Send an Inquiry

To receive a quote for your project, please fill in the following information, and we’ll get back to you promptly.

Submit Inventory List

Please fill in the following information, and we’ll get back to you promptly.

Home > Products > Discrete Semiconductor Products > Transistors > Bipolar (BJT) > Single Bipolar Transistors

onsemi 2N4123BU

Name: 2N4123BU
Brand: onsemi
SKU: 2N4123BU
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

Compare

Part No.:: 2N4123BU

Manufacturer:: onsemi

Category:: Single Bipolar Transistors

Package:: TO-226-3, TO-92-3 (TO-226AA)

Datasheet:: 2N4123BU.pdf

Description:: TRANS NPN 30V 0.2A TO-92-3

Quantity:: RFQ Add To RFQ list

Payment:

Shipping:

Inventory:6,038

Please send an inquiry. Send us your inquiry, and we will respond immediately.

Product details

Overview

2N4123BU from Fairchild Semiconductor is an NPN general-purpose bipolar junction transistor (BJT) designed for low-to-medium current amplification and switching applications up to 200 mA collector current. It features a V_CEO of 30 V, V_CBO of 40 V, and a DC current gain (h_FE) of 50–150 at I_C = 2.0 mA, with f_T = 250 MHz. It is commonly used in signal amplification stages, digital logic interfaces, and low-power relay drivers.

For engineers reviewing the 2N4123BU datasheet, pinout, applications, or equivalent options, key selection considerations include its TO-92 package compatibility, saturation voltage (V_CE(sat) = 0.3 V at I_C = 50 mA), noise figure (6.0 dB at 100 µA), thermal resistance (R_θJA = 200 °C/W), and suitability for discrete amplifier or switch designs operating below 30 V.

Technical Context

The 2N4123BU operates as a silicon NPN BJT with a maximum junction temperature of +150°C and storage range from –55°C to +150°C. Its small-signal current gain (h_fe) varies from 50 at 2 mA to 2.5 at 10 mA under 100 MHz test conditions, indicating frequency-dependent gain roll-off.

It exhibits low output capacitance (C_ob = 4.0 pF at V_CB = 5.0 V) and input capacitance (C_ib = 8.0 pF at V_EB = 0.5 V), supporting stable high-frequency operation. The device's noise figure of 6.0 dB at 100 µA and 1 kHz–15.7 kHz bandwidth makes it suitable for low-noise preamplifier stages in audio and sensor signal chains.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CEO	30 V - Maximum safe collector-emitter voltage before breakdown; defines upper rail limit in amplifier or switch circuits.
I_C (max)	200 mA - Continuous collector current rating; supports medium-current loads like LED drivers or small relays.
h_FE	50–150 - DC current gain range at V_CE = 1.0 V; determines base drive requirements for saturation.
f_T	250 MHz - Transition frequency; enables use in RF amplifier stages up to ~100 MHz with gain margin.
V_CE(sat)	0.3 V at I_C = 50 mA - Low saturation voltage reduces conduction loss in switching applications.
NF	6.0 dB at I_C = 100 µA - Measured noise performance critical for low-level analog signal amplification.
R_θJA	200 °C/W - Junction-to-ambient thermal resistance in TO-92; informs heatsinking needs at >300 mW dissipation.

Pinout & Package

2N4123BU is housed in a through-hole TO-92 package with standard three-terminal configuration: emitter, base, and collector. Lead spacing and body dimensions conform to JEDEC TO-92 outline (3.3 mm × 4.3 mm × 4.6 mm).

Pin/Terminal	Circuit Role	Design Meaning
E (Emitter)	Current sink terminal	Reference node for biasing; connects to ground or low-impedance return path in common-emitter configurations.
B (Base)	Control input	Receives current-limited drive to modulate collector current; requires series resistor to limit I_B per h_FE target.
C (Collector)	Current source terminal	Delivers amplified or switched load current; connects to supply rail via load in common-emitter topology.

Key Features

Feature	Design Value
Low V_CE(sat)	0.3 V enables <100 mW conduction loss at 50 mA, improving efficiency in battery-powered switches.
High f_T	250 MHz supports wideband amplification up to VHF, suitable for IF stages and oscillator buffers.
Low noise figure	6.0 dB at 100 µA allows clean amplification of microvolt-level signals from piezoelectric or electret sensors.
Wide temperature range	–55°C to +150°C operation supports deployment in industrial controls and automotive under-hood modules.
TO-92 compatibility	Standard footprint enables drop-in replacement in legacy designs using 2N2222A, PN2222, or BC547 equivalents.

Applications

Audio Preamplifier Stage	Microcontroller GPIO Driver
Use Scenario: Amplifying weak microphone or piezo sensor outputs before ADC sampling in portable audio recorders. IC Role / Device Role / Timing Role: Discrete NPN amplifier in common-emitter configuration with emitter degeneration for linearity. Use Value: 6.0 dB noise figure and 50–150 h_FE ensure high SNR and predictable gain control without op-amp complexity.	Use Scenario: Driving LEDs, small solenoids, or optocoupler inputs from 3.3 V or 5 V MCU pins with limited sink/source capability. IC Role / Device Role / Timing Role: Digital switch configured in common-emitter mode with base resistor limiting I_B. Use Value: 0.3 V V_CE(sat) minimizes power loss and heat rise, while 200 mA I_C rating accommodates peak loads beyond MCU pin limits.
DC-DC Converter Feedback	Industrial Sensor Interface
Use Scenario: Level-shifting and isolating error amplifier output signals in isolated flyback or forward converter feedback loops. IC Role / Device Role / Timing Role: Linear-mode level translator between optocoupler output and PWM controller reference input. Use Value: 30 V V_CEO withstands transient spikes on secondary-side rails; low C_ob (4.0 pF) preserves loop stability.	Use Scenario: Conditioning analog outputs from RTDs, thermistors, or bridge-based pressure sensors in PLC input modules. IC Role / Device Role / Timing Role: Discrete emitter-follower buffer providing low-impedance drive to ADC front-end or instrumentation amplifier. Use Value: High h_FE and low V_BE(sat) (0.95 V) maintain accuracy across –40°C to +85°C ambient, per industrial temp grade.

Audio Preamplifier Stage

Microcontroller GPIO Driver

Use Scenario: Amplifying weak microphone or piezo sensor outputs before ADC sampling in portable audio recorders.

IC Role / Device Role / Timing Role: Discrete NPN amplifier in common-emitter configuration with emitter degeneration for linearity.

Use Value: 6.0 dB noise figure and 50–150 h_FE ensure high SNR and predictable gain control without op-amp complexity.

Use Scenario: Driving LEDs, small solenoids, or optocoupler inputs from 3.3 V or 5 V MCU pins with limited sink/source capability.

IC Role / Device Role / Timing Role: Digital switch configured in common-emitter mode with base resistor limiting I_B.

Use Value: 0.3 V V_CE(sat) minimizes power loss and heat rise, while 200 mA I_C rating accommodates peak loads beyond MCU pin limits.

DC-DC Converter Feedback

Industrial Sensor Interface

Use Scenario: Level-shifting and isolating error amplifier output signals in isolated flyback or forward converter feedback loops.

IC Role / Device Role / Timing Role: Linear-mode level translator between optocoupler output and PWM controller reference input.

Use Value: 30 V V_CEO withstands transient spikes on secondary-side rails; low C_ob (4.0 pF) preserves loop stability.

Use Scenario: Conditioning analog outputs from RTDs, thermistors, or bridge-based pressure sensors in PLC input modules.

IC Role / Device Role / Timing Role: Discrete emitter-follower buffer providing low-impedance drive to ADC front-end or instrumentation amplifier.

Use Value: High h_FE and low V_BE(sat) (0.95 V) maintain accuracy across –40°C to +85°C ambient, per industrial temp grade.

Equivalent & Alternatives

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

Alternative Part	Technical Difference	Application Difference	Selection Advice
PN2222A	V_CEO = 40 V, h_FE = 100–300, R_θJA = 200 °C/W - higher voltage rating and wider gain spread.	Better suited for 40 V rail systems; may require gain binning in precision biasing circuits.	Select when higher V_CEO or tighter production gain consistency is required.
BC547B	V_CEO = 45 V, f_T = 300 MHz, NF = 4.0 dB - improved high-frequency response and lower noise.	Preferred in RF preamps and low-noise sensor nodes where sub-5 dB NF is critical.	Choose for enhanced RF performance or lower-noise analog front-ends; verify TO-92 pinout alignment.

Compared with PN2222A and BC547B, the 2N4123BU offers balanced trade-offs: moderate voltage rating (30 V), proven thermal behavior in TO-92, and documented 6.0 dB noise at microamp bias-making it optimal for cost-sensitive, mid-bandwidth industrial signal conditioning where strict V_CEO headroom is not required.

Availability

2N4123BU is available at Aetrix Electronics and suitable for audio preamplifiers, microcontroller interface circuits, DC-DC feedback paths, and industrial sensor signal conditioning requiring stable component supply and long-term obsolescence management.

Supply support for 2N4123BU 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 2N4123BU belongs to Fairchild's legacy general-purpose BJT family, engineered for reliability in linear amplification and robust switching across consumer, industrial, and computing applications.

FAQ

What is the maximum continuous collector current for the 2N4123BU?

The 2N4123BU has a maximum continuous collector current (I_C) rating of 200 mA at T_A = 25°C. Derating applies above 25°C per the 5.0 mW/°C specification. This rating supports use in medium-current switching and amplification tasks such as LED drivers and relay coils, provided thermal design accounts for R_θJA = 200 °C/W in TO-92 mounting.

Does the 2N4123BU have a specified noise figure, and under what conditions?

Yes, the 2N4123BU 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 bandwidth 10 Hz to 15.7 kHz. This value is documented in the "Small Signal Characteristics" section of the Fairchild datasheet and confirms suitability for low-noise preamplifier stages in audio and sensor interfaces.

Is the 2N4123BU pin-compatible with the 2N2222A?

No-the 2N4123BU and 2N2222A share the same TO-92 package but differ in pinout order. The 2N4123BU uses E-B-C (Emitter-Base-Collector) pin assignment, whereas the 2N2222A uses E-C-B. Swapping them without board revision risks circuit malfunction. Always verify pin mapping using the official Fairchild 2N4123BU datasheet before substitution.

What is the transition frequency (f_T) of the 2N4123BU, and how is it tested?

The 2N4123BU has a current gain–bandwidth product (f_T) of 250 MHz, measured at I_C = 10 mA, V_CE = 20 V, and f = 100 MHz. This parameter reflects the frequency at which short-circuit current gain drops to unity and validates usability in VHF amplifiers, oscillator buffers, and broadband signal conditioning where gain-bandwidth trade-offs must be managed.

Can the 2N4123BU be used in surface-mount designs?

No-the 2N4123BU is exclusively offered in the through-hole TO-92 package. It lacks SMT variants such as SOT-23 or SOT-223. For surface-mount alternatives with similar electrical characteristics, consider the BC547B (SOT-23) or MMBT2222A (SOT-23), both requiring layout and thermal validation due to differing R_θJA and pinout.

2N4123BU Specifications

Product attributes: Attribute value

Manufacturer:: onsemi

Series:: -

Package/Case:: TO-226-3, TO-92-3 (TO-226AA)

Packaging:: Bulk

Product Status:: Obsolete

Transistor Type:: NPN

Current - Collector (Ic) (Max):: 200 mA

Voltage - Collector Emitter Breakdown (Max):: 30 V

Vce Saturation (Max) @ Ib, Ic:: 300mV @ 5mA, 50mA

Current - Collector Cutoff (Max):: 50nA (ICBO)

DC Current Gain (hFE) (Min) @ Ic, Vce:: 50 @ 2mA, 1V

Power - Max:: 625 mW

Frequency - Transition:: 250MHz

Operating Temperature:: -55°C ~ 150°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: TO-92-3

2N4123BU FAQ

1.How can I place an order for 2N4123BU through Aetrix?

Please submit a Request for Quotation (RFQ) for 2N4123BU on Aetrix. Our sales agent will provide a competitive quotation and guide you through the order confirmation once you accept the terms.

2.Are the price and stock information for 2N4123BU reliable?

The price and inventory of 2N4123BU are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for 2N4123BU is usually 5 days.

3.What payment methods are accepted for 2N4123BU?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for 2N4123BU transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for 2N4123BU?

2N4123BU orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your 2N4123BU order is processed, you will receive an email with the shipment details and tracking number.

Note: Tracking information may take up to 24 hours to appear. Express delivery typically takes 3–5 business days.

5.How can I obtain technical support or documentation for 2N4123BU?

For technical support, including 2N4123BU datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your 2N4123BU requirements.

6.How does Aetrix verify that 2N4123BU is sourced from the original manufacturer or authorized distributors?

All 2N4123BU products on Aetrix are procured from qualified distributors and authorized channels. Our dedicated quality assurance team conducts strict verification, including traceability checks and, if necessary, third-party testing. This ensures that 2N4123BU meets industry standards.

7.What is the process for return or replacement of 2N4123BU?

All 2N4123BU units undergo pre-shipment inspection (PSI). If there is an issue with 2N4123BU, returns or replacements are accepted under the following conditions:

1.Quantity discrepancies, incorrect items, or visible external defects (such as breakage or corrosion), acknowledged by Aetrix.

2.The issue is reported within 90 days of delivery.

3.The 2N4123BU part is unused and in its original packaging.

Return procedure for 2N4123BU:

1.Submit a request within 90 days.

2.Obtain a Return Material Authorization (RMA) from Aetrix.

Tech Hub

2026-09-09 Counterfeit Electronic Components: How to Detect, Authenticate, and Test Suspect Parts

Counterfeit components can hide behind convincing markings and passing basic function tests. This engineering reference covers source traceability, external inspection, X-ray, XRF, electrical testing, …

2026-09-08 Obsolete Electronic Components: How to Source and Evaluate Replacements

A practical engineering and sourcing framework covering lifecycle verification, lifetime-buy calculations, replacement qualification, supplier checks and counterfeit-risk controls.

2026-09-07 TTL vs CMOS Logic Levels: 74HC and 74HCT Compatibility Explained

TTL and CMOS logic families differ in thresholds, loading, output drive, power and timing. This engineering guide compares 74HC and 74HCT, calculates noise margins and checks 3.3 V/5 V compatibility.

2026-09-04 Logic Level Shifter: How to Connect 3.3V and 5V Logic Safely

A practical engineering guide to 3.3V and 5V logic compatibility, input thresholds, resistor dividers, translator ICs, MOSFET level shifting, I2C pull-ups, timing limits and power-sequencing risks.

2026-09-03 74HC595 vs TPIC6B595: Output Drive, Pinout, Current, and Load Selection

The 74HC595 uses push-pull logic outputs, while the TPIC6B595 uses 50 V open-drain DMOS sinks for higher-power loads. This guide compares timing, current limits, 3.3 V interfacing, load wiring, thermal…

2026-09-02 74HC595 Pinout, Timing, Output Current, and Applications

The 74HC595 converts serial data into eight stable parallel outputs. This guide covers pin functions, shift and storage timing, OE and MR behavior, drive-current limits, cascading, voltage compatibilit…

2026-09-01 Latch vs Flip-Flop: Timing, Differences and Design Choices

A technical comparison of level-sensitive latches and edge-triggered flip-flops, covering timing windows, setup and hold limits, master–slave operation, time borrowing, race-through, HDL inference and…

2026-08-30 D Latch Circuit: Truth Table, Timing, ICs and Applications

A D latch stores one bit while Enable controls when data can pass. This reference covers gate-level operation, truth tables, transparency, setup and hold timing, LE versus OE, common ICs and practical …

2026-08-28 SR Latch Explained: NOR and NAND Circuits, Truth Tables, and Applications

An SR latch stores one bit through cross-coupled feedback. This engineering reference covers NOR and NAND implementations, truth tables, forbidden-state recovery, gated operation, switch debouncing, fa…

2026-08-27 Latch Circuit: SR and D Latches, Truth Tables, Timing, and Applications

Latch circuits retain one bit through feedback. This technical reference covers SR and D latches, truth tables, transparency, timing limits, latch-versus-flip-flop behavior, applications and common log…