Hi all
A small number (approx 0.1% of this group) are experimenting with alternative PA transistors and methods to improve the robustness of QRP Labs PAs. I applaud their efforts, reading their?adventures with?great interest, and we should always encourage and admire experimentation and improvement.?
However I want to re-assure the remaining 99.9% of you that the BS170 is an excellent choice, reliable, durable, robust and inexpensive. I believe it is difficult or impossible to find a better transistor today if you include availability, price, packaging and characteristics as criteria.
BS170 MOSFETs have been used in QRP transceivers for 20+ years. For very good reasons. QRP Labs has been using them for 10+ years, first in the Ultimate-series QRSS/WSPR transmitters then in the transceiver kits starting with QCX. To date over 20,000 QCX transceivers sold have three BS170s in parallel Class-E, and almost 10,000 QDX/QMX transceivers sold have the quad of push-pull BS170's in the Class-D design. Years of experience have shown the BS170 largely "just works", reliably, day in day out for years.?
Generally when it comes to MOSFETs, the bigger the little lump of silicon in the middle, the more current it can handle, and?watts it can dissipate, but the higher the capacitances involved, which requires lower impedance drive particularly at higher frequencies. There's no free lunch. "Real" RF transistors may have little tweaks to improve this or that parameter or characteristic curve shape. In a Class-E or Class-D PA, hard switching is all we need anyway, and again the BS170 is great for this.?
The BS170 lies at a particular sweet point. The specifications of the device are just right for our QRP efforts at HF. The QDX can even produce over 2 Watts right up on 4m band (70 MHz). The 500mA current rating and 833mW dissipation rating make them well suited to the 5W PAs of QCX, QDX and QMX transceivers.?
Very importantly, they are low cost, easily available everywhere, and the TO92 through-hole packaging makes them perfect for that large majority of us that don't have equipment and/or inclination and/or experience for SMD soldering, in those situations where transistor replacement is needed.?
Yes, indeed there are circumstances where they could fail sometimes. There are people who push their?transceivers for absolute maximum power output, run with higher battery voltages, or perhaps SWR mismatches. I personally don't recall having lost a QCX/QDX/QMX PA except for that night there was a big storm and I was too lazy to get out of bed and unplug the antenna, and static zapped them. But in that case, it's easy and cheap to replace them.?
I don't see anything much not to love about a few BS170s working hard, when you want 5W at HF. If you want more power, don't waste time going for 6 or 7W. Do it properly and go for at least a 5x or 10x increase in power. For that matter the IRF510 comes next, a whole new praise topic, again easy, robust, reliable, inexpensive, and commonly available ;-)?
