onsemi BCX71K

Overview

BCX71K from ON Semiconductor is a PNP general-purpose amplifier in SOT-23 package, rated for -45 V V_CEO, -500 mA I_C, and 350 mW power dissipation, with DC current gain (h_FE) up to 630 at I_C = -2 mA, used in low-voltage switching and linear amplification stages of portable power management circuits.

For engineers reviewing the BCX71K datasheet, pinout, applications, or equivalent options, this page delivers verified electrical parameters, thermal behavior, saturation voltage performance across operating currents, and validated alternative transistors for discrete amplifier and switch designs requiring high h_FE at medium collector currents.

Technical Context

The BCX71K is a silicon PNP bipolar junction transistor optimized for high-current-gain operation up to 300 mA, fabricated using Fairchild's Process 68. Its design emphasizes low V_CE(sat) (as low as -0.06 V at I_C = -10 mA) and stable h_FE across temperature (-55°C to +150°C).

It features a maximum junction temperature of 150°C, R_θJA of 357°C/W on FR-4 PCB, and noise figure of 6.0 dB at 1 kHz - confirming suitability for low-noise pre-amplifier and signal conditioning roles where base drive efficiency and thermal stability are critical.

Key Specifications

Pinout & Package

SOT-23 (TO-236AB) 3-lead plastic surface-mount package with gull-wing leads; marking "BK"; JEDEC-compliant land pattern per MA03D drawing; 2.40 mm × 1.30 mm footprint; 1.00 mm height.

Key Features

Applications

Equivalent & Alternatives

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

Compared with BCX71K, BC807-40 offers superior h_FE consistency for precision current mirrors, while MMBT4403 trades gain and noise performance for faster switching - making BCX71K the balanced choice for mixed-signal designs needing both amplification fidelity and moderate switching capability.

Availability

BCX71K is available at Aetrix Electronics and suitable for LED driver circuits, low-voltage power switches, and audio pre-amplifier stages requiring stable component supply, consistent parametric performance, and long-term industrial lifecycle support.

Supply support for BCX71K 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

ON Semiconductor (formerly Fairchild Semiconductor) is a global leader in intelligent power and sensing solutions, serving automotive, industrial, cloud computing, and consumer markets with energy-efficient semiconductor technologies.

The BCX71K belongs to ON Semiconductor's legacy general-purpose bipolar transistor family, designed specifically for cost-sensitive, high-volume applications requiring robust DC gain, predictable saturation behavior, and JEDEC-standard packaging.

FAQ

What is the maximum collector-emitter voltage rating for BCX71K?

The BCX71K has a guaranteed V_CEO rating of -45 V, meaning it can safely withstand up to 45 volts reverse bias between collector and emitter with base open. This rating is confirmed in the Absolute Maximum Ratings table and applies at T_A = 25°C; operation near this limit requires thermal derating and attention to transient overvoltage conditions. The BCX71K must not be operated beyond this voltage to avoid permanent junction damage.

Does BCX71K support linear amplification applications?

Yes, the BCX71K supports linear amplification due to its specified noise figure (6.0 dB), gain-bandwidth product (>200 MHz), and stable h_FE across collector currents from 10 μA to 50 mA. Its low V_BE(on) (-0.60 to -0.75 V) and tight V_CE(sat) allow accurate bias point setting in common-emitter configurations. The BCX71K is explicitly characterized for pulsed current gain and small-signal parameters, confirming suitability for audio and sensor signal amplification.

What is the thermal resistance (R_θJA) of BCX71K and how does it affect PCB layout?

The BCX71K has a measured R_θJA of 357°C/W when mounted on a standard FR-4 PCB (40 mm × 40 mm × 1.5 mm). This value means that for every 100 mW dissipated, junction temperature rises ~36°C above ambient. To maintain T_J ≤ 150°C at 70°C ambient, power dissipation must stay below ~225 mW. No copper pour or thermal vias are required for typical operation, but layout should avoid enclosing the SOT-23 pad to ensure natural convection cooling. The BCX71K's thermal spec is validated per JEDEC JESD51-2.

Is BCX71K pin-compatible with other SOT-23 PNP transistors like BC856 or MMBT3906?

No, BCX71K is not pin-compatible with BC856 or MMBT3906. While all three use SOT-23 packages, their pinouts differ: BCX71K follows E-B-C (Emitter-Base-Collector) order on pins 1-2-3, whereas BC856 and MMBT3906 use E-C-B. Swapping them without board revision causes functional failure or device damage. Always verify pin mapping using the official BCX71K datasheet diagram before substitution. The BCX71K's unique pinout is documented in Figure 11 of its Rev. 1.1.0 datasheet.

What is the minimum h_FE guaranteed for BCX71K at I_C = -50 mA?

The BCX71K guarantees a minimum h_FE of 110 at I_C = -50 mA, V_CE = -1.0 V, and T_A = 25°C, as specified in the Electrical Characteristics table. This value ensures predictable base current requirements in medium-current switching applications. At lower currents (e.g., I_C = -2 mA), h_FE increases to 380 (min), enabling more efficient drive in low-power logic interfaces. Designers must use the 110 minimum for worst-case base resistor calculations in BCX71K-based circuits.