If you're tired of placing an NPN BJT + two discrete resistors every time you need a simple low-side switch or level translator in your 3.3 V / 5 V logic circuits, meet the MMUN2211LT1G (from onsemi) — one of the most popular single-package Bias Resistor Transistors (BRT) or digital transistors.
This little SOT-23 part integrates everything you need for many "digital" switching jobs.
Quick Specs at a Glance
- Type: NPN pre-biased (digital / BRT)
- Vce(max): 50 V
- Ic(max): 100 mA
- Built-in resistors: R1 (base) = 10 kΩ, R2 (base-emitter) = 10 kΩ → ratio 1:1
- Package: SOT-23-3 (super common, ~2.9 × 1.3 mm)
- Power dissipation: up to ~246 mW (depending on PCB heatsinking)
- VBE(on) typical: around 0.8–1.0 V at low current
- Price: usually $0.01–$0.03 @ 1k+ qty (2026 pricing)
In short: apply > ~1.4–1.8 V to the base pin → transistor turns on cleanly, no external resistors needed.
Typical Use Cases (Where You'll See It Most)
MCU / GPIO → LED / small load switching
GPIO high (3.3 V or 5 V) → directly drives base → sinks up to ~100 mA through collector.Open-drain / open-collector level shifting
Pulls a 5 V or 12 V line down to ground when driven from 3.3 V logic.Relay / buzzer / small motor driver pre-stage (with external power BJT/MOSFET)
I²C / SMBus pull-up disabling or bus isolation switches
Replacing 2N3904 + 2× 10k resistors in space-constrained consumer, IoT, and automotive designs
Schematic example (very common LED driver):
3.3V logic GPIO ───┬───[MMUN2211LT1G base pin]
│
[10k internal]
│
[10k internal]─── emitter ─── GND
│
collector ─── LED ─── 330 Ω ─── +5V
No extra parts — cleaner BOM, smaller layout, fewer solder joints.
Pinout (SOT-23-3 looking from top, flat side facing you)
1 = Base (input)
2 = Emitter (GND)
3 = Collector (output)
(Most datasheets label it this way — double-check onsemi's marking "A8A" on top.)
Pros vs Classic Discrete Approach
- Saves 2 discrete resistors → lower BOM cost & count
- Reduces PCB area (one 3-pin device vs three)
- Less pick-and-place time during assembly
- Very consistent turn-on threshold (factory-matched resistors)
- AEC-Q101 qualified variants available (SMMUN2211LT1G)
Gotchas / When NOT to Use It
- Only 100 mA max collector current — don't try to switch >80–90 mA continuously
- Fixed 10k/10k ratio — if you need different gain or threshold, go discrete or choose another BRT family member (MMUN22xx series has many ratios)
- Not suitable for high-frequency switching (> few hundred kHz) — internal resistors add some delay
- For negative logic or PNP side, look at MMUN21xx series
Common Cross / Equivalent Parts
- DTC114E / DTC114EE (Rohm, very similar 10k/10k)
- PDTC114ET (Nexperia)
- KRC111 / KRC111S (KEC)
- Many Chinese clones with marking "A8A" or "114"
Bottom Line
If your next board has 5–20 places where an MCU pin just needs to sink moderate current or pull something low, MMUN2211LT1G (or one of its siblings) will almost always win on cost, space, and simplicity.
Stock is excellent at DigiKey, Mouser, LCSC — usually under 2¢ in reel quantities.
Have you used this family in a recent project? Which BRT ratio do you reach for most often — 10k/10k, 4.7k/47k, or something else?
Drop a comment — happy to discuss alternatives or draw more application circuits!
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