Increasing U3S output on 6m #u3s


Greetings QRP Labs fans,

I’m using only one BS170 and ~8 volts in my U3S’s PA and am getting about 0.2W output on 6m.  I’d like to raise that.

The Si5351a’s output impedance spec is 50 ohms, which matches well with the BS170’s 60 pF maximum input capacitance at 50 MHz.  Operating multiple BS170s in parallel would therefore overload the Si5351A.

I noticed that the QCX’s RF output section uses a 74ACT00 between the Si5351a and the BS170s.  At the highest intended operating frequency of the QCX, the 3 BS170s in parallel would only overload the Si5351a slightly, so I suspect the buffer's function might be to amplify the drive on the BS170s.

I’m thinking of inserting a 74ACT00 between the Si5351a and the PA MOSFETs on the U3S so that I can run all 3 BS170s at 50 MHz.  The first NAND gate’s output would connect to the other 3 NAND gates’ inputs, thus providing 3, stronger drive signals to the PA section.

If I connect those NAND gate outputs in parallel, they might work against each other during the transitions to the extent that they don’t switch at the same time.  But it would simplify the rest of the circuit.  Alternatively, I could connect them to the BS170 gates individually, which would move the synchronization issue to the BS170 drains.  If I go the latter route, I suppose I would duplicate R6 for the other BS170 gates.  Should I introduce ballast resistors at the BS170 drains to absorb any synchronization transients?  Or should I connect the 3 NAND outputs in parallel and leave the BS170s in parallel, too?

Comments welcome!

-Halden VE7UTS


Some success!
I found a simpler way to boost the U3S’ output on 6m, and it helped even more on 2m.  I installed a BSN20-7 MOSFET in place of the BS170.  I chose this transistor for its lower input capacitance and Ron while also checking the maximum Vds and gate threshold voltage specs.  Unfortunately, it’s in an SOT23 package and not available in TO-92,  so I made an adapter.  Grooves cut with knife in the copper foil of a scrap of printed circuit board create separate pads for gate, source, and drain.  The drain pad is large to serve as a heat sink as this is the best thermal path out of the device.  See photo below.  I drilled holes for wires and installed it in place of the BS170.  I adjusted the bias potentiometer for maximum output and then backed it off about 10%.  Perhaps luckily, this point was the same for both 6m and 2m bands.

WSPR reports showed a 5 dB increase on 6m and 7 dB on 2m.  Per an oscilloscope connected across a dummy load, the power output is now 0.68 W on 6m and 0.25 W on 2m.

I hope this helps someone make a similar modification.  Having a few more WSPR transmitters on 6m may alert us to a few more QSO opportunities there.
-Halden VE7UTS