onsemi 2N4126BU

Overview

2N4126BU from Fairchild Semiconductor is a PNP general-purpose bipolar junction transistor (BJT) designed for low-to-moderate current amplification and switching applications, with confirmed V_CEO = 25 V, I_C = 200 mA continuous, h_FE = 120–360 at I_C = 2 mA, and f_T = 250 MHz - used in discrete audio preamplifier stages and logic-level interface switching circuits.

For engineers reviewing the 2N4126BU datasheet, pinout, applications, or equivalent options, key selection considerations include its TO-92 package compatibility, PNP polarity constraints, saturation voltage behavior under β = 10 drive, thermal derating profile (5.0 mW/°C above 25°C), and noise figure of 4.0 dB at 100 µA/5 V.

Technical Context

The 2N4126BU operates as a single PNP BJT with fixed emitter-base and collector-base junction structures optimized for linear amplification and saturated switching. Its DC current gain exhibits strong temperature dependence (h_FE drops from ~250 at −40°C to ~180 at 125°C), and its f_T remains stable across V_CE = 20 V bias, confirming suitability for RF-coupled small-signal stages up to mid-VHF.

Thermal performance is defined by R_θJA = 200°C/W on FR-4 (1.6" × 1.6" × 0.06"), requiring heatsinking only above ~125 mW dissipation. Switching times (t_d, t_r, t_f, t_s) are characterized at I_C = 10–100 mA with base drive ratios of 10, supporting reliable 100 kHz–1 MHz digital switching.

Key Specifications

Pinout & Package

2N4126BU is supplied in the through-hole TO-92 package (JEDEC TO-92 variant), with standardized lead orientation: flat side facing viewer, leads downward, left-to-right pin order Emitter–Base–Collector (E-B-C). Package outline conforms to JEDEC TO-92 dimensions (0.1977 g/unit, 0.098" max base-to-lead-bend).

Key Features

Applications

Equivalent & Alternatives

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

Compared with MMBT4126 and 2N3906, the 2N4126BU offers identical high-frequency performance to MMBT4126 in a through-hole package ideal for prototyping and low-volume manufacturing, while outperforming 2N3906 in both bandwidth and guaranteed minimum h_FE, making it preferable for precision analog and RF-adjacent signal paths.

Availability

2N4126BU is available at Aetrix Electronics and suitable for audio preamplification, logic-level interface switching, and industrial sensor biasing requiring stable component supply across extended temperature ranges and long-lifecycle programs.

Supply support for 2N4126BU 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 manufacturer specializing in power management, analog, and discrete devices before its acquisition by ON Semiconductor in 2016; legacy products maintain full datasheet compliance and second-source availability.

The 2N4126BU belongs to Fairchild's legacy general-purpose BJT family, engineered for reliability in industrial control, consumer audio, and instrumentation applications where predictable h_FE, low noise, and stable saturation behavior are critical.

FAQ

What is the maximum operating temperature for the 2N4126BU?

The 2N4126BU has a specified operating junction temperature range of −55°C to +150°C. Its storage temperature matches this range. When mounted on a standard FR-4 PCB (1.6" × 1.6" × 0.06"), thermal resistance R_θJA is 200°C/W, meaning at 25°C ambient, the device reaches 150°C junction temperature at approximately 625 mW dissipation - the absolute maximum rated power. Derating begins immediately above 25°C ambient at 5.0 mW/°C.

Is the 2N4126BU pin-compatible with the 2N3906?

Yes, the 2N4126BU and 2N3906 share the same TO-92 package and identical pinout (Emitter–Base–Collector, flat side facing viewer). However, they are not functionally interchangeable without circuit review: the 2N4126BU specifies higher minimum h_FE (120 vs. 100), higher f_T (250 MHz vs. 100 MHz), and lower V_CE(sat) (0.4 V vs. 0.2 V typical but not guaranteed), so substituting 2N3906 may degrade gain stability and high-frequency response in the 2N4126BU's intended applications.

Does the 2N4126BU support RF amplification up to 100 MHz?

Yes - the 2N4126BU's f_T of 250 MHz and measured C_cb of 4.5 pF at 100 kHz indicate strong suitability for RF amplification up to at least 100 MHz. Typical pulsed h_FE curves show usable gain (>10) beyond 50 MHz at I_C = 10 mA, and noise figure remains low (≤4.5 dB) up to 10 kHz, supporting wideband IF and VHF preamp designs when properly biased and matched.

What is the typical V_BE(sat) for the 2N4126BU under switching conditions?

The 2N4126BU specifies V_BE(sat) = 0.95 V maximum at I_C = 50 mA and I_B = 5.0 mA (β = 10). This value is consistent across −40°C to +125°C in typical characterization data, confirming reliable turn-on margin for TTL- and CMOS-driven base circuits. At lower currents (e.g., I_C = 10 mA), V_BE(sat) drops to ~0.82 V, easing drive requirements in low-power switching applications.

Can the 2N4126BU be used in linear regulator pass-transistor configurations?

Yes - the 2N4126BU is routinely applied as an emitter-follower pass transistor in linear regulators. Its h_FE ≥ 120 at I_C = 100 mA ensures sufficient current gain for feedback-loop stability, and its V_CEO = 25 V supports input voltages up to 20 V. Thermal design must account for R_θJA = 200°C/W; for example, at 150 mW dissipation and 25°C ambient, junction temperature rises to 55°C - well within safe limits.