Adventures in Amplification: Random MOSFETs, Fiery Endings, and Valuable Lessons
If there’s one thing every amateur radio operator learns eventually, it’s this: amplifiers are deceptively tricky little beasts. Sure, the concept sounds simple—take a small signal, make it bigger, and boom, you’re the loudest guy on the band. But in practice? Let’s just say my recent experiments with random MOSFETs, specifically the IRF150, taught me a thing or two about humility… and the importance of keeping a fire extinguisher nearby.
The Plan: Build an Amplifier for the QMX
The QMX transceiver is a remarkable little rig, but with its modest output, I thought, Why not experiment with an external amplifier? The idea was simple: take a few spare IRF150 MOSFETs (because, hey, I had them lying around) and build something that could boost the QMX’s signal.
Now, I know what some of you are thinking: “IRF150s? Those aren’t even RF MOSFETs!” And you’d be right. But I wasn’t aiming for perfection here—I was aiming for fun. Plus, the IRF150 is cheap, readily available, and, well, expendable. What’s the worst that could happen? (Spoiler: a lot.)
The First Attempt: Smoke Signals, Literally
The first version of my amplifier was… ambitious. I breadboarded a basic Class AB circuit, using the IRF150s as the power devices. With a little bit of real maths and some careful strategic planning (guessing and hoping it wouldn't blow), I figured out the biasing and slapped together a heatsink that looked more like an art project than a functional piece of hardware.
Excited to see it in action, I connected the QMX, set everything up, and hit the PTT. For a glorious half-second, I thought I’d nailed it with the scope reporting that I had turned 5W of RF into nearly 75w. Then the magic smoke escaped.
When a MOSFET fails, it doesn’t do so quietly. One of the IRF150s exploded with a loud pop, sending a plume of smoke into the air. The heatsink? It was warm enough to fry an egg. And my workbench? Well, let’s just say it now has a charming scorch mark as a permanent reminder of my optimism.
Lessons Learned (Sort Of)
Undeterred, I went back to the drawing board. The first failure taught me a few things:
- Biasing Matters: Getting the bias current wrong is a surefire way to turn your MOSFETs into very expensive resistors.
- Heatsinks Aren’t Optional: IRF150s aren’t exactly known for their efficiency, and my makeshift heatsink wasn’t cutting it.
- Don’t Rush Testing: Maybe blasting it with full power on the first go wasn’t the smartest move.
With these lessons in mind, I rebuilt the circuit, this time adding better bias control (using a variable resistor this time), a larger heatsink, and even a fuse (because apparently, I do learn from my mistakes).
The Second Attempt: A (Mostly) Functional Amplifier
The second version of the amplifier fared much better. I carefully adjusted the bias, double-checked all the connections, and powered it up slowly using a current-limited power supply. This time, it didn’t immediately blow up, which I took as a good sign.
The output wasn’t spectacular—around 25 watts from a 5-watt input—but it worked! For a while, anyway. After about 10 minutes of testing, one of the MOSFETs overheated and failed. No fire this time, thankfully, but it was clear that the IRF150s just weren’t cut out for the job.
Why Bother with Random MOSFETs?
At this point, you might be wondering why I didn’t just use proper RF MOSFETs. The answer is simple: experimentation. Playing around with random parts is a fantastic way to learn. Sure, it’s not always efficient (or safe), but the lessons you learn from failure stick with you far more than those you learn from success.
For example, I now have a much deeper appreciation for thermal management, impedance matching, and the quirks of MOSFETs. I also have a growing collection of fried components, which I’m considering turning into a “Wall of Shame” for my workshop.
Takeaways for Fellow Tinkerers
If you’re thinking about experimenting with amplifiers, here are a few tips to avoid (or at least minimize) the fiery endings (inevitable, but still probably best avoided):
- Start Small: Don’t jump straight into high power. Begin with low-voltage, low-current circuits to get a feel for how everything works.
- Use Proper Components: While it’s fun to experiment with random parts, using MOSFETs designed for RF applications will save you a lot of headaches.
- Invest in Cooling: Heat is the enemy of electronics. A good heatsink and active cooling can make all the difference.
- Test Gradually: Use a current-limited power supply and slowly increase the input power to avoid catastrophic failures.
Closing Thoughts
In the end, my IRF150 amplifier experiments were a mixed bag. I didn’t end up with a reliable amplifier for the QMX, but I did gain a lot of knowledge (and some great stories). And honestly, that’s what amateur radio is all about—experimenting, learning, and having fun along the way.
Would I recommend building an amplifier with random MOSFETs? Probably not. But if you’re like me and can’t resist the urge to tinker, go for it—just keep a fire extinguisher handy.
And, of course, I’ve got to give my number one friend, critic, and supporter some credit for keeping me "grounded" (and occasionally laughing at my failures with me). I don't think she gets why I feel the need to take apart literally everything I own before putting it back together, and I’m pretty sure she thinks I’m a bit mad, but hey, maybe that’s part of the charm. Either way, she’s definitely one of the reasons I keep chasing down these ridiculous projects.
Until next time, 73s and happy experimenting! -Daniel