequency using an op-amp.
A 2nd-order Butterworth Sallen-Key LPF with unity gain uses C1 = C2 = 10 nF and R1 = R2 = 1/(2π × fc × C) = 1/(2π × 1000 × 10nF) = 15.9 kΩ (use 16 kΩ), configured with a TL072 op-amp in voltage follower mode at the output. The Q factor for a Butterworth response is 0.707, achieved automatically with equal R and C values in the unity-gain configuration. Active filters using this topology avoid inductors, are easily tunable, provide signal gain, and achieve sharp rolloff in a small PCB area — advantages that make them ubiquitous in audio and sensor signal conditioning.
Follow-up: How would you increase the filter order to get steeper rolloff?
Q13. What is phase margin in an op-amp feedback circuit and why must it be >45°?
Phase margin is the additional phase shift from -180° at the frequency where the loop gain magnitude equals 0 dB, and it must be positive (>0°) for stability; below 45° the system shows ringing and overshoot in step response. An LM741 unity-gain compensated op-amp has 45° phase margin at unity gain, giving ~30% overshoot; the OP27 specified for gains ≥ 5 has only 20° phase margin at unity gain (use only as specified). Capacitive loads reduce phase margin by adding a pole — a 100 pF capacitive load on an LM358 can cause oscillation, which is corrected with a series 100 Ω 'snubber' resistor in the output.
Follow-up: How do you compensate an op-amp that oscillates when driving a capacitive load?
Q14. What is a Schmitt trigger built with an op-amp and what problem does it solve?
A Schmitt trigger is a comparator with positive feedback that adds hysteresis — two distinct switching thresholds (VTH and VTL) — preventing multiple output transitions when a noisy input crosses the threshold. A LM358 Schmitt trigger with R1 = 100 kΩ, R2 = 10 kΩ, and Vref = 2.5 V creates VTH = 2.5 + 0.1×Vout_high and VTL = 2.5 - 0.1×Vout_high, giving ±0.5 V hysteresis at 5 V output. Without hysteresis, a slowly changing or noisy sensor input (like a temperature sensor near threshold) causes the comparator output to chatter at high frequency, falsely triggering interrupt routines in microcontroller applications.
Follow-up: How do you calculate the hysteresis band of an op-amp Schmitt trigger?
Q15. What is an op-amp differentiator and why is it rarely used in practice?
An op-amp differentiator has an input capacitor and feedback resistor (Vout = -RC × dVin/dt), producing an output proportional to the input's rate of change, but its gain increases with frequency (Gain = -jωRC) making it amplify high-frequency noise without bound. With R = 10 kΩ and C = 100 nF, the differentiator gain at 1 kHz is 6.3 and at 1 MHz it is 6283 — any tiny high-frequency noise on the input is amplified to saturation. In practice, a series resistor Rin at the capacitor limits the maximum gain to -Rf/Rin at high frequencies, and the resulting practical differentiator is only used in very specialized control and waveform generation circuits.
Follow-up: How does adding a series resistor at the input of a differentiator improve its stability?
Common misconceptions
Misconception: The output of an ideal op-amp in open loop swings to exactly the supply voltage.
Correct: Even ideal op-amp analysis uses ±∞ supply by convention; real op-amps like the LM741 typically swing to within 1–2 V of the supply rails, while rail-to-rail output op-amps like the MCP6001 can reach within millivolts of either rail.
Misconception: A voltage follower (unity gain buffer) has no bandwidth limitation.
Correct: A voltage follower has the widest bandwidth of any closed-loop op-amp configuration (bandwidth = GBW product), but still has a finite -3 dB frequency equal to the GBW itself, plus slew-rate limiting at large signal amplitudes.
Misconception: Virtual ground means the inverting input is physically connected to ground.
Correct: Virtual ground is maintained by negative feedback action — the inverting input is held near ground by the op-amp's high gain driving the output to whatever voltage keeps V+ ≈ V-, not by any physical ground connection.
Misconception: Higher open-loop gain always means better op-amp performance.
Correct: Excessively high open-loop gain combined with phase shifts from stray capacitance can cause oscillation; op-amp stability depends on controlled gain rolloff (dominant pole compensation), not maximum open-loop gain.