onsemi MMBT4126

Overview

MMBT4126 from Fairchild Semiconductor is a PNP general-purpose bipolar junction transistor designed for low-to-moderate current amplification and switching in discrete analog and digital circuits, with confirmed V_CEO = 25 V, I_C = 200 mA continuous, h_FE = 120–360 at 2 mA, f_T = 250 MHz, and V_CE(sat) = 0.4 V at I_C = 50 mA / I_B = 5 mA - used in signal conditioning stages of industrial sensor interfaces.

For engineers reviewing the MMBT4126 datasheet, pinout, applications, or equivalent options, this page delivers verified package mapping (SOT-23), terminal assignment (C-B-E), thermal resistance (R_θJA = 357 °C/W), noise figure (NF = 4.0 dB), and substitution guidance grounded in manufacturer-specified electrical and mechanical data.

Technical Context

The MMBT4126 operates as a single PNP BJT with fixed emitter-base and collector-base junctions, supporting linear amplification (h_fe = 120–480 at 2 mA) and saturated switching (V_BE(sat) = 0.95 V, β = 10). Its f_T = 250 MHz and C_ibo = 10 pF confirm suitability for audio-frequency and low-MHz switching applications.

Thermal performance is defined for SOT-23 mounting on FR-4 PCB (1.6" × 1.6" × 0.06"), with R_θJA = 357 °C/W and P_D = 350 mW at 25°C, derating 2.8 mW/°C above ambient - enabling operation up to +150°C junction temperature under controlled board layout conditions.

Key Specifications

Pinout & Package

SOT-23 plastic surface-mount package with molded epoxy body, marked "ZF" on top surface. Dimensions per JEDEC MO-178AA: A0 = 3.15 mm, B0 = 2.77 mm, W = 8.0 mm, t = 0.228 mm.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MMBT4403 and BC807-25, the MMBT4126 offers the best balance of bandwidth (250 MHz), noise (4.0 dB), and thermal resistance (357 °C/W) in the 25 V PNP class - making it optimal for audio-frequency amplification and fast-switching applications where gain-bandwidth product and noise floor are prioritized over raw current handling.

Availability

MMBT4126 is available at Aetrix Electronics and suitable for industrial sensor interfaces, portable medical devices, audio pre-amplifiers, and low-voltage PNP switching applications requiring stable component supply across extended temperature ranges and long production lifecycles.

Supply support for MMBT4126 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 components before its acquisition by ON Semiconductor in 2016; known for robust BJT and MOSFET process technologies.

The MMBT4126 belongs to Fairchild's legacy SOT-23 PNP transistor family, engineered for cost-sensitive, high-reliability general-purpose amplification and switching in consumer, industrial, and computing systems.

FAQ

What is the maximum collector current rating for the MMBT4126?

The MMBT4126 has a maximum continuous collector current (I_C) rating of 200 mA at T_A = 25°C, with derating applied above that temperature. This rating is validated under FR-4 PCB mounting conditions (1.6" × 1.6" × 0.06") and supports reliable operation in switching and amplification roles up to 150 mA in typical designs. The MMBT4126 must not be operated beyond this limit without thermal analysis confirming junction temperature remains ≤150°C.

Is the MMBT4126 pin-compatible with the 2N4126?

No - the MMBT4126 is the SOT-23 surface-mount version, while the 2N4126 is the through-hole TO-92 variant; both share identical electrical specifications and PNP polarity but differ in package, pinout, and thermal characteristics. The MMBT4126 uses SOT-23 pin 1 = Emitter, pin 2 = Base, pin 3 = Collector, whereas the 2N4126 in TO-92 uses E-B-C pin order when viewed with flat side facing up. The MMBT4126 is not a drop-in replacement for 2N4126 without PCB redesign.

What is the noise figure of the MMBT4126 and under what conditions is it measured?

The MMBT4126 has a noise figure (NF) of 4.0 dB, measured at I_C = 100 µA, V_CE = 5.0 V, R_S = 1.0 kΩ, and frequency range 10 Hz to 15.7 kHz. This value reflects its suitability for low-noise pre-amplification tasks such as sensor signal conditioning and audio front-ends. The MMBT4126 maintains this NF performance across its specified operating temperature range, making it appropriate for precision analog designs where signal integrity is critical.

Does the MMBT4126 support operation at elevated temperatures?

Yes - the MMBT4126 is rated for junction temperatures from -55°C to +150°C, with storage temperature matching that range. Its thermal resistance R_θJA = 357 °C/W (SOT-23) allows operation up to +85°C ambient in typical PCB layouts with modest copper pour, and up to +125°C ambient with enhanced thermal design. The MMBT4126 retains specified h_FE, V_CE(sat), and leakage parameters across this full range, supporting deployment in automotive and industrial environments.

How does the MMBT4126 compare to NPN alternatives like the MMBT4401 in circuit design?

The MMBT4126 serves as the PNP complement to NPN transistors such as the MMBT4401, sharing identical SOT-23 packaging, thermal metrics (R_θJA = 357 °C/W), and bandwidth (f_T ≈ 250 MHz), enabling matched performance in push-pull or complementary amplifier stages. Unlike the MMBT4401, the MMBT4126 conducts with negative base-emitter voltage and sinks current from the collector - making it ideal for high-side switching and level-shifting where sourcing from V_CC is required. Circuit biasing and signal polarity must be inverted relative to NPN usage.