onsemi BC557A

Overview

BC557A from onsemi is a PNP silicon small-signal amplifier transistor in TO-92 package, rated for V_CEO = –45 V, I_C = –100 mA continuous, and h_FE = 120–420 at I_C = –10 µA. It delivers low V_CE(sat) (–0.075 V typical) and f_T = 320 MHz, enabling use in audio preamplifiers, level shifters, and discrete logic inverters.

For engineers reviewing the BC557A datasheet, pinout, applications, or equivalent options, key selection criteria include its guaranteed DC current gain range (A-series), thermal resistance (R_θJA = 200 °C/W), saturation voltage under defined drive conditions, and TO-92 mechanical compatibility with legacy through-hole designs.

Technical Context

The BC557A operates as a general-purpose PNP bipolar junction transistor optimized for linear amplification and switching in low-power analog and digital circuits. Its design emphasizes stable h_FE across collector currents from 10 µA to 100 mA and low noise figure (2.0 dB at 1 kHz).

It features a maximum junction temperature of +150°C, R_θJC = 83.3 °C/W, and Cob = 3.0–6.0 pF at V_CB = –10 V - parameters critical for high-frequency stability and thermal management in compact PCB layouts.

Key Specifications

Pinout & Package

BC557A is housed in a standard TO-92 (Case 29, Style 17) plastic package with 3 leads, compatible with manual soldering and wave soldering processes. Lead spacing matches JEDEC TO-92 footprint (0.1" pitch), and mechanical dimensions comply with ON Semiconductor's Issue AL drawing.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar PNP small-signal amplifier applications.

Compared with BC557B and 2N3906, the BC557A offers the narrowest h_FE spread among BC557 variants (120–420), making it optimal for designs requiring predictable, moderate-gain performance without overdesigning base drive or feedback networks.

Availability

BC557A is available at Aetrix Electronics and suitable for audio preamplifiers, discrete logic interfaces, and current mirror reference circuits requiring stable component supply, long-term obsolescence resilience, and full traceability.

Supply support for BC557A 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

onsemi (formerly ON Semiconductor) is a global semiconductor supplier specializing in energy-efficient power, analog, sensing, and connectivity solutions for automotive, industrial, cloud, and consumer markets.

The BC557A belongs to onsemi's legacy BC5xx family of general-purpose bipolar transistors, designed for cost-sensitive, high-reliability through-hole applications in analog signal conditioning and discrete logic.

FAQ

What is the maximum collector-emitter voltage rating for BC557A?

The BC557A has a maximum collector-emitter voltage (V_CEO) rating of –45 VDC. This value is tested under specified conditions (I_C = –2.0 mA, I_B = 0) and defines the absolute upper limit for safe operation in common-emitter configuration. Exceeding this voltage risks avalanche breakdown and permanent device failure. The BC557A datasheet confirms this rating applies specifically to the BC557 series, distinct from BC556 (–65 V) and BC558 (–30 V).

Is BC557A pin-compatible with BC557B or BC557C?

Yes, BC557A is fully pin-compatible with BC557B and BC557C - all share identical TO-92 (Style 17) packaging with Collector/Base/Emitter on pins 1/2/3 respectively. However, their DC current gain (h_FE) bins differ: BC557A is 120–420, BC557B is 180–500, and BC557C is 270–800. Substitution is electrically safe but may require bias network verification if gain sensitivity is critical.

What is the typical noise figure of BC557A and under what conditions is it measured?

The BC557A has a typical noise figure (NF) of 2.0 dB, measured at I_C = –0.2 mA, V_CE = –5.0 V, R_S = 2.0 kΩ, f = 1.0 kHz, and ∆f = 200 Hz. This low-noise performance is confirmed in the official onsemi BC556/D datasheet and makes the BC557A suitable for low-level signal amplification where signal integrity is critical, such as electret microphone preamps.

Does BC557A have a specified transition frequency (f_T) and how is it relevant to circuit design?

Yes, BC557A has a specified transition frequency (f_T) of 320 MHz, measured at I_C = –10 mA, V_CE = –5.0 V, and f = 100 MHz. This parameter indicates the frequency at which the transistor's small-signal current gain drops to unity, directly limiting usable bandwidth in amplifier and oscillator designs. For stable gain, practical operating frequencies should remain ≤ f_T/10 (i.e., ≤32 MHz) unless compensation is applied.

What thermal resistance values apply to BC557A and how do they impact PCB layout?

BC557A has R_θJA = 200 °C/W (junction-to-ambient) and R_θJC = 83.3 °C/W (junction-to-case). These values mean that without forced airflow or heatsinking, power dissipation above ~3 mW raises junction temperature by >1°C - necessitating careful copper pour sizing and proximity to heat sources. The TO-92 package relies heavily on PCB copper area for thermal relief; doubling pad size can reduce R_θJA by ~15–20%.