STMicroelectronics SD1731

Overview

SD1731 from STMicroelectronics is a 50 V, 220 W PEP NPN RF power bipolar transistor in M174 plastic package, optimized for 30 MHz HF SSB communications with 13 dB gain and 40% collector efficiency in common-emitter configuration using gold metallization and emitter ballasting for ruggedness.

For engineers reviewing the SD1731 datasheet, SD1731 pinout, SD1731 application, or SD1731 equivalent, this device serves as a high-power RF amplifier stage in amateur radio transceivers, military HF radios, and fixed-station SSB exciters where thermal stability, intermodulation performance (–30 dBc IMD), and ruggedized biasing are critical selection criteria.

Technical Context

The SD1731 operates in common-emitter mode with V_CE = 50 V and I_CQ = 150 mA quiescent point, delivering 220 W PEP output at 30 MHz. Its emitter ballasting enhances safe operating area (SOA) under pulsed RF conditions and improves second-breakdown immunity.

Thermal design is enabled by low R_TH(j-c) = 0.55 °C/W and R_TH(c-s) = 0.2 °C/W, supporting high-duty-cycle SSB operation when mounted on a heatsink maintained at ≤25°C. The 330 pF C_OB at 50 V enables stable matching network design at HF frequencies.

Key Specifications

Pinout & Package

The SD1731 is housed in the M174 plastic epoxy-sealed 4-lead flange package with isolated collector tab, designed for direct heatsink mounting and high-power RF grounding. Pin 1 is Collector (tab-connected), Pins 2 and 4 are Emitter (dual-emitter configuration for current sharing), and Pin 3 is Base.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar RF power amplifier applications.

Compared with MRF454 and BLF177, the SD1731 offers optimal balance of proven ruggedness, 220 W PEP capability, and drop-in compatibility with legacy HF amplifier PCBs using M174 footprints and common-emitter bias schemes.

Availability

SD1731 is available at Aetrix Electronics and suitable for amateur radio transceivers, military HF manpack radios, and fixed-station SSB exciters requiring stable component supply despite its obsolete status through controlled legacy sourcing.

Supply support for SD1731 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, designing and manufacturing analog, power, and RF components for industrial, automotive, and communications markets.

The SD1731 belongs to ST's legacy RF bipolar transistor family engineered specifically for high-reliability HF linear amplification, emphasizing ruggedness, thermal stability, and spectral purity in mission-critical radio systems.

FAQ

Is the SD1731 still in active production?

No, the SD1731 is marked "Obsolete Product(s)" in ST's official documentation (Rev. 2, June 2004). However, Aetrix Electronics maintains traceable inventory from authorized legacy channels and offers full datasheet compliance verification for ongoing production use.

What is the correct bias voltage for the SD1731 in Class AB operation?

The SD1731 requires a fixed 150 mA quiescent collector current (I_CQ) at V_CE = 50 V, achieved via base-emitter forward bias typically set between 0.75 V and 0.85 V depending on junction temperature. Bias stability is enhanced by emitter ballasting and gold-metallized contacts.

Can the SD1731 be used at frequencies other than 30 MHz?

Yes - while optimized for 30 MHz SSB, the SD1731 maintains usable gain and efficiency from 1.8 MHz to 54 MHz per ST's test data. Performance degrades above 54 MHz due to decreasing h_FE and rising C_OB; matching networks must be re-tuned for each band.

What thermal interface material is recommended for the M174 flange?

A thermally conductive silicone-based grease (e.g., Dow Corning TC-5123) or 0.1 mm thick thermally conductive pad (e.g., Laird T-flex 400) is recommended. Mounting torque must be 0.5–0.7 N·m to ensure uniform pressure without cracking the epoxy seal while maintaining R_TH(c-s) ≤ 0.2 °C/W.