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Q: Can I use an op-amp as a comparator?

Generally no - unless you're in a very specific, slow-speed, low-differential-voltage situation and you've tested thoroughly. Most op-amps have internal input clamp diodes that conduct at ±0.7V differential, slow saturation recovery, and outputs incompatible with logic levels. For any production design, use a dedicated comparator.

Q: What happens if I use an op-amp as a comparator at high speed?

The output will be slow, possibly wrong, and the part may be damaged over time. The compensation capacitor slows recovery from saturation, input diodes may conduct, and some op-amps draw excessive current when the output is railed. In a fault-detection circuit, the op-amp will be 10-100× slower than a comparator.

Q: LM358 or LM393 - which one for battery-low detection?

LM393 (comparator) - faster, cleaner switching, and the open-collector output directly drives an MCU GPIO or LED. The LM358 can do the job in a pinch since it lacks input clamp diodes, but its saturation recovery is slower. For production, use the LM393.

Q: Why do comparators have open-collector outputs?

Flexibility. You pull up to whatever logic voltage you need - 1.8V, 3.3V, 5V, 12V - with one resistor. Multiple open-collector outputs can also be wired-OR together, making window comparators trivial with a single Power-Good signal.

Q: Do I need hysteresis on a comparator?

Almost always yes - if your input signal changes slowly or has any noise. Without hysteresis, a comparator with a slow-moving input will oscillate at the transition point. Add positive feedback with two resistors. The hysteresis window should be 2-3× larger than your expected peak-to-peak noise.

Q: What's the difference between LMV393 and LM393 - which comparator should I stock?

LMV393 for low-voltage (2.7-5.5V) and low-power (50µA). LM393 for standard (2-36V) and higher voltage. If your designs span both 3.3V and 12V rails, stock both. The LM393DR covers the widest range of applications as a general-purpose comparator.

Q: Can a comparator be used as an op-amp?

No - a comparator will oscillate if you try to close a negative feedback loop around it. Comparators are intentionally uncompensated for maximum switching speed. Without the internal Miller capacitor that makes op-amps stable, a comparator in closed-loop operation becomes an oscillator, not an amplifier.

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Comparator vs Op-Amp — Full Comparison & Selection Guide

2026/7/30 11:26:24

Comparator vs Op-Amp - Full Comparison & Selection Guide | ICMASS

A comparator tells you which of two voltages is bigger and gives a clean logic-level answer. An op-amp amplifies the difference between two voltages and gives you a linear output proportional to that difference. Same triangle symbol in the schematic. Completely different job.

Need a digital high/low decision - undervoltage lockout, zero-crossing, window comparator? Use a comparator. Need amplification, filtering, buffering? Use an op-amp. Using an op-amp as a comparator is the most common analog shortcut that comes back to bite you in production.

Comparator vs Op-Amp: Side-by-Side Comparison

Parameter	Comparator	Op-Amp
Primary function	Compare two voltages, output logic HIGH/LOW	Amplify voltage difference, output linear proportional signal
Operation mode	Open-loop (no negative feedback). Hysteresis via positive feedback	Closed-loop (negative feedback sets gain)
Output type	Open-collector/drain or push-pull; saturates to rail	Push-pull, linear; swings between rails but stays in active region
Output states	Binary: HIGH or LOW	Analog: any voltage between the rails
Switching speed	200ns–1µs (propagation delay)	1–20µs (recovery from saturation)
Internal compensation	No Miller capacitor - maximized for speed	Miller compensation capacitor - ensures closed-loop stability
Differential input range	Wide: can withstand large V_IN+ − V_IN−	Often limited: back-to-back protection diodes clamp at ~±0.7V
Input protection diodes	Generally none between inputs	Common on bipolar and many CMOS op-amps (check datasheet)
Phase reversal risk	Low; designed to avoid it	Present on many non-RRIO op-amps when inputs exceed CM range
Hysteresis	Built-in on some; easy to add with positive feedback	Requires positive feedback (making it a comparator in practice)
Logic-level compatible	Yes - open-collector pulls to any logic rail	No - output swings to analog rails; needs level translation
Supply current (typ)	50µA–500µA (low-power comparators)	50µA–2mA (varies widely; general-purpose ~0.5–1mA/ch)
Key datasheet specs	Propagation delay, overdrive, hysteresis, output sink current	GBW, slew rate, phase margin, V_OS, CMRR, PSRR
Typical gain	200V/mV (open-loop, but irrelevant - it's always saturated)	100–130dB open-loop (used closed-loop at set gain)
Price (relative)	Comparable; dedicated comparators often slightly cheaper	Comparable; general-purpose op-amps among cheapest ICs
Pinout (dual SOIC-8)	Same industry-standard pinout as dual op-amp	Same industry-standard pinout as dual comparator

The pinout is identical. So why different parts? The silicon is optimized for opposite goals. An op-amp's compensation capacitor gives stability with feedback - but slow saturation recovery. A comparator skips it: blazing fast switching, but oscillates if you close a feedback loop around it.

Key Differences (Why You Can't Just Swap Them)

1. Output Stage: Open-Collector vs Push-Pull Linear

Comparators use open-collector (or open-drain) outputs. The output pin can only pull LOW - it needs an external pull-up resistor to go HIGH. This lets you pull up to any voltage: 3.3V logic, 5V logic, 12V, even 24V for driving MOSFET gates directly. One comparator, any logic level.

Op-amps use push-pull outputs designed for linear operation. They source and sink current, but aren't optimized for slamming rail-to-rail at speed. When the output saturates, internal transistors take microseconds to recover - by which time your overcurrent protection should have already tripped.

2. Speed: Propagation Delay vs Saturation Recovery

Switching/Recovery Time (ns, lower is faster)

LMV393 Comparator 200–600ns

TLV3501 High-Speed Comparator 4.5ns

LM358 Op-Amp (saturation recovery) 2–15µs

A comparator is designed to switch and recover in nanoseconds. An op-amp, when driven into saturation, takes 2–15µs to recover - 10–100× slower. For a 100kHz SMPS fault-detection circuit, 2µs is the difference between a protected MOSFET and a dead one.

3. Input Stage: Differential Voltage Tolerance

Many op-amps have internal back-to-back protection diodes between IN+ and IN−. In closed-loop operation, the inputs sit at nearly the same voltage - diodes never conduct. In open-loop comparator use, a large differential forward-biases them, causing cross-input current and potential damage. Comparators omit these diodes, allowing large differential voltages safely.

4. When You Actually Can Use an Op-Amp as a Comparator

There's one legitimate scenario: you have a spare op-amp channel in a quad package, your comparison speed is measured in milliseconds, the differential input voltage stays under ±0.3V, and adding a separate comparator IC would mean a BOM line and PCB spin.

In that case, use an LM358 or LM324 - they lack input clamp diodes. Add hysteresis. Test across temperature. And never do this with a precision op-amp like the OPA227 or NE5532 - the input diodes will destroy signal integrity and possibly the part.

When to Choose a Comparator

Any speed-sensitive threshold detection. Undervoltage lockout, overcurrent protection, zero-crossing detection. Every microsecond of delay is more stress on your power stage.
Digital logic interface needed. The open-collector output pulls directly to your logic rail - 1.8V, 3.3V, 5V - with one pull-up resistor. No level translation circuit needed.
Window comparator. Two comparators, one package, wired-OR outputs. Tells you whether a signal is inside or outside a voltage window with a single logic signal.
Large input differential expected. Any circuit where the two inputs might be volts apart in normal operation.
Relaxation oscillator or PWM generator. Comparators switch cleanly with hysteresis - the LM393/LMV393 is the standard building block for this.

When to Choose an Op-Amp

Amplification, filtering, buffering, integration. Anywhere you need a linear relationship between input and output.
Closed-loop control. Error amplifiers in power supplies, PID controllers, current-sense amplifiers - all need the linear gain and phase margin that only an op-amp provides.
Sensor signal conditioning. Thermocouple amplifiers, photodiode transimpedance amps, strain gauge bridges - you're amplifying microvolts, not comparing them.
Audio and precision AC. Active filters, summing amplifiers, balanced line drivers - all rely on the op-amp's linearity, not its saturation behavior.
Impedance buffering. Voltage followers between high-Z sources and low-Z loads need the op-amp's feedback-stabilized unity gain.

Recommended Parts

Category	Model	Key Spec	Best For
Low-Voltage Comparator	LMV393IDR	2.7–5.5V, 50µA, 200–600ns	Battery-powered threshold detection, 3.3V MCU window comparator
Standard Comparator	LM393DR	2–36V, 400µA, 300ns–1.5µs	12V/24V industrial, automotive, line-powered
General-Purpose Op-Amp	LM358DR	3–32V, 0.8mA, 1MHz GBW, 0.5V/µs	DC/low-frequency signal conditioning, unity-gain buffer
Low-Voltage RRIO Op-Amp	LMV358IDR	2.7–5.5V, 210µA, 1MHz GBW, RRIO	3.3V/5V single-supply near-rail signals
Low-Voltage Comparator w/ Protection	LM393LVDR	1.65–5.5V, 35µA, 600ns, phase-reversal protection	New designs where inputs may go negative

Frequently Asked Questions

Q1: Can I use an op-amp as a comparator?

A: Generally no - not unless you're in a specific, slow, low-differential-voltage situation and you've tested thoroughly. Most op-amps have input clamp diodes (conduction at ±0.7V), slow saturation recovery, and logic-incompatible outputs. The LM358/LM324 lack input diodes but are still slower than real comparators. For production, use a dedicated comparator.

Q2: Why do comparators and op-amps use the same schematic symbol?

A: Historical convention - they share the same differential-input, high-gain architecture. But the internal silicon is optimized differently. Think of it like a pickup truck and a race car: both have an engine and four wheels, but you wouldn't use one for the other's job.

Q3: What happens if I use an op-amp as a comparator at high speed?

A: Slow output, possible wrong state, potential cumulative damage. The compensation capacitor delays saturation recovery. Input diodes may conduct. Some op-amps draw excessive current when railed. In fault detection, the op-amp is 10–100× slower than a comparator - by the time it trips, the FET is already dead.

Q4: LM358 or LM393 - which one for battery-low detection?

A: LM393 (comparator). Faster switching, and the open-collector output drives an MCU GPIO or LED directly. The LM358 works in a pinch since it lacks input clamp diodes, but saturation recovery is slower. For a battery-low indicator with a spare LM358 already on the board, it's acceptable. For production, use the LM393.

Q5: Why do comparators have open-collector outputs?

A: Flexibility. One pull-up resistor sets any logic level - 1.8V, 3.3V, 5V, 12V. Multiple open-collector outputs wire-OR naturally: any comparator pulling low pulls the shared node low. For a window comparator, tie both outputs to one pull-up - single Power-Good signal, no extra logic.

Q6: Do I need hysteresis on a comparator?

A: Almost always - if your signal changes slowly or has noise. Without hysteresis, a comparator with a slow input oscillates at the transition, producing a burst of HF pulses instead of a clean edge. Add positive feedback: a resistor from output to non-inverting input. Set the hysteresis window 2–3× larger than your peak-to-peak noise.

Q7: What's the difference between LMV393 and LM393 - which comparator should I stock?

A: LMV393 for low-voltage (2.7–5.5V, 50µA). LM393 for standard (2–36V). If your designs span both 3.3V and 12V rails, stock both. The LM393DR covers the widest range. The LMV393IDR is right when battery life or 3.3V-only operation matters.

Q8: Can a comparator be used as an op-amp?

A: No. A comparator oscillates if you close a negative feedback loop around it. Comparators are intentionally uncompensated for maximum speed. Without the Miller capacitor that stabilizes op-amps, a comparator in closed-loop mode becomes an oscillator - not an amplifier. The two parts are not interchangeable in either direction.