onsemi BCX79

Overview

BCX79 from Fairchild Semiconductor is a PNP general-purpose bipolar junction transistor designed for amplification and switching in low-to-medium power circuits, with 45 V V_CEO, 500 mA I_C, and 625 mW P_D at 25°C. It operates across –55°C to +150°C and is commonly used in discrete amplifier stages and load-switching applications in industrial control modules.

For engineers reviewing the BCX79 datasheet, pinout, applications, or equivalent options, key selection considerations include its TO-92 package, PNP polarity, DC current gain (h_FE = 120–1000), saturation voltage (V_CE(sat) = 0.6 V @ 100 mA), and thermal resistance (R_θJA = 200 °C/W).

Technical Context

The BCX79 is a silicon planar epitaxial PNP transistor fabricated using Fairchild's Process 68, optimized for stable DC and small-signal performance up to ~100 MHz. Its h_FE varies with operating point-120 min at 2 mA, 80 min at 10 mA, and 40 min at 100 mA-indicating intentional gain trade-off for higher current capability.

Switching behavior is characterized by 150 ns turn-on and 800 ns turn-off times under defined test conditions, while noise figure is specified at 6.0 dB (V_CE = 5 V, I_C = 0.2 mA, R_S = 2 kΩ), supporting audio and low-noise preamplifier use cases where moderate bandwidth suffices.

Key Specifications

Pinout & Package

BCX79 is housed in a standard through-hole TO-92 plastic package with Emitter-Base-Collector (E-B-C) lead configuration, compatible with manual soldering and wave soldering processes.

Key Features

Applications

Equivalent & Alternatives

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

Compared with BCX79, BC807-40 offers SMT compatibility and tighter h_FE spread but requires layout redesign, while PN2907A provides higher voltage tolerance and identical TO-92 fit but trades off gain flexibility and saturation performance.

Availability

BCX79 is available at Aetrix Electronics and suitable for industrial control systems, audio signal conditioning circuits, and relay driver modules requiring stable component supply and long-term obsolescence management.

Supply support for BCX79 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.

The BCX79 belongs to Fairchild's general-purpose bipolar transistor family, engineered for cost-effective, reliable amplification and switching in non-critical industrial and consumer electronics.

FAQ

What is the maximum collector-emitter voltage rating for BCX79?

The BCX79 has a maximum collector-emitter voltage (V_CEO) rating of 45 V at T_A = 25°C. This value is confirmed in the Absolute Maximum Ratings table and applies under continuous DC conditions. Exceeding this voltage risks avalanche breakdown and permanent device failure. The BCX79 must be operated within this limit in all circuit configurations, including inductive load switching where voltage spikes may occur.

Is BCX79 suitable for audio amplifier applications?

Yes, BCX79 is suitable for low-frequency audio amplifier applications due to its 6.0 dB noise figure at 1 kHz, 1.6–8.5 kΩ input impedance (h_ie), and stable h_FE across bias points. It is commonly used in microphone preamplifiers and line drivers where moderate gain and low added noise are required. However, it is not intended for high-fidelity RF or wideband applications beyond ~10 MHz.

What is the pin configuration of BCX79 in its TO-92 package?

The BCX79 uses an Emitter-Base-Collector (E-B-C) pinout in the standard TO-92 package, with leads arranged left-to-right when viewing the flat side with leads pointing downward. Pin 1 is Emitter, Pin 2 is Base, and Pin 3 is Collector. This matches the physical marking "E B C" shown in the original Fairchild datasheet diagram and is consistent across all BCX79 manufacturing lots.

Does BCX79 have a documented replacement or second-source part?

The BCX79 does not have an official second-source designation, but PN2907A is a functionally comparable TO-92 PNP transistor with identical pinout and overlapping specifications-including 60 V V_CEO, 500 mA I_C, and 625 mW P_D. While not a drop-in replacement due to differences in h_FE range and V_CE(sat), PN2907A is widely accepted as a design alternative where higher voltage margin is acceptable.

What thermal derating applies to BCX79 above 25°C ambient?

The BCX79 has a thermal derating factor of 5.0 mW/°C above 25°C ambient temperature, as specified in its Absolute Maximum Ratings. This means its total power dissipation must be reduced by 5 mW for every 1°C rise in ambient temperature. At 75°C ambient, for example, allowable P_D drops to 625 mW − (50 × 5 mW) = 375 mW. This linear derating ensures junction temperature remains ≤150°C.