What Is the LM311? Flexible Comparator: Pinout, Emitter/Collector Output and Uses
The LM311 is a single, flexible voltage comparator whose output transistor collector and emitter are both brought out to separate pins, plus a strobe pin. This enables level shifting, tying the output to different voltages and disabling it; its response time of ~200 ns is faster than the LM393. Its output can sink ~50 mA and can drive an LED or a relay driver stage directly.
The LM311 is a single, highly flexible voltage comparator. Unlike the LM393/LM339, both the collector and the emitter of its output transistor are brought out to separate pins, and it has a strobe pin. This enables level shifting to different voltages, push-pull-like driving and temporarily disabling the output. For the concept, see what is a comparator.
What is the LM311 used for?
Precise threshold detection, zero-crossing detectors, level shifting (e.g. adapting an analog threshold to a logic family), oscillators and one-shot circuits. Because the output can sink up to ~50 mA, it can drive small loads (an LED, a relay driver stage) directly.
LM311 pinout (DIP-8)
| Pin | Function |
|---|---|
| 1 | Output emitter / GND |
| 2 | Input + (non-inverting) |
| 3 | Input − (inverting) |
| 4 | V− (VEE) |
| 5 | Balance |
| 6 | Balance / Strobe |
| 7 | Output collector |
| 8 | V+ (VCC) |
Key electrical specifications
| Parameter | Typical value |
|---|---|
| Type | Single comparator (flexible output) |
| Supply | Single 5 V or dual ±15 V (total ≤ 36 V) |
| Output type | Separate collector + emitter (open) |
| Output current | up to ~50 mA |
| Response time | ~200 ns (faster than the LM393) |
Why the LM311 (instead of the LM393)?
Separate collector and emitter give the flexibility to tie the output to a different reference/voltage and to level-shift; the strobe pin disables the output on command; response time is faster. For a simple threshold the LM393 is enough; when you need flexibility/speed, pick the LM311.
Limitations
- The output is open, so (depending on use) a pull-up/load resistor is needed.
- Not a linear amplifier — use an op-amp (e.g. the LM358) for amplification.
- Without hysteresis it can chatter on noisy transitions.
LM311 equivalents and alternatives
- LM393: dual, simple and cheap comparator.
- LM339: quad comparator.
- TLV3501: a much faster (sub-ns) comparator.
Find it in our catalog
To compare price and stock for the LM311 and its alternatives, use the LM311 search.