onsemi 2N4125TAR

Overview

2N4125TAR from Fairchild Semiconductor is a PNP general-purpose bipolar junction transistor used for low-to-mid current amplification and switching in linear and digital circuits, with V_CEO = 30 V, I_C = 200 mA continuous, h_FE = 50–150 at I_C = 2 mA, and V_CE(sat) = 0.4 V at I_C = 50 mA / I_B = 5 mA. It operates across –55°C to +150°C and is commonly deployed in discrete logic interfaces, sensor signal conditioning, and relay drivers.

For engineers reviewing the 2N4125TAR datasheet, pinout, applications, or equivalent options, key selection criteria include its PNP polarity, TO-92 package thermal resistance (R_θJA = 200 °C/W), low-noise performance (NF = 5.0 dB at 10 Hz–15.7 kHz), and verified saturation behavior under β = 10 switching conditions.

Technical Context

The 2N4125TAR is a silicon planar epitaxial PNP BJT optimized for general-purpose analog amplification and saturated-switching operation. Its DC current gain (h_FE) exhibits strong temperature dependence-ranging from ~250 at –40°C to ~50 at 125°C-and supports stable biasing in emitter-follower and common-emitter configurations with V_BE(sat) = 0.95 V at I_C = 50 mA.

Small-signal parameters include h_fe = 50–200 (1 kHz–100 MHz), C_ob = 4.5 pF at V_CB = 5 V, and input capacitance C_ib = 10 pF at V_BE = 0.5 V, enabling predictable frequency response up to several MHz in audio and control-loop applications.

Key Specifications

Pinout & Package

2N4125TAR is housed in a through-hole TO-92 package (3-lead, plastic, upright configuration) with standard lead orientation: flat side facing viewer, leads down, left-to-right = Emitter–Base–Collector.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MPSA92 and BC557, the 2N4125TAR offers balanced performance for mid-current, low-voltage PNP switching and amplification in through-hole designs, with tighter h_FE control than BC557 and better thermal efficiency than MPSA92 in ambient-limited enclosures.

Availability

2N4125TAR is available at Aetrix Electronics and suitable for discrete amplifier stages, relay drivers, and logic level shifters requiring stable component supply, long-lifecycle support, and consistent parametric performance across production batches.

Supply support for 2N4125TAR 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.

The 2N4125TAR belongs to Fairchild's legacy general-purpose bipolar transistor product line, engineered for cost-sensitive, reliability-critical linear and switching applications in industrial controls and consumer electronics.

FAQ

What is the maximum collector-emitter voltage rating for the 2N4125TAR?

The 2N4125TAR has a maximum collector-emitter voltage (V_CEO) rating of 30 V under open-base conditions at T_A = 25°C. This rating ensures safe operation in low-voltage switching and amplification circuits, including 12 V and 24 V industrial systems. Exceeding this voltage risks avalanche breakdown and permanent device damage. Derating is recommended for operation above 25°C ambient.

Does the 2N4125TAR support linear amplification applications?

Yes, the 2N4125TAR supports linear amplification with verified small-signal parameters: h_fe = 50–200 (1 kHz–100 MHz), C_ob = 4.5 pF, and NF = 5.0 dB at 10 Hz–15.7 kHz. Its h_FE consistency and low leakage enable stable biasing in common-emitter and emitter-follower configurations. The 2N4125TAR is routinely used in microphone preamps and sensor signal conditioning where fidelity and low distortion are critical.

What is the thermal resistance from junction to ambient for the 2N4125TAR?

The 2N4125TAR has a junction-to-ambient thermal resistance (R_θJA) of 200 °C/W in its standard TO-92 package at T_A = 25°C. This value assumes free-air convection and no heatsinking. For sustained operation at full 625 mW dissipation, ambient temperature must be held below ~75°C. In practice, design margins recommend limiting power to ≤300 mW in enclosed or elevated-temperature environments to maintain T_J < 125°C.

Is the 2N4125TAR pin-compatible with other TO-92 PNP transistors like the 2N3906?

No, the 2N4125TAR is not pin-compatible with the 2N3906. While both use TO-92 packages, their pinouts differ: 2N4125TAR (flat side facing viewer, leads down) is E-B-C left-to-right, whereas 2N3906 is E-C-B. Swapping them without board revision will cause circuit failure. Always verify pin assignment using the official Fairchild 2N4125TAR datasheet before substitution.

What is the typical noise figure of the 2N4125TAR and under what conditions is it measured?

The typical noise figure of the 2N4125TAR is 5.0 dB, measured at V_CE = 5.0 V, I_C = 100 µA, R_S = 1.0 kΩ, and frequency range 10 Hz to 15.7 kHz. This specification confirms its suitability for low-level analog signal amplification where added noise must be minimized-such as in electret microphone front-ends or thermocouple amplifiers. Performance degrades at higher source impedances or frequencies outside this band.