QDX FDT86256 PA MOD KitSold By JasonKits QRP
Acknowledgment: Thanks to Barb WB2CBA for sharing this mod and making it public to the ham community. The PA mod accommodates two FDT86256 Mosfets and provides some heatsink PCB space with footprint…Read More…
Acknowledgment: Thanks to Barb WB2CBA for sharing this mod and making it public to the ham community.
The PA mod accommodates two FDT86256 Mosfets and provides some heatsink PCB space with footprints matching the motherboard BS170 for easy installation.
The FDT86256 offers several advantages over the BS170. The most notable advantage is its higher breakdown voltage (Vds), which stands at an impressive 150V, in contrast to the BS170's 60V.
Another significant benefit of the FDT86256 is its ability to handle higher power levels, with a rating of 10 watts, compared to the BS170's 800 milliwatts.
However, it's worth noting a drawback of the FDT86256 when compared to the BS170, and that's its higher input capacitance (Ciss), which measures 55 picofarads (pf). This higher capacitance can potentially impact its performance at higher frequencies. However, in a Class D configuration, it performs comparably to a pair of BS170s. Typically, the Ciss capacitance of a BS170 is around 24pf. When used in parallel pairs, the total capacitance reaches 48pf, which is not significantly different from that of a single FDT86256. This encouraged me to consider the FDT86256 as a replacement for a BS170 pair.
An additional benefit of this PA Mod is that it allows for the operation of the QDX under 13.8 Volts. Under this voltage. QDX can comfortably reach the 5-watt limit without worrying about damaging the finals. However, it's important to note a caveat: the 5 Volt regulator on the QDX board tends to get hot at this voltage, so exercise caution when operating at 13.8V.
Supplied as a kit! PCB and two MOSFETs or Assembled Unit According to the selected option. (QDX not included )
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