What Is a General Purpose NPN Transistor?

General-purpose NPN transistor ratings: 40 V, 200 mA, gain 100-300 in SOT-23. Learn base-resistor sizing and when a MOSFET is the better choice.

Request a Quote

What Is a General Purpose NPN Transistor?

Updated: October 28, 2026

A general-purpose NPN transistor is a three-terminal current-controlled switch/amplifier (collector–base–emitter) used for small-signal and modest switching jobs—examples include classic 2N3904-class parts.

Key Takeaways

“General purpose” describes a ratings envelope, not a technology: low voltage, moderate current, useful gain, and a package that is easy to place.
The numbers are given as bands, not single values. A current gain of 100 to 300 is a spread, and a design that works at the nominal 200 will not necessarily work at the low end.
The power rating is an ambient figure measured on a defined board, so the same part carries less current in a warm enclosure or on less copper.
The base needs a resistor, and its value comes from the collector current divided by the gain you are willing to accept.
Whether to use a BJT or a MOSFET is a question about drive, not about era — each has a bias that suits it. Related reading on switching and amplifying devices is collected in the MOSFET, IGBT and transistor hub.

The Ratings, Read Properly

Start from the limits, because they define what the device can be asked to do. The Nexperia PMBT3904 and MMBT3904 datasheets describe a 40 V, 200 mA NPN switching transistor in a small SOT23 surface-mounted package, with a PNP complement in the PMBT3906.

Parameter	Specified value	What it constrains
Collector-base voltage V_CBO	60 V (open emitter)	The highest voltage across the junction with the base disconnected
Collector-emitter voltage V_CEO	40 V (open base)	The rail the device can switch
Emitter-base voltage V_EBO	6 V (open collector)	The reverse drive the base junction tolerates — low, and often exceeded by mistake
Collector current I_C	200 mA, with a 200 mA peak and a 100 mA peak base current (1 ms pulse)	The load current, and the drive the base can be given
DC current gain h_FE	100 to 300 at V_CE = 1 V, I_C = 10 mA	How much base current the load requires — at the worst end of the band
Total power dissipation P_tot	250 mW at T_amb ≤ 25 °C, on FR4 with a standard footprint	The thermal budget, and it is an ambient figure
Junction temperature	150 °C maximum; ambient range −65 °C to 150 °C	Derating for the enclosure the part lives in
Package and qualification	SOT23, 1.9 mm pitch, 2.9 × 1.3 × 1 mm; AEC-Q101 qualified	Board area and the automotive evidence trail

The gain row is the one most often misread. A specification of 100 to 300 means two parts from the same reel can need three times as much base current as each other for the same collector current, so a circuit designed around a gain of 200 will misbehave at the bottom of the band. The usual fix is to drive the transistor harder than the minimum implies: choose a forced gain well below the guaranteed minimum and let the excess base current guarantee saturation.

Sizing the Base Resistor

The calculation is short, and doing it explicitly is what stops a marginal design reaching production.

Take a 100 mA load switched from a 3.3 V logic output, and aim for a forced gain of 10 — that is, base current of 10 mA, well below the 200 mA current rating and below the 100 mA peak base current limit. With V_BE of roughly 0.7 V in saturation (a design assumption, not a datasheet maximum):

R_B = (3.3 V − 0.7 V) / 10 mA = 260 Ω, so a 270 Ω standard value.
Base current actually delivered: (3.3 − 0.7) / 270 = 9.6 mA — a forced gain of just over 10 at 100 mA.
Worst-case check at the low end of the gain band: the base drive still exceeds I_C/10, which is the condition for saturation rather than linear operation.
Check the logic output: 9.6 mA is within a typical microcontroller pin’s capability, but not if the same port is driving several loads.

This is a model with stated assumptions — the 0.7 V base-emitter figure and the forced gain are design choices, not datasheet values — and the method is what matters: pick the forced gain, compute the base current, then confirm that both the driving pin and the transistor’s dissipation can supply it. The dissipation in this example is roughly 100 mA × 0.2 V of saturation voltage, comfortably inside the 250 mW rating at a 25 °C ambient — but not necessarily inside it at 85 °C, which is where the ambient-based rating has to be derated.

When a BJT, and When Something Else

The class remains useful because its bias requirements are undemanding. A bipolar transistor turns on at a base-emitter voltage that most logic families can provide directly through a resistor, which makes it convenient for small loads, level shifting, driving an indicator, and interfacing a sensor. Where the drive is already a voltage-referred signal and the load current is in the hundreds of milliamps, that simplicity is hard to beat.

A MOSFET, by contrast, is a voltage-driven device with no steady-state gate current, but it needs enough gate voltage to reach a specified on-resistance, and its behaviour at low gate drive is a curve rather than a threshold. Where a design needs very low conduction loss at high current, or needs to be driven from a fixed gate-drive rail, the MOSFET wins; where it needs a simple, cheap switch from a logic pin, the general-purpose NPN still does the job.

Field checklist

Bias the base with a resistor; do not rely on β alone.
Check V_CE(sat) if used as a switch.
For power switching, consider a MOSFET instead.

NPN versus PNP in one glance

NPN conducts when the base is driven positive relative to the emitter (conventional current into the base). PNP mirrors the polarities. Many teaching pages start with NPN low-side switches; PNP often appears as high-side switches with different drive needs.

Saturation and why VCE(sat) matters

As a switch, you care about VCE(sat) at your collector current and the base current needed to reach it. Undersized base drive leaves the transistor in a hot linear region. Check hFE only as a starting point—switching designs should use forced beta from the datasheet examples.

Frequently asked questions

What is a general-purpose NPN used for?

Small amplifiers, logic-level switching, and LED/relay drivers within its voltage/current limits.

NPN or MOSFET?

MOSFETs win many power switches; BJTs remain common for cheap small-signal and certain analog jobs.

Why does my NPN get hot when “on”?

Often insufficient base current, so it is not fully saturated.

Example part numbers?

2N3904-class and similar—always match the exact datasheet for your footprint.

Continue through this engineering topic

Topic hub: General-purpose NPN transistor — related selection guides and application notes in this cluster.