onsemi 2N4125BU

Overview

2N4125BU from Fairchild Semiconductor is a PNP bipolar junction transistor designed for general-purpose amplification and switching in low-to-moderate current circuits, with confirmed V_CEO = 30 V, I_C = 200 mA continuous, h_FE = 50–150 at I_C = 2 mA, and TO-92 package. It serves in discrete amplifier stages, level-shifting interfaces, and load-switching applications up to 100 mA.

For engineers reviewing the 2N4125BU datasheet, pinout, applications, or equivalent options, key selection criteria include its PNP polarity, 30 V breakdown rating, 200 mA collector current capability, thermal resistance (R_θJA = 200 °C/W), and suitability for linear amplification or saturated switching below 100 mA.

Technical Context

This device operates as a silicon PNP BJT with fixed-emitter bias topology, supporting common-emitter and common-base configurations. Its DC current gain (h_FE) varies from 50 at 2 mA to 25 at 50 mA, and saturation voltages are specified at V_CE(sat) = 0.4 V and V_BE(sat) = 0.95 V under I_C/I_B = 10.

Small-signal parameters include h_fe = 50–200 (1 kHz–100 MHz), input capacitance C_ib = 10 pF, output capacitance C_ob = 4.5 pF, and noise figure NF = 5.0 dB at 100 µA/1.0 kHz. Thermal derating begins above 25°C at 5.0 mW/°C.

Key Specifications

Pinout & Package

2N4125BU is housed in a TO-92 plastic package with standard PNP lead configuration: Emitter (lead 1), Base (lead 2), Collector (lead 3), viewed from flat side with leads down.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar PNP general-purpose amplifier applications.

Compared with 2N4125BU, the 2N3906 offers higher guaranteed h_FE and voltage margin, while the BC557 trades current capability for broader gain range-neither is pin-compatible without layout verification, and both require revalidation of thermal and saturation behavior.

Availability

2N4125BU is available at Aetrix Electronics and suitable for audio pre-amplifiers, logic level shifters, relay drivers, and current-limited LED drivers requiring stable component supply and long-term industrial availability.

Supply support for 2N4125BU includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Fairchild Semiconductor was a U.S.-based semiconductor company specializing in power management, analog, and discrete devices before its acquisition by ON Semiconductor in 2016.

The 2N4125BU belongs to Fairchild's legacy general-purpose bipolar transistor product line, engineered for cost-sensitive, medium-speed amplification and switching in consumer, industrial, and automotive-qualified auxiliary circuits.

FAQ

What is the maximum collector-emitter voltage rating for the 2N4125BU?

The 2N4125BU has a maximum collector-emitter voltage (V_CEO) rating of 30 V under open-base conditions at 25°C. This rating decreases with rising junction temperature and must not be exceeded to prevent avalanche breakdown. Derating curves in the official datasheet show allowable V_CEO reduction above 25°C ambient, and operation beyond 30 V-even briefly-risks permanent device failure. Always verify voltage margins in the final circuit with worst-case supply and transient conditions applied to the 2N4125BU.

Does the 2N4125BU support linear amplification applications?

Yes, the 2N4125BU supports linear amplification with verified small-signal parameters: h_fe = 50–200 (1 kHz–100 MHz), input capacitance C_ib = 10 pF, and noise figure NF = 5.0 dB at 100 µA/1.0 kHz/1 kΩ source. Its h_FE variation across collector current (50 at 2 mA, 25 at 50 mA) requires emitter degeneration for stable gain. The 2N4125BU is commonly used in single-stage pre-amplifiers where moderate bandwidth and low-noise performance are prioritized over ultra-high linearity.

What is the thermal resistance of the 2N4125BU in free-air conditions?

The 2N4125BU has a junction-to-ambient thermal resistance (R_θJA) of 200 °C/W in standard TO-92 mounting on FR-4 PCB with no copper pour. This means each watt of power dissipated raises the junction temperature by 200°C above ambient. At its 625 mW maximum power rating, junction temperature rises 125°C above ambient-so operation above 25°C ambient requires derating per the 5.0 mW/°C slope. For sustained 200 mW dissipation, ambient must stay below ~75°C unless additional copper or airflow is provided for the 2N4125BU.

Can the 2N4125BU replace the 2N3906 in existing designs?

The 2N4125BU is not a direct replacement for the 2N3906 due to differences in h_FE (2N3906 min = 100 at 10 mA vs. 2N4125BU min = 50 at 2 mA) and V_CEO (40 V vs. 30 V). While both are TO-92 PNP transistors, substituting the 2N4125BU may cause insufficient gain or premature breakdown in circuits relying on the 2N3906's higher voltage or tighter h_FE spec. Any swap requires validation of bias point stability, saturation margin, and thermal rise in the target application using the actual 2N4125BU characteristics.

What are the absolute maximum ratings for base-emitter reverse voltage on the 2N4125BU?

The absolute maximum emitter-base reverse voltage (V_EBO) for the 2N4125BU is 4.0 V at 25°C. Exceeding this rating risks zener-like breakdown of the base-emitter junction, potentially degrading h_FE or causing leakage. In switching applications with inductive loads or fast turn-off, flyback voltage spikes across the base-emitter junction must be clamped-e.g., with a 3.3 V Zener-to protect the 2N4125BU. This 4.0 V limit applies regardless of pulse duration or duty cycle, as confirmed in the Absolute Maximum Ratings table of the official 2N4125BU datasheet.