MOSFETS for HF PAs -what do I need to know to start contemplating this? - Page 1
Forum section: electronics / rf-microwave

author: cdev
author_rank: Super Contributor
timestamp: April 19, 2022, 12:58:57 pm
content: I am looking around at QRP right now in anticipation of ham licensing. My biggest obstacle now is my lack of mobility. A situation that many possibly share. But I do have at least one local electronics distributor whop is well stocked when it comes to power transistors.Also, I have a bunch of copper clad PCB material in good shape, as well as heatsinks, various magnetics and so on.I may not build my own rig, so much as build a kit, but I do want to build my own PA. I want to do this in case I accidentally destroy it with an experimental antenna I want to be able to fix it, redo it, etc. I dont want to be timid when it comes to my antenna, I want to be able to fix my output finals. ANd of course if I am running SSB (A3) I want my modulation to be linear. And clean.So I will need DC power, clean DC power? Presumably the current matters as to its ability to amplify.. cleanly?And so on.. I do have lots of large healthy capacity electrolytic capacitors and power transformers capable of delivering what I suspect is enough power.To be specific, I am hoping to be able to amplify a QRP signal cleanly and legally. But I am not a wealthy guy, I am actually on a tight budget. Not currently having a spectrum analyzer, which are to me expensive pieces of kit I probably wont have for awhile..My wife is fairly tech savvy, (she is actually a computing pro, working ion a related field) and she is likely to realize if I am wasting money, and she would and does(and I would expect her to) call me out if I was wasting money, so I have to make every purchase count! Optimize it..Although currently it seems she has no interest in ham radio, that may change. She might even get into it someday. I want her to realize that I am doing my best to DIY and save money as I go. And get maximal use out of the "junk" I already have acquired.. (too much as far as she is concerned!)I do have a 50MHz o-scope, however..which I think is within calibration specs.. I am wondering if that will be enough to adjust a MOSFET power amplifier? Also, I am wondering if there are any specific designs I should look at.As far as output power, I have fairly modest goals of simply generating enough to have my signal get out and being able to make contacts..without it being frustrating. I have no need to be particularly loud, or strong.Although there is a lot of noise where I live, our QTH I suspect is an adequate one for being a ham. It might even be a good location.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 19, 2022, 01:34:13 pm
content: Have you looked at this?http://golddredgervideo.com/kc0wox/wa2ebyamp/

author: cdev
author_rank: Super Contributor
timestamp: April 19, 2022, 02:01:38 pm
content: Sure, I can do that (test the filters properly) with my nanovna2Would it be adequate to verify the low pass filters functionality? Can I use the scope to test for linearity? I should be able to view the signal envelope in real time with my RF probe.The o-scope is linear, and reads out in volts/division.? I have a 2200 series Tek 2211.Maybe some local ham has a spectrum analyzer who could verify clean signal and lack of splatter/junk?I think I am likely going to be using an exciter that has LPFs too. I have a lot of home built filters already. I like to build filters for my cheap SDRS.

author: Grandchuck
author_rank: Super Contributor
timestamp: April 19, 2022, 02:44:52 pm
content: This is one way to test linearity. If you don't have two audio generators you could build something.

author: A.Z.
author_rank: Super Contributor
timestamp: April 20, 2022, 06:38:03 am
content: as for two tone tests, it's also possible to use a computer audio cardhttps://vk6ysf.com/ssb_2tone_test.htmotherwisehttps://dk7ih.de/a-simple-2-tone-generator-for-testing-the-linearity-of-ssb-transmitters/HTH

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 20, 2022, 07:34:07 am
content: What band?Most of SW is reachable with fairly pedestrian power transistors, let alone at QRP levels. IRF510 and bigger have been demonstrated many times, at 10s of W; a MJE340 say, should get several watts, or even just 2N7002 or MMBT3904 for 100s mW. Or ye olde 2N3866 but you probably want something cheaper than that old bastard (unless you have some on hand already). For receivers, you may want to pick up some JFETs or RF BJTs (MMBTH10, BFR92?).If you've a stockpile (or access to) old CRT TVs and monitors, the video outputs are usually quite good performing BJTs, of modest sizes (up to 5W say). Later monitors adopted hybrids or ICs, but modest-resolution units used discrete circuits. I have a small baggie of 100V 100mA transistors pulled from old, iMacs I think, years ago. I suppose even the integrated versions could be used directly, with a bit of matching; you even get three of them! (Easy enough to wire in parallel with a pair of 0° hybrids. Or 0 and 180, or even get freaky and use four 120°'s.)VHF and up, pretty much you'll have to buy something. I suppose video transistors could work 2m well enough, but you're very much looking at purpose made LDMOS or whatever. Or just some damn MMIC -- they're not exactly expensive or hard to find, and quite easy to use.Tim

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: April 20, 2022, 11:42:46 pm
content: IRF 520 are available and inexpensive. I buy big lots and they are about $ 1. More if smaller lots. Mouser is a good source.Grandchuck's suggestion is a good one. Yhere are lots of IRF 520 linears very much like this one. If you need more power then you can line up these 520s and drive them with another 520..I have seen ten 520s lined up.You need a PS capable of more watts than you are planning on using. If you build a linear PS test it into a load (a big resistor). If you are planning for a 100 W RF linear, then you need maybe 150 watt capability.E=IR, So at 14 volts you need a load of about 1.3 ohms. You can make one with a bunch of resistors in a paint can with mineral oil in it. Maybe seven 20 watt resistors of 10 ohm value in parallel for testing your PS. You will need a big transformer if you build a linear supply. Big transformers are expensive. If you have one that can supply the power you need, you're in like Flint.Could be cheaper to go to Hamfest and find a 25 amp Linear supply. They are easy to fix and cheap if not working. Sometimes cheap if they are working. Just make sure the transformer is OK.I bought a non working linear 35 amp unit for $20 (there was almost nothing wrong with it, easy repair) and a working one for $80, both in last 6 mos. Nice to have one with a volt meter and an amp meter so you can see how much power you are using when you build your RF linear.There are good switching PS but there are more that are not very good at all and all of them make RF noise. They are much more efficient, however.With mosfets you are not confined to the "12 volt" supplies. you can use more volts. But then you need a PS for the QRP rig and then another for the linear.Maybe someone here knows how many volts the IRF 520 can handle. (or 510)Some of the new mosfet RF linears use 50 volts.You will need a 50 OHM load for testing you RF linear amplifier, Again, you can make one in a paint can, These things were very popular in the past and still sold new, commonly called Cantennas. You can pick one up at a Hamfest for $ 5.You can make a "take off" from the Cantenna by wrapping about 5 turns of wire around the outside of the coax feeding the load. The wires are connected to your scope or a spectrum analyzer or a freq counter or whatever. Some Cantennas came with a "take off' already installed and a RCA jack for connecting to test equipment.If the CW wave on your scope is not smooth, there may be a problem such as harmonics.Good Luck!!!

author: cdev
author_rank: Super Contributor
timestamp: April 21, 2022, 12:21:34 am
content: This looks like it would be easy to source at NTE's warehouse not far from me in Bloomfield, NJ.I used to go to the predecessor at the same exact place.. It was a high point of my young adulthood. I went to high school not far away from there..so used to go there on my way back from school. US electronics buffs who live near NYC, I think this is the best electronics store in the NYC area, at least it is that I know of. NYCused tohave lots of electronics stores in Lower Manhattan but they are now long gone.. Frys is still hanging on but really just a ghost of its former glory.. It was dingy even in 2006 when I left California..There is also Micro Center here - not too far away, they are a large chain electronics and computer store.. And B&H Camera in NYC is planning to move to NJ, but I dont know where.. My guess, It will probably be somewhere along the Route 46/Route 3 corridor/Also near Route 80..The NTE warehouse is right off of Bloomfield Ave.. renowned as "Guinea Gulch" in its many mentions on the Sopranos TV show. Indeed, there was an Italian restaurant in the North Ward of Newark I think, where the local Mafia boss and his cronies used to meet.. the bus went right past it.. I remember all the black Cadillacs that used to park there.. Anyway, things have changed a lot since then, huh? Now the Bloomfield Ave area is much nicer and much safer than it was in the 70s and 80s..Although there still are a great many Italian restaurants..Which when I was a kid, Italian and Chinese restaurants were the only places one could get decent vegetables with your dinner.. They kept the flame of green semi healthy food alive in restaurants.. Decades before Chez Panisse.. Cross the bridge from Newark, (over the railroad tracks) and then one is in Bloomfield.. and around a block from what is now NTE.. But for my entire life this was a cluster of electronics related stores.. It had a barn like feel, lots and lots of stuff in bins.. and a will call window where you filled out a little form and they got your parts and called out your name to pick them up and pay. Instant gratification. Its been there literally forever..This is actually a really great area to live in if you are going to live in the US. And dont want to pay a kings ransom in rent every month.. This area still has a healthy middle class.. And good jobs..Where they now have lots of power transistors.. at good prices.. although I'm sure I could find them for less, the online sources might be sketchy..NTE2382https://www.nteinc.com/specs/2300to2399/pdf/nte2382.pdf

author: cdev
author_rank: Super Contributor
timestamp: April 21, 2022, 12:41:01 am
content: I've been gradually building up the various things that I am going to need, bit by bit..The goal is in sight.. it seems, along with the skills.. Dummy load is coming together.. actually..I now have a deceptively tiny(fingernail sized, its that small) (supposedly!) 250 watt capable 50 ohm resistor designed for use as a dummy load. It measures exactly 50 ohms. Its basically so small that I know it has to be heat sinkedvery well.. How do I do this? Treat it as if it was a CPU, I am guessing, a hot one..I cant put it in a can of mineral oil, I dont think.. although maybe I can, somehow..I would rather just heat sink the thing really well.. Presumably I could use a PC heatsink/fan combo in some way? How does this sound, make a 50 ohm trace going across a small piece of FR-4 solder the trace to the top side of the resistor using the provided thin copper foil.. Solder the bottom to the other side and also solder the resistor to another pice of PCB beneath or better yet, a square piece of solid copper. (use it as a heat spreader) Affix the solid copper to as large a heat sink as I can, right where the CPU would usualy go.. and in exactly the same way.. I have several years worth of unused CPU fans.. some quite capable, from the power guzziling CPUS of the past.. Actually, I dont need a HSF per se, although they are good for this as they concentrate their heat wicking abilities on a small area.. which is what I want to do. Any heat sinks I can find thats not "too big" or mechanically unwieldy. But a HSF is good because of the focus on cooling one spot.. and also likely to be manageably compact.. I should mount it with the heat sink on the side, fins facing vertically and a sort of air funnel so that convection carries air past the fins naturally. I can use a thermistor so that the fan doesn't amp up until it needs to. Like a computer.Properly contained heatsinked, and interconnected the little white flanged resistor seems potentially great for testing an RF power amplifier, baluns, any RF related gadget.. in prep for getting on the air. Basically its like a cantenna (Heathkit) potentially, if I am good to it and dont abuse it.Question. Keeping in mind the real but hopefully remote possibility of a thermal oscillation, feedback caused, possibly, I am guessing thermistor might be a dumb mistake (its putting too much confidence in a slow ramp up in heat triggering the fan when I may not have the luxury of time..Its legendary how fast solid state RFfinals can just pffft.. and let the magic smoke out..and SO guessing would be better to run the fan continuously on high for the first power testing. Use Arctic Silver or similar.. And it would also be good to use my current controlled power supply and set the current output to the expected current plus no more than 5 or 10 percent. This is where a current controlled bench supply is really essential.. Set an alarm that goes off if the current starts going up in any unexpected manner.. to trip off..

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: April 21, 2022, 11:52:14 am
content: I would just run the fans continuously.. It's not as though you are going to run this thing for a long time anyway. Why complicate things?Run some power (even DC) into your RF load without the fan to see how it heats up.You are absolutely correct, heatsinking the small 50 ohm resistor is the critical point. In General, resistor elements in themselves are small. There are exceptions, wire wound resistors are large usually.The cases on resistors are much larger than the resistive element, cases are relatively large so that is part of the heat sink. The case part is relatively smaller on surface mount devices obviously.Thermal runaway is a real consideration in heatsinking the RF mosfets. All the mosfet amps I have seen have fans also. And BIG heatsinks.You will not have an exact 50 ohm load when finished. The capacitance and inductance of your assembly will affect the impedance of the unit. But that does not mater much at HF Freq.. You just need to be close. I would think your proposed RF load might not work well at VHF and UHF, but I don't think you need to worry about that now. The cantennas do not work too well at VHF either.It is even a big challenge to get an antenna system to be a 50 ohm load.Bench power supplies that have current limiting are typically lower power. Modern good PS turn off when limit is exceededHigh power output "bench" PS with variable amps (adjustable from the front panel) are not common, I have one, It weighs 80#..

author: cdev
author_rank: Super Contributor
timestamp: April 21, 2022, 02:20:21 pm
content: Quote from: Wallace Gasiewicz on April 21, 2022, 11:52:14 amI know what you mean. I have a Sorensen adjustable triple DC power supply rated at 60 watts per. It doesnt weigh 80 lbs but itispretty heavy. I would estimate 50 lbs maybe. It is definitely a boat anchor. Thats the one I would be using because (aside from the hum it puts out while operating, which isnt RF noise, its just the hum of the transformer.. ) its very quiet as far as RF goes. I recently freshened up the heat sink thermal compound on the massive heat sinks it has. Just because it is almost as old as I am and they had dried out... It will be ideal because its possible to set the output current limiting very precisely, within a few ma.