Comparator Symbol
Definition: The Comparator symbol represents an analog circuit element — drawn as a triangle (op-amp outline) with a non-inverting input (In+), an inverting input (In−), and a single digital-level output — used in schematics to denote a device that compares two voltages and drives its output high or low depending on which input is larger, as defined under IEC 60617 and ANSI/IEEE 315.
Also known as: voltage comparator, analog comparator, lm311 symbol, lm339 symbol, op-amp comparator.
What the Comparator symbol means
The comparator symbol denotes an integrated circuit or subcircuit whose sole function is to determine which of two analog input voltages is greater and to output a logic-level signal accordingly. When the voltage at In+ exceeds the voltage at In−, the output is driven to the high state; when In− exceeds In+, the output is driven low. The comparator is therefore the bridge between the analog and digital domains in a circuit.
Unlike an operational amplifier used with feedback, a comparator is optimised for open-loop operation: its output switches decisively between two rail voltages without settling to an intermediate level. Common comparator ICs include the LM311 (single), LM393 (dual), and LM339 (quad), all of which share the same schematic symbol convention.
How to identify the Comparator symbol
The comparator symbol is an equilateral triangle pointing to the right, identical in outline to an operational amplifier symbol. The non-inverting input (In+, marked with a '+' or '+' label) enters the left side of the triangle closer to the top, and the inverting input (In−, marked with a '−' or '−' label) enters the left side closer to the bottom. The output pin exits from the apex on the right. Unlike a full op-amp symbol, the comparator symbol typically omits explicit power supply pins (V+ / V−) in schematic shorthand, though those pins physically exist on the IC.
Function in a circuit
A comparator continuously monitors the voltage difference between its two inputs. When the non-inverting input (In+) voltage rises above the inverting input (In−) voltage, the output switches to the high state (logic 1, near the positive rail). When In− exceeds In+, the output switches to the low state (logic 0, near the negative rail or ground). The output transition is very fast — typically nanoseconds to microseconds — because the comparator operates without the slew-limiting feedback network used in amplifier circuits. An open-collector or open-drain output variant (used in the LM393/LM339) allows multiple comparators to be wire-OR'd together and requires an external pull-up resistor.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-13 (analogue elements) depicts the comparator using the standard amplifier triangle with '+' and '−' input annotations. The output symbol may include an active-low bubble if the output is inverted. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 uses the same right-pointing triangle with non-inverting and inverting input markers. The open-collector output is indicated by a small circle at the output pin tip per IEEE 91/IEEE 315. |
| Key difference | The triangle glyph is identical in IEC and ANSI/IEEE representations; the primary difference is that IEC documents may include explicit power-supply pin stubs on the triangle, while ANSI/IEEE schematics typically suppress them in favour of a cleaner symbol. |
Terminals / pins
| Pin | Name |
|---|---|
| in_pos | In+ |
| in_neg | In- |
| out | Out |
Typical values
Supply voltage: typically 3.3 V to 36 V (single-supply) or ±15 V (split-supply). Response time (propagation delay): 1 ns–1 µs depending on part. Input offset voltage: < 1 mV (precision types) to ~5 mV (general purpose). Open-collector sink current: up to 50 mA. Hysteresis (if Schmitt feedback applied): designer-specified, commonly 10 mV–500 mV.
Where the Comparator symbol is used
- Zero-crossing detection — an AC waveform is compared against a 0 V reference to produce a digital pulse at each zero crossing for phase-locked loops or power metering.
- Threshold / window detection — battery management systems use a comparator to trigger a cutoff relay when cell voltage drops below a set threshold.
- PWM generation — a sawtooth or triangle wave is compared against a DC control voltage to produce a pulse-width-modulated output for motor speed control.
- Analog-to-digital conversion — flash ADCs use banks of comparators (one per quantisation level) to convert an input voltage to a binary code in a single clock cycle.
- Overcurrent protection — a sense resistor voltage is compared to a reference; when the drop exceeds the threshold the comparator output triggers a latch or MOSFET gate to cut power.
- Schmitt trigger implementation — positive feedback from the comparator output to the non-inverting input creates hysteresis, converting a slowly-changing or noisy signal into a clean digital edge.
Example
In a 12 V lead-acid battery monitor, In− is connected to a precision 10.5 V reference voltage divider and In+ is connected to the battery terminal through a resistor divider. When the battery discharges below 10.5 V, In+ drops below In−, the LM393 open-collector output pulls low through a 10 kΩ pull-up, and that logic-low signal activates a warning LED and disables the load relay — all represented in the schematic using the standard comparator triangle symbol.
Key facts
- The Comparator symbol is a right-pointing equilateral triangle with a non-inverting input (In+) at the upper-left, an inverting input (In−) at the lower-left, and an output at the right apex; power supply pins are typically suppressed in schematic shorthand.
- A comparator differs from an op-amp in that it is designed for open-loop switching, not linear amplification; its output is a two-state digital signal, not a proportional analog voltage.
- Common comparator ICs include the LM311 (single, 200 ns), LM393 (dual, open-collector, 1.3 µs), and LM339 (quad, open-collector, 1.3 µs); all share the same schematic symbol.
- An open-collector comparator output requires an external pull-up resistor (typically 1 kΩ–100 kΩ) to the desired logic supply voltage; without it the output floats in the high state.
- Hysteresis can be added to a comparator by connecting a resistor from the output back to the non-inverting input (In+), creating a Schmitt trigger that prevents output chatter on noisy signals.
- The schematic designator for a comparator is U (general IC) or sometimes A; the circuit is represented under IEC 60617-13 (analogue elements) and ANSI/IEEE 315-1975.
- Pins on this symbol: In+ (non-inverting input, x=0 y=15), In− (inverting input, x=0 y=35), Out (output, x=60 y=25).
- The comparator symbol is the primary functional block in flash ADC topologies, where 2^N − 1 comparators are arranged in parallel to achieve single-cycle conversion at very high sample rates.
Frequently asked questions
What does the comparator symbol look like in a schematic?
The comparator symbol is a right-pointing triangle (identical to an op-amp outline) with two inputs on the left side — the non-inverting input (In+) near the top and the inverting input (In−) near the bottom — and one output at the right apex. Power supply connections are usually omitted from the drawing for clarity.
What does the comparator symbol mean in a circuit diagram?
The comparator symbol indicates a device that compares two analog voltage levels and produces a binary output. If In+ is higher than In−, the output is high (logic 1); if In− is higher than In+, the output is low (logic 0). It marks the interface between the analog and digital sections of a circuit.
How is a comparator symbol different from an op-amp symbol?
Both use the same triangle glyph, but context and annotation distinguish them. A comparator symbol rarely shows feedback resistors and its output is labelled as a logic-level or open-collector node. An op-amp symbol typically appears with feedback components forming a defined closed-loop gain. Some schematics add an explicit 'COMP' label beneath the triangle to remove ambiguity.
What is the IEC vs ANSI difference for the comparator symbol?
The comparator symbol is essentially identical under IEC 60617-13 and ANSI Y32.2 / IEEE 315-1975: both use a right-pointing triangle with '+' and '−' input markers. IEC notation may include explicit supply-pin stubs on the triangle body, while ANSI/IEEE schematics typically suppress power pins for a cleaner diagram.
What is an open-collector comparator output and why does it need a pull-up resistor?
An open-collector output (used by LM393 and LM339) has an internal transistor that can only pull the output pin to ground (logic 0); it cannot actively drive the output high. An external pull-up resistor connected to the supply rail is required so the output rises to the high logic level when the transistor is off. Without the pull-up, the high state is undefined.
What designator letter is used for a comparator on a schematic?
The reference designator for a comparator IC is U (universal integrated circuit designator) per ANSI/IEEE 315. In some European and legacy drawings the designator A (amplifier) is used. The schematic value label typically shows the part number (e.g. U1 LM339).
How do I add hysteresis to a comparator symbol in a circuit?
Hysteresis is added by connecting a positive-feedback resistor from the comparator output back to the non-inverting input (In+). This creates two different threshold voltages — one for the rising edge and one for the falling edge — preventing output chatter on slowly-changing or noisy input signals. The circuit is then called a Schmitt trigger and the hysteresis band is set by the ratio of the feedback resistor to the reference divider resistors.
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