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

STMicroelectronics BF421-AP

Name: BF421-AP
Brand: STMicroelectronics
SKU: BF421-AP
Price: 2.40 USD
Availability: InStock
Rating: 5 (2 reviews)

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Part No.:: BF421-AP

Manufacturer:: STMicroelectronics

Category:: Single Bipolar Transistors

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

Datasheet:: BF421-AP.pdf

Description:: TRANS PNP 300V 0.5A TO-92-3

Quantity:: RFQ Add To RFQ list

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Inventory:2,596

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Product details

Overview

BF421-AP from STMicroelectronics is a silicon epitaxial planar PNP high-voltage transistor in TO-92 package, rated for V_CEO = −300 V, I_C = −500 mA, and P_tot = 830 mW at T_C = 25 °C. It serves as a cathode current control device in RGB video amplifier circuits and hook-switch/dialer stages in telephone wireline interfaces.

For engineers reviewing the BF421-AP datasheet, BF421-AP pinout, BF421-AP application, or BF421-AP equivalent, key selection criteria include high-voltage PNP switching capability, −300 V breakdown rating, TO-92 through-hole compatibility, thermal resistance (R_thj-amb = 150 °C/W), and complementary pairing with BF420 for push-pull designs.

Technical Context

The BF421-AP operates as a high-voltage, medium-current PNP bipolar junction transistor optimized for linear and switching applications where reverse polarity and high collector-base standoff are required. Its f_T = 60 MHz and C_RE = 1.6 pF at V_CB = −30 V support stable operation in video-frequency amplification and telephony signal conditioning.

Designed for DC-coupled cathode drive and line interface switching, it features V_CE(sat) = −0.6 V at I_C = −30 mA/I_B = −5 mA and h_FE = 50 minimum at I_C = −25 mA/V_CE = −20 V - enabling predictable gain and low-saturation loss in active load configurations.

Key Specifications

Parameter	Value and Actual Design Meaning
V_CEO	−300 V: Enables safe operation in high-voltage analog signal paths such as CRT RGB cathode drivers and telecom line interface circuits.
I_C	−500 mA: Supports moderate current switching for hook-switch actuation and cathode current regulation without forced cooling.
P_tot	830 mW at T_C = 25 °C: Defines maximum continuous power dissipation under controlled case temperature conditions.
h_FE	50 min at I_C = −25 mA/V_CE = −20 V: Ensures sufficient DC current gain for base-driven switching and linear amplification stages.
f_T	60 MHz: Validates usable bandwidth for RGB video signal amplification up to ~10–20 MHz fundamental content.
C_RE	1.6 pF at V_CB = −30 V: Limits Miller capacitance impact on high-frequency stability in common-emitter configurations.
R_thj-amb	150 °C/W: Indicates thermal limitation for ambient-air convection cooling in TO-92 mounted PCBs.

Pinout & Package

BF421-AP is housed in a standard TO-92 plastic package with axial leads suitable for through-hole PCB assembly. The package conforms to mechanical dimensions specified in STMicroelectronics' preliminary data sheet (Feb 2003), including lead pitch (P0 = 12.5–12.9 mm), body height (H = 18.5–20.5 mm), and lead diameter (d = 0.48 mm).

Pin/Terminal	Circuit Role	Design Meaning
E (Emitter)	Current source terminal for PNP operation	Connected to positive rail in common-emitter switching; sets reference for V_EB bias and current sourcing path.
B (Base)	Control input for minority-carrier injection	Receives negative-going drive to turn on; requires current-limiting resistor due to h_FE-dependent base current demand.
C (Collector)	Output current sink terminal	Connected to load (e.g., CRT cathode or telephone line); withstands −300 V reverse bias when off.

Key Features

Feature	Design Value
High-voltage PNP structure	−300 V V_CEO/V_CBO enables direct interface with legacy CRT anode supplies and telecom line voltages.
Complementary pairing	Matched electrical specs and package with NPN BF420 allow symmetrical push-pull output stages without layout asymmetry.
Video-bandwidth capability	60 MHz f_T and 1.6 pF C_RE support stable small-signal amplification across RGB video bandwidths.
TO-92 through-hole compatibility	Standard leadform and mechanical dimensions ensure drop-in replacement in existing wireline and display driver PCBs.
Thermal robustness	150 °C max junction temperature and −65 to +150 °C storage range suit industrial and telecom environments.

Applications

RGB Video Amplifier

Telephone Hook Switch

Use Scenario: Driving individual red/green/blue cathodes in analog CRT monitors to modulate beam intensity.

IC Role / Device Role / Timing Role: PNP current source controlling cathode current magnitude in DC-coupled video output stage.

Use Value: −300 V V_CEO withstands CRT anode voltage transients; low V_CE(sat) minimizes power loss during full-on conduction.

Use Scenario: Controlling loop current flow during telephone handset off-hook detection and dial pulse generation.

IC Role / Device Role / Timing Role: High-voltage PNP switch enabling/disabling DC loop current on tip/ring lines.

Use Value: −300 V breakdown rating accommodates ringing voltage surges up to ±120 V RMS without avalanche conduction.

Dialer Circuit

Line Interface Buffer

Use Scenario: Generating timed pulse trains for rotary dial emulation in legacy PBX systems.

IC Role / Device Role / Timing Role: Fast-switching PNP element in monostable or astable pulse generator topology.

Use Value: 60 MHz f_T ensures clean edge fidelity for 10–20 pulse-per-second dial signals without rounding or delay.

Use Scenario: Isolating microcontroller GPIO from high-voltage telephone line signaling.

IC Role / Device Role / Timing Role: Level-shifting buffer translating logic-level control to −48 V line interface domain.

Use Value: −5 V V_EBO and −300 V V_CBO provide margin against line transients while maintaining digital control integrity.

Equivalent & Alternatives

The following parts are listed as comparable options for similar high-voltage PNP transistor applications.

Alternative Part	Technical Difference	Application Difference	Selection Advice
BF420	NPN complement; identical V_CEO, I_C, and package but opposite polarity and gain profile.	Used in push-pull pairs or NPN-biased circuits; not interchangeable without circuit redesign.	Select BF420 only when NPN topology or complementary symmetry is required.
BC639	Lower V_CEO (−100 V), higher h_FE (40–250), same TO-92 package and I_C rating.	Suitable for lower-voltage consumer audio/switching; insufficient for −300 V line or CRT applications.	Choose BC639 only if system voltage remains below −100 V and higher gain is prioritized over breakdown safety margin.

Compared with BF421-AP, BF420 provides matched complementary behavior essential for balanced output stages, while BC639 trades critical high-voltage capability for wider DC gain spread - making BF421-AP uniquely suited for legacy telecom and CRT systems demanding −300 V standoff.

Availability

BF421-AP is available at Aetrix Electronics and suitable for RGB video amplifier circuits, telephone wireline interface modules, and dialer circuit implementations requiring stable component supply and long-lifecycle availability.

Supply support for BF421-AP 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

STMicroelectronics is a global semiconductor leader headquartered in Geneva, Switzerland, specializing in power management, analog, MEMS, and discrete devices for industrial, automotive, and communications markets.

The BF421-AP belongs to ST's high-voltage bipolar transistor product line, engineered specifically for legacy video display and telephony infrastructure where robust −300 V PNP switching and TO-92 manufacturability are mandatory.

FAQ

What is the maximum safe operating voltage for BF421-AP in common-emitter configuration?

The BF421-AP supports up to −300 V between collector and emitter with base open (V_CEO), verified under pulsed test conditions per STMicroelectronics' preliminary data sheet. Continuous operation near this limit requires strict thermal management and derating per R_thj-amb = 150 °C/W to avoid junction overheating.

Is BF421-AP pin-compatible with BF420?

No - BF421-AP (PNP) and BF420 (NPN) share identical TO-92 mechanical footprint and pin assignment (E-B-C), but their polarity and biasing requirements differ fundamentally. Direct substitution without circuit modification will result in non-functional or damaged operation.

Can BF421-AP be used in surface-mount designs?

No - BF421-AP is exclusively offered in TO-92 through-hole package (bulk or ammopack). STMicroelectronics does not manufacture or qualify a surface-mount variant. Conversion to SMT requires adapter boards or redesign using functionally similar SOT-23 or SOT-89 PNP transistors with verified −300 V ratings.

What is the significance of the "-AP" suffix in BF421-AP?

The "-AP" suffix denotes ammopack shipment format: components are supplied in tape-and-reel compatible ammo packs for automated through-hole insertion equipment. This differs from standard bulk TO-92 delivery and ensures consistent lead orientation and feeding reliability in high-volume assembly lines.

BF421-AP Specifications

Product attributes: Attribute value

Manufacturer:: STMicroelectronics

Series:: -

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

Packaging:: Cut Tape (CT)

Product Status:: Obsolete

Transistor Type:: PNP

Current - Collector (Ic) (Max):: 500 mA

Voltage - Collector Emitter Breakdown (Max):: 300 V

Vce Saturation (Max) @ Ib, Ic:: 600mV @ 5mA, 30mA

Current - Collector Cutoff (Max):: 10nA (ICBO)

DC Current Gain (hFE) (Min) @ Ic, Vce:: 50 @ 25mA, 20V

Power - Max:: 830 mW

Frequency - Transition:: 60MHz

Operating Temperature:: 150°C (TJ)

Grade:: -

Qualification:: -

Mounting Type:: Through Hole

Supplier Device Package:: TO-92-3

BF421-AP FAQ

1.How can I place an order for BF421-AP through Aetrix?

Please submit a Request for Quotation (RFQ) for BF421-AP 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 BF421-AP reliable?

The price and inventory of BF421-AP are updated periodically and may fluctuate due to market conditions. Stock and pricing data are typically refreshed every 24 hours. Quotation validity for BF421-AP is usually 5 days.

3.What payment methods are accepted for BF421-AP?

We accept Wire Transfer, PayPal, Credit Card, Western Union, MoneyGram, and Escrow for BF421-AP transactions.

Note: Certain payment methods may incur a processing fee.

4.How is shipping managed for BF421-AP?

BF421-AP orders can be shipped via leading logistics carriers, including DHL, UPS, FedEx, TNT, or Registered Mail.

Once your BF421-AP 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 BF421-AP?

For technical support, including BF421-AP datasheets, pinout diagrams, or application guidance, please contact our engineering support team. They can provide detailed documentation and assistance for your BF421-AP requirements.

6.How does Aetrix verify that BF421-AP is sourced from the original manufacturer or authorized distributors?

All BF421-AP 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 BF421-AP meets industry standards.

7.What is the process for return or replacement of BF421-AP?

All BF421-AP units undergo pre-shipment inspection (PSI). If there is an issue with BF421-AP, 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 BF421-AP part is unused and in its original packaging.

Return procedure for BF421-AP:

1.Submit a request within 90 days.

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

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