Looking for HF Driver Transistor for Transmitter Project

Discussion in 'General Technical Questions and Answers' started by KK5JY, Oct 3, 2017.

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  1. KK5JY

    KK5JY Ham Member QRZ Page

    I'm doing some experiments with a home-brew transmitter project. I have everything working, but the weak link is the driver stage before the final transistor.

    The PA board is this: http://qrp-labs.com/pa

    I did all of the recommended modifications, including replacing the switching FET with a real RD15HVF1 RF transistor, which is supposed to extend the "flat" gain output range of the PA board into the low VHF range.

    The exciter is the sinusoidal output of an AD9850 DDS. This is very low power, so I'm using an intermediate driver stage amplifier, consisting of a single BS170, very similar to the driver circuit used by the QRP Labs U3S transmitter project. This amplifies the DDS output enough to drive the 5W PA, at least on the low bands.

    This arrangement works for frequencies up to about 30m, where the driver transistor gain is starting to roll-off. The BS170 can drive the 5W PA board on 80m and 40m to about 4.5W, but the output is down to 1W by the time I get to 10m.

    Obviously, the BS170 wasn't meant for HF RF work, so I'm looking for a transistor that I could use to replace it. The circuit needs a part with reasonably flat gain across the HF bands. I'm sure I can dig around on Mouser and find something that looks good on paper, but I wonder if any of you have a favorite intermediate HF power FET that you prefer for this kind of application?
  2. KT1F

    KT1F Ham Member QRZ Page

    I don't know but QRP Labs sure seem to make some nice stuff at low prices. I don't think I've seen them before.
  3. KK5JY

    KK5JY Ham Member QRZ Page

    Nice kits for people like me who are just starting working on RF. I got that PA module and five LPFs, and the relay board that switches the LPF. The latter has a lot of options, and they aren't terribly well-documented, but an hour with an ohm meter and a battery was all it took to figure out the right settings.
  4. WA4SIX

    WA4SIX Ham Member QRZ Page

    Find an old CB & use the 1969(?) out of it. It is the same driver as a TS-440. Great parts in a CB.

    KK5JY likes this.
  5. KD8DEY

    KD8DEY Ham Member QRZ Page

    I remember reading somewhere (Oh *$&% there bubba goes again) That for RF work, to assure adequate gain. that the cut off frequency should be AT Least 8x the operating frequency that the transistor is operating at.
    Also Gain can be approximated by dividing the cut off frequency by the operating frequency....
    KK5JY likes this.
  6. W4KJG

    W4KJG Subscriber QRZ Page


    First -- congratulations on taking on such a project. We need a lot more people doing what you are doing, and then learning by asking questions when things don't follow the text book explanations. I just hope that the never-built-anything expert nay-sayers don't swoop in like vultures to ruin this project for you.

    I looked at the data sheet for the amplifier. Image 4 of 9 on the data sheet begins to tell a story. I can't tell from the writeup, but it appears that Images 3 of 9 and 4 of 9 are best case, under the best of conditions. I hate to say this, but I have a feeling that what you are measuring is probably about right. I don't fault QRP for providing that data. DON'T FRET OVER IT!

    I looked at the data sheet for the BS170. It has an input capacitance of 60 pF at 1 MHz. That is starting to get kind of high for an HF transistor.

    I'm using ADS9850s in a number of my crazy projects. They are wonderful. As you've found, they need just a little more oomph to drive certain circuits. I seldom use mine above about 10 MHz, but I do have some test jigs using them that work up to 30 MHz.

    My favorite buffer amplifier up to 20-30 MHz is one suggested by N3ZI to follow the output of his DDS: N3ZI DDS suggested Buffer Amplifier. As he suggests, splitting R3 and adding a capacitor between, to ground, extends the frequency range of the amplifier. I have no problem getting about 1 v p-p at 28 MHz out of this driver from the output of an ADS9850. The transistors are really cheap on Amazon and other places.

    I don't have a schematic for the QRP-Labs amplifier. But, I keep an ever-dwindling/regularly-refilled drawer of IRF-510s. They are extremely inexpensive, and they can work extremely well when controlled. They are very suicidal, especially in RF applications with touchy bias requirements. I regularly find ways to make fire-cracker noise with them, while also releasing the smoke out of them.

    Get your project basically working, no matter what the power output might be. Don't worry about it. Remember the difference between 5 watts (+37 dBm) and 1 watt (+30 dBm) is only about one S-Unit on the receiving end.

    With a working DDS, concentrate on getting the bank of LPFs working. Make sure the DDS and LPFs are separate modules. With that, you've got about 95% of the hard work accomplished. Just start plunking amplifiers in between until you find what you like.

    Maybe tie them into a project with a cheap RTL-SDR receiver/upconverter, and you'll have something most of us couldn't have possibly envisioned or dreamed of as recently as the 70s or 80s -- really, the early 2000s.

    Good luck,
    KK5JY likes this.
  7. KK5JY

    KK5JY Ham Member QRZ Page

    This is actually "the plan." I'm building a companion to my RTL-SDR. It could be any SDR, but the idea is to do uber-cheap HF radio. :cool: I'm building it up as a set of inexpensive modules so other kitters could use the ones they like, and replace the ones they don't, so that the kit is easily adapted. Since receivers are now sub-$20, there's nothing stopping us from having kits that let you have a 5W xcvr for HF for $50 to $70.

    This looks like a good project for tomorrow. I have bags full of 2222's of different types, so I'll go find the ones with the best specs, and give that website a good study. The other amplifier I tried was an LM7171 op-amp, but it also shows a lot of roll-off in the top 20MHz of the HF range, despite the data sheet specifications.

    Already there. :cool: Now I'm just optimizing the gain of the transmitter. That was the last step, and I knew it would take some time, so I saved it for last. ;)
  8. KK5JY

    KK5JY Ham Member QRZ Page

    I built the amplifier the way he had it in the schematic, and it has slightly more gain than the BS170 circuit I was using, but not much. The 2222 surprisingly has a GBP of 300MHz, but it apparently isn't very flat.

    It did provide enough extra gain on 80m to get the 5W PA output up over 5 actual watts, though. :cool:

    I'll try the split R3 modification next, to see how it compares.
  9. SM0AOM

    SM0AOM Ham Member QRZ Page

    If you need about 200 mW output and 10-13 dB gain, I would recommend a common-emitter 2N5109 or 2N3866 stage using RF inverse feedback.

    Using this, it is possible to tailor the gain and frequency response exactly to your needs.
    The W1FB/W7ZOI book "Solid State Design for the Radio Amateur" contains the whole design process.

    KK5JY and WA7PRC like this.
  10. KK5JY

    KK5JY Ham Member QRZ Page

    Yes, that would do it. I think that's about what I'm currently getting on the low bands, so if that would get me the same thing up through 10m, it would be just about right.

    Do you know what the most recent edition of that book is? I went looking for it, but the most recent one I can find is a PDF scanned from an edition from the mid-1980's. Apparently it was a League publication at some point?

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