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What a 2N3906 PNP transistor does
PNP transistor (2N3906) — The NPN’s upside-down twin: it switches the + side, and turns on when its base is pulled low.
What it is
A PNP transistor is the NPN's mirror twin. Same job: a small current at one leg lets a much bigger current flow between the other two. Same three legs, same little black half-cylinder. Hold the flat face toward you, legs down: left to right they are Emitter, Base, Collector, the same order as the 2N2222.
The difference is which way everything points. In an NPN the big current flows in at the collector and out at the emitter, and you turn it on by lifting the base above the emitter. In a PNP the big current flows in at the emitter and out at the collector, and you turn it on by pulling the base below the emitter. So a PNP's emitter goes to battery +, and the load hangs off its collector down to −.
Why own both? An NPN switches the − side of a load; a PNP switches the + side. Put one of each together and you can push a signal either way, which is how motor drivers, H-bridges and the output stage of nearly every chip work. The 2N3906 is the PNP that comes in the same assortment bag as the 2N2222 and 2N3904.
How it works
The 0.7 V rule, upside down. The emitter-to-base junction is a diode. When the base sits about 0.7 V below the emitter, it conducts and the transistor turns on. With the emitter on a 4.5 V rail that means the base must be pulled down to about 3.8 V. Base at 4.5 V (same as the emitter): off.
Gain. Collector current is roughly base current times the gain (hFE, or β), about 100 to 300 for a 2N3906; the site's simulations use 200. To pass 11 mA to an LED you need only 11 ÷ 200 ≈ 0.055 mA out of the base.
Pulling the base down. A 10 kΩ from the base to − (through a button or a sensor) carries 3.8 V ÷ 10 000 Ω ≈ 0.38 mA once the junction clamps the base at 3.8 V. A 10 kΩ pull-up from + to the base supplies (4.5 − 3.8) ÷ 10 000 ≈ 0.07 mA of that. The rest, about 0.31 mA, is base current. Times 200 is 62 mA of possible collector current, far more than the 11 mA the LED lane allows, so the transistor is saturated: fully on, about 0.1 V from emitter to collector.
Load current. Collector at about 4.4 V, LED lane of 220 Ω + red LED (2.0 V) to −: (4.4 − 2.0) ÷ 220 ≈ 10.9 mA. A buzzer (≈ 150 Ω) on the collector instead: (4.4 − 0) ÷ 150 ≈ 29 mA. Both well inside the 200 mA rating.
The pull-up matters. With nothing pulling the base down, the pull-up resistor holds it at the emitter voltage and the transistor is cleanly off. Leave the base floating and it will pick up stray voltage and flicker.
Limits. 200 mA collector current (a third of the 2N2222's), 40 V emitter-collector, 625 mW. At 4.5 V and LED currents you are nowhere near any of them.
The numbers
| Pinout (flat face toward you) | E – B – C — emitter to +, load on the collector |
|---|---|
| Turn-on | base ≈ 0.7 V below the emitter |
| Gain (hFE) | 100 – 300 — sims use 200 |
| Max collector current | 200 mA — 2N3906 datasheet |
| Saturation voltage (E–C) | ≈ 0.1–0.25 V — at 10 mA |
| Base current from 3.8 V via two 10 kΩ | ≈ 0.31 mA |
See it working
The The Upside-Down Transistor circuit, simulated live. Open it on the Workbench to change anything.
simulating…
Mistakes everyone makes
- Wired like an NPN (emitter to −, load on top). The PNP never turns on, or conducts weakly backwards. Fix: emitter to +, load between the collector and −.
- Base driven toward +. That's how you turn an NPN on; it turns a PNP off. Fix: pull the base toward − through a resistor to turn it on.
- No pull-up. The lamp flickers or stays half on because the base floats. Fix: 10 kΩ from + to the base.
- No resistor in the base pull-down. A button straight from base to − dumps (4.5 − 0.7) ÷ 0 Ω through the junction and the transistor dies. Fix: 10 kΩ (or 1 kΩ) in series.
- Asking it to drive a motor. 200 mA is the limit, and a stalled hobby motor wants more. Fix: use the 2N2222 (600 mA) with a flyback diode for motors; keep the 2N3906 for LEDs, buzzers and signals.
Questions people ask
- What is the pinout of a 2N3906?
- TO-92 plastic package, flat face toward you, legs down: emitter, base, collector from left to right. It matches the 2N2222 and 2N3904, which is why the three live in the same assortment bag.
- What is the difference between a PNP and an NPN transistor?
- Opposite polarity. An NPN turns on when its base is about 0.7 V above its emitter and passes current collector to emitter; a PNP turns on when its base is about 0.7 V below its emitter and passes current emitter to collector. NPNs switch the − side of a load, PNPs switch the + side.
- Can I swap a 2N3906 for a 2N2222?
- Not in the same circuit. They are opposite types. You can redesign the circuit around the other one: put the PNP's emitter on + and pull its base low instead of high. A 2N3906 handles 200 mA where the 2N2222 handles 600 mA, so motors stay with the NPN.
Projects that use this part
- 🙃 The Upside-Down Transistor — A PNP transistor switches the + side of a lamp and turns on when you pull its base DOWN. Same button, same LED, whole circuit flipped. L3 · 30 min
verified against: onsemi — 2N3906 datasheet · SparkFun — Transistors · Kuphaldt — Lessons in Electric Circuits, Bipolar Junction Transistors