onsemi BC238

Overview

BC238 from Fairchild Semiconductor is an NPN epitaxial silicon transistor designed for low-noise amplification and general-purpose switching in analog and digital circuits. It features V_CEO = 25 V, I_C = 100 mA, P_C = 500 mW, h_FE = 120–800 (classified A/B/C), and f_T = 85–250 MHz. It is commonly used in audio preamplifiers, signal conditioning stages, and discrete logic interface circuits.

For engineers reviewing the BC238 datasheet, pinout, applications, or equivalent options, key selection considerations include its TO-92 package, V_CE(sat) ≤ 0.2 V at I_C = 10 mA, low noise figure (2 dB @ 1 kHz for BC238), and h_FE classification-dependent gain range - critical for bias stability and small-signal linearity in amplifier designs.

Technical Context

The BC238 operates as a medium-speed, low-power bipolar junction transistor with epitaxial base construction enabling improved f_T and reduced base resistance. Its DC current gain spans 120–800 depending on h_FE grade (A/B/C), and it supports linear operation up to ~100 MHz with verified f_T of 85 MHz at V_CE = 5 V, I_C = 10 mA.

It exhibits low V_BE(on) (0.55–0.7 V) and tight V_CE(sat) (0.07–0.6 V across operating conditions), supporting efficient saturation in switching applications while maintaining low distortion in Class-A amplifiers due to its specified 2 dB noise figure at 1 kHz.

Key Specifications

Pinout & Package

BC238 is housed in a standard through-hole TO-92 package with 3 leads: Collector (lead 1), Base (lead 2), Emitter (lead 3), per JEDEC TO-92 outline. Lead spacing and body dimensions comply with Fairchild's published mechanical drawing (Rev. A2, August 2002).

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar NPN transistor applications.

Compared with BC547 and 2N3904, BC238 offers the lowest noise figure (2 dB) among TO-92 NPN transistors in its class, making it uniquely suited for sensitive analog front-ends - though it trades off voltage headroom and maximum current capability.

Availability

BC238 is available at Aetrix Electronics and suitable for audio preamplifier stages, discrete logic level shifters, and LED constant-current drivers requiring stable component supply and long-term obsolescence management.

Supply support for BC238 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 BC237/238/239 family was developed for cost-sensitive, high-reliability analog and switching applications demanding low noise, stable h_FE, and TO-92 manufacturability - particularly in consumer audio, instrumentation, and industrial control.

FAQ

What is the maximum collector-emitter voltage rating for BC238?

The BC238 has a maximum collector-emitter voltage (V_CEO) rating of 25 V at T_A = 25°C. This value is explicitly specified in the Fairchild BC237/238/239 datasheet (Rev. A2, August 2002) under Absolute Maximum Ratings. Exceeding this voltage risks avalanche breakdown and permanent device failure. Derating is required above 25°C ambient per the datasheet's thermal curves.

Does BC238 have a defined noise figure, and under what conditions is it measured?

Yes, BC238 has a specified noise figure of 2 dB, measured at V_CE = 5 V, I_C = 0.2 mA, f = 1 kHz, and R_G = 2 kΩ. This value applies to BC237 and BC238 variants (not BC239, which is rated at 4 dB). The low noise performance makes BC238 suitable for microphone preamplifiers and other low-level analog signal paths where SNR preservation is critical.

What hFE classification options are available for BC238, and how do they affect circuit design?

BC238 is offered in three h_FE classifications: A (120–220), B (180–460), and C (380–800). These bins enable gain-matched pairing in differential pairs or consistent biasing across production batches. Designers must select resistors based on the minimum h_FE of the chosen grade to ensure reliable turn-on and avoid cutoff under worst-case conditions.

Is BC238 pin-compatible with BC237 or BC239, and can they be interchanged in a PCB layout?

Yes, BC238 shares identical pinout (Collector-Base-Emitter, lead 1–2–3) and TO-92 package dimensions with BC237 and BC239. However, electrical differences exist: BC237 has higher V_CEO (45 V) and lower noise (2 dB), while BC239 offers higher f_T (250 MHz) but higher noise (4 dB). Interchange is mechanically possible but requires validation of voltage, gain, and noise requirements in the target application.

What is the typical current gain bandwidth product (fT) of BC238, and how does it vary with operating conditions?

The typical f_T of BC238 is 85 MHz when measured at V_CE = 5 V and I_C = 10 mA. At lower currents (e.g., I_C = 0.5 mA), f_T rises to 150 MHz. This variation reflects the device's transconductance dependence on bias point. Designers targeting wideband amplification should operate BC238 near its 10 mA I_C sweet spot to balance gain, bandwidth, and thermal stability.