onsemi 2N4123TFR

Overview

2N4123TFR from Fairchild Semiconductor is an NPN general-purpose bipolar junction transistor (BJT) designed for low-to-medium current amplification and switching applications up to 100 mA collector current. It features a V_CEO of 30 V, V_CBO of 40 V, V_EBO of 5.0 V, h_FE of 50–150 at I_C = 2.0 mA, and f_T of 250 MHz - enabling use in audio preamplifiers, signal conditioning stages, and discrete logic-level switching circuits.

For engineers reviewing the 2N4123TFR datasheet, pinout, applications, or equivalent options, key selection considerations include its TO-92 package compatibility, guaranteed h_FE range across operating conditions, thermal resistance (R_θJA = 200 °C/W), noise figure (6.0 dB at 100 µA), and saturation voltage performance (V_CE(sat) ≤ 0.3 V at I_C = 50 mA / I_B = 5.0 mA).

Technical Context

The 2N4123TFR operates as a silicon NPN BJT with a maximum junction temperature of +150 °C and storage range from –55 °C to +150 °C. Its DC current gain (h_FE) is specified at two bias points: 50–150 at V_CE = 1.0 V, I_C = 2.0 mA and 25–150 at V_CE = 1.0 V, I_C = 50 mA - indicating usable gain stability across moderate current levels.

Small-signal parameters include f_T = 250 MHz at I_C = 10 mA/V_CE = 20 V, Cob = 4.0 pF at V_CB = 5.0 V, and NF = 6.0 dB under standardized 10 Hz–15.7 kHz conditions - confirming suitability for RF-coupled amplifiers and broadband analog front-ends where gain-bandwidth and noise are critical.

Key Specifications

Pinout & Package

2N4123TFR is housed in a through-hole TO-92 package with standard three-terminal configuration. Pin assignment follows JEDEC TO-92 outline: Emitter (pin 1), Base (pin 2), Collector (pin 3), viewed from flat side with leads down.

Key Features

Applications

Equivalent & Alternatives

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

Compared with PN2222A and BC547B, the 2N4123TFR offers a balanced trade-off: guaranteed mid-range h_FE, proven TO-92 reliability, and verified 250 MHz f_T - making it optimal for prototyping, educational kits, and industrial controls where manual assembly and parameter consistency are prioritized.

Availability

2N4123TFR is available at Aetrix Electronics and suitable for audio preamplifiers, logic-level switching circuits, and RF interstage coupling applications requiring stable component supply, long-term manufacturability, and legacy-compatible through-hole packaging.

Supply support for 2N4123TFR 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. Its legacy products remain widely supported and qualified for industrial use.

The 2N4123TFR belongs to Fairchild's classic NPN general-purpose transistor family, engineered for broad utility in analog amplification, digital switching, and signal conditioning - emphasizing parameter consistency, thermal resilience, and ease of integration in mixed-signal systems.

FAQ

What is the maximum continuous collector current rating for the 2N4123TFR?

The 2N4123TFR has a maximum continuous collector current (I_C) rating of 200 mA, as specified in its absolute maximum ratings table. This value applies at T_A = 25 °C with proper heatsinking; actual usable current depends on ambient temperature, PCB copper area, and duty cycle. For sustained 100 mA operation - the typical design target cited in its application description - thermal derating must be applied above 25 °C using the 5.0 mW/°C derating factor from its 625 mW P_D rating.

Does the 2N4123TFR have a guaranteed minimum h_FE at I_C = 50 mA?

Yes, the 2N4123TFR guarantees a minimum h_FE of 25 at V_CE = 1.0 V and I_C = 50 mA, per its "ON CHARACTERISTICS" table. This ensures predictable base drive requirements when used as a saturated switch or medium-current amplifier. The h_FE upper limit remains 150 under those same conditions, providing a defined gain window for circuit simulation and worst-case analysis.

Is the 2N4123TFR suitable for RF amplifier designs up to 100 MHz?

Yes, the 2N4123TFR is suitable for RF amplifier designs up to approximately 100 MHz. Its specified f_T of 250 MHz - measured at I_C = 10 mA and V_CE = 20 V - provides adequate gain margin for stable operation in Class-A or Class-C amplifiers within that band. Designers should account for its Cob of 4.0 pF and h_fe roll-off above 10 MHz, using neutralization or emitter degeneration where necessary to ensure unconditional stability.

What is the noise figure of the 2N4123TFR and under what conditions is it measured?

The 2N4123TFR has a noise figure (NF) of 6.0 dB, measured at V_CE = 5.0 V, I_C = 100 µA, R_S = 1.0 kΩ, and over a bandwidth of 10 Hz to 15.7 kHz. This specification makes it appropriate for low-noise preamplifier stages in audio and sensor signal chains where preserving signal integrity at microvolt-level inputs is essential. NF increases at higher currents or source impedances, so bias optimization is recommended for critical applications.

Can the 2N4123TFR replace the 2N3904 in existing designs?

The 2N4123TFR can replace the 2N3904 in many general-purpose amplifier and switching applications, but with important distinctions: it offers higher I_C capability (200 mA vs. 200 mA max, but 2N3904 typically used ≤100 mA), similar V_CEO (30 V), and higher f_T (250 MHz vs. 300 MHz). However, its h_FE range is narrower (50–150 vs. 100–400), and its V_BE(sat) is slightly higher (0.95 V vs. ~0.85 V). Verify base drive and thermal margins before substitution.