RF Power Amplifier Construction

hi i'm ron wa7 g IL and in this video i want to show you how i built this little amplifier RF amplifier for my qrp radios to give them a little bit of a boost well this is the schematic of the amplifier we're going to build and I need to acknowledge everybody involved in this I didn't design it the amplifier section itself was designed some time ago by diz W ADIZ and he runs and owns that kits and parts dot-com very good source for toroid Xand other parts that you need in your homebrew activities I highly recommend you go to his site and kits and parts dot-com especially for these input and output in binocular course like the BN 4333 12 that's a large one and the being 43202 next up wa2 Eby designed this circuit it was part of an article he wrote in qst March in April 1999 in an article he did about an amplifier that he built slightly different from this one but it I used the RF sensing circuit from that it keys these relays and bypasses the amp when it's not in service when ARF's present it turns on the relays and and passes RF through I designed the circuits over here that's a switching circuit to bring the 12 volts into the 5 volt regulator to provide the bias supply only who an RF is present so it's not on all the time and a little DC filter input and finally the low-pass filter output that you will be required to use as a 7 element circuit I'm acknowledging cons G 0 u PL of qrp - labs and dot-com he has he actually sells these in a smaller lower power version but he has all the details on the coil winding and capacitance values and he acknowledges in there that he got it from others as well so I hope everybody's properly acknowledged it's not my work I'm showing her it's just something as homebrew you do find circuits at work and acknowledge those did it and go for it build it so here we go I started with an aluminum heatsink found in my junk box drilled out the four corners tapped him for four 40 screws also drilled two holes in these locations and did the same then I got a piece of single sided copper clad board cut a couple of rectangular holes in it screw the whole thing down and mounted the transistors by simply screwing their tabs to the heatsink these are Mitsubishi Rd 16 h hf 1 RF MOSFET transistors the center pin is the source lead and it's also connected to the tab and the source is grounded in these designs and so there's no need for any kind of insulation padding between the device and the heatsink very simple build thing I had to do is do funny bends on the leads the drains go this way and the gates go this way so I had to bend these leads opposite each other compared to these okay here's the schematic and at this point we have the two transistors mounted and their source leads grounded well the next thing to make is the output transformer I'm using this be in 4333 12 binocular core is recommended for this design and you can get this from kits and parts comm for about two dollars I think we're gonna whine one term primary and three turns secondary and the primary turn will be a piece of rg-58 shielding with binocular course one turn is like a hairpin turn in and out both holes so that is one turn like that secondary will be three turns starting and stopping on this side I'm just using this plastic coated wire if this was a higher power than the 20 to 30 watt class you'd want to use something like a Teflon coated wire for that the completed output transformer with a one to three turns ratio this will be the input from the drains of the two transistors mount about right there and one side of this will go to ground the other side will be the up and now solder this there's one drain connected the other drain connected and one side of the output grounded you don't need any flux if you use good solder with flux like the Kester or 60/40 and and make sure your boards good and clean and that solder is very very easily to ground to the schematic again we've added this transformer the large core being 4333 12 as the output transformer the input transformer is wound on a B and 43202 binocular core like this one's much smaller than the output but it's also carrying much less power this will also have a 1 to 3 ratio but it would be 1 2 3 turns ratio so it would be actually 2 turns to 6 turns to on the primer on the input and 6 on its output so I'm going to start that way this is 26 gauge wire and I'll wind 6 turns on this and here's this input transformer with one of the windings the 6th turn winding of 26 gauge wire now I'm going to feed this one in and out twice so for 2 turns on on the input side I'm using a plastic covered wire just basically because I didn't have any other colors of magnet wire don't want to get them confused and that's what the finished transformer input transformer should look like it'll mount right here and there's a couple other components and some capacitors to isolate a DC isolated from the gates but it basically connects RF straight to the gates DC wise is not coupled to the gates because we'll be running bias in there too for the transistor bias settings this will be the input one side and ground just like the output and the other side is the input I like to place parts ahead of time for soldering some things to make sure I've got room for everything so here's where the input transformer will sit almost exactly this is a relay there'll be two of them one sitting approximately here and one sitting approximately over here and these two relays will either allow the amp to be in the circuit or bypass it for the receive direction of the path I'll be saving some room for RF sensing circuitry right here to drive those relays and some room over here for the bias circuits when I mounted the large transformer the 3 and the 2 real stiff wire connections here on the grounded wire here made this thing very very tight it's not not moving on the board this one's gonna have very fine wire on this side with capacitors kind of flying from it to there now this isn't a circuit board this is this is what's called dead body style construction so the only thing that would hold this thing steady to ground is one of the one of the these two wires have been grounded so I always like in a case like that to stick it down a little bit with some hot glue right where I want it and now it's tight okay if already gone ahead now and mounted the relays and connected the normal open contact down here on the bottom to the input and to the output the Commons of these two relays will be the actual RF input an output and then the normally closed contacts will just tie across to each other to pass the receive through when this thing is not energized you see between the input transformer and the gates we have some point 0 1 micro farad and ceramic capacitors and those again will isolate DC so that we can put DC bias on these two gates without it short now through this coil I did a few things there and grounded the one side of that I did a few things without the video running so I just want to bring you up to date where this stands it's all pretty basic so with this kind of construction you have parts flying in the air like here well is a very stiff pin here this is not very stiff but it's very short and this is tight and you can push on this it won't move so there's no much chance of this getting you know falling down and shorting to ground or anything okay onto next steps now in the schematic we've added the input transformer on the B and 43202 a binocular core and we've added the two points or 1 micro farad capacitors to the transistor gates they've also attacked the relays down and soldered these two connections in and out I've gone ahead now and added the two trimpots here and here for the bias adjustment these are 20k multi-turn I think 20 turned pots or something not critical just whatever we got one pin is grounded on both of them but that wasn't adequate to hold them stable enough to tune so there's a little bit of hot glue sticking them down to the board and I also added the input and output connectors by just soldering on a pieces perf board I mean the copper clad board and put the connectors on it these are soldered straight to the common point on these relays so it's good and rugged so far now it's time to get down to the actual wiring of more things we have to still add the bias circuitry and the RF sensor that's gonna trigger these relays when it's time to transmit now the two trimpots are installed along with the 470 ohm resistors and the bypass capacitors well I changed my mind and moved these two pots in closer to the transistors I get real close view here I added from the center pin 470 ohm resistor across to the gate of that one and of that one so that's how the gate bias is injected into the transistor this is the coupling capacitor you saw before taking the RF in and blocking DC from getting back into here now I need to put five volts on these two loop ends of these two you know this pin and this pin of these two pots I'm going to build that regulator circuit right down in that area I'll be using this 78 elbow five zero five five volt regulator I have the bent the center pin the ground pin I can see that because this will be mounted on its top dead bug style and that little part will be soldered to the board you know I'll come back to show you what that done I went ahead and added the regulator right down here that's putting five volts on to the top of this pot and on top of this pot yeah it's got a bypass capacitor but if we put 12 volts from here to ground we will get adjustable bias on both gates I'm gonna add a little bit more bypassing over here per this schematic and go from there so the next thing after the bias is to build the RF sensing circuit I literally have to take a sample right there and build a little solid-state switch that will drive these two relays oh that's the next step I'm going to install this 4.7 K resistor fuzzy there down here on the board this is part of the RF sensing circuit and it also being a carbon resistor it's pretty rugged and it'll be a good standoff down here for other parts that have to come through the air to get to it in this type of construction and there it is now on to the other parts we have to put a capacitor and a resistor in series from this RF input over to there so I want to show you where we're at now I've got the from the RF input series capacitor 1k resistor and then 4.7 K resistor to ground from there and there's a diode cathode and up to ground from the same point in another diode this little voltage doubler here detector so it'll turn the AC RF here into DC signal and we'll have a small filter capacitor and a transistor will hit the base of a transistor switch this is a good time to show the ruggedness here this part will not move and this is still a little wobbly because it's you know soldered anything yet but the next phase is to put a transistor down here bottom side I'm top side down just like we did the regulator and in that particular case of switching transistor NPN and the emitter will be grounded so that'll be the leg that holds it tight and everything else will solder in the air now I've added a few more components here and this diode we remember from the last time actually goes to the base of the transistor the emitter lead of that transistor is grounded there's a point zero zero 1 microfarad bias bike basket back it's a filter capacitor in this case the filter out the DC signal coming out of this diode and this is the collector lead and there will be some capacitors across from the collector to the base for some timing just how long the relay holds in after it seizes up but yes since this is the switch contact now that will pull ground down on the one side of the coil are these two relays to energize them now I've finished the relay and drive our circuit right here it's kind of hard to see in the video but that is the transistor we were looking at before this dead bug mounted style this is its collector lead this capacitor is a timing capacitor on the release of the relays this is the connection to this relays with the negative side and this is to this relays negative side this one has a back EMF protection diode across the coil and they both have RF bypass the ground just because they're in the vicinity of stuff here and I thought it might be a good idea so that's that we now have bias we have keying circuitry so the last thing to do is to figure out a way to get DC down into this thing now the RF sensing circuit is built this way through year two this transistor well it's tied to the relays to energize them this circuit came from wa2 Eby from an article he wrote March in April 19 1999 qst where he built a amplifier similar this using high RF 510 transistors I've modified a few things here but that's this is a basic circuit where I got that the next step is to provide a little transformer like this that has a center tap for the DC to come in and then two leads to take it to the transistors Cologne drain circuit so I want to show how to make one of those this is not the right ferrite or the right wire but take two strands of wire run them around for whatever the turns count has supposed to be eight or ten I don't remember I'll just do a couple here for illustration and like when you get all your turns on there even then separate these wires 10 the ends discover which ones are which because they'll be a continuity on one and the other and then spread them out with continuity this way then take one and diagonally across another one like this it's like I went from here to there twist these the other that would be the center town it's that easy I didn't want to unwind this coil our he had made because you don't have any more of this larger wire diameter also the drawing called 4ft 5061 and i didn't have any so i stacked a couple of ft 3716 ones it seems to work alright this thing will be mounted down here literally connecting the two drains just like that and DC will come in the center of it okay now we've got the transformer in here and it's time to get DC to it I'm gonna build a little filter here just a little bit of extra added protection that's right that's the view here there's this transformer here's where the filters gonna go I want to demonstrate another means that's used in dead bug style construction and that's called island cutting to do that got this special drill bit that I bought it Harbor Freight it's just a rotary saw basically with a spring-loaded sharp tip on here the tip doesn't drill through anything it just gives a guide point to keep this from wobbling around it wobbles enough as it is I'm gonna make two two little round islands here and show you how that works didn't quite get it all the way through clean cut all the way around is what I meant but not through the whole thing just get the copper removed one more time they're not quite getting as you can see right here okay that's two very clean pads that we can solder to that are not grounded okay the filters built here these two pads are isolated from everything else so these capacitors go to ground and then coil goes across I'll bring 12 volts into this thing here and then bring 12 volts here up to here in the various other places it needs to go and then it'll be finished at this point everything is in except for this little DC switch and the regulator is just powered on all the time for the testing I'm going to be doing so the amplifier is it complete and working and switchable and then we'll go into some testing okay all the DC wirings done this thing is connected to power I checked the with the old meter make sure didn't have shorts before it did that I'm running the ammeter here in series with the DC positive input to the thing pulling about three milliamps right now I'm gonna have to add go ahead and energize this thing to get DC well look to get the relays connected so everything's good now I want to adjust the bias and it's not tricky at all you see we have a quiescent 68 and I want to put about 20 20 milliamps per device and tricky getting that in the slot there it goes so we just turned this until it reaches the gate threshold nothing will happen then it starts up turn to about 88 or 89 milliamps to get around 20 okay so that's close enough that's the first one now when you bring the other one up and say my mouth I'll be going to about a hundred ten or so milliamps when it reaches a threshold it'll start climbing and there it is roughly close enough now this is made much simpler multi turn pots okay so now we're gonna plug the signal source and you'll see a jump in the current as as a little bit of Drive is in there okay so it went up quite a bit now that means we do have the amplification going on some sort here the scope is not on screen here but I'm gonna be looking at the scope and tell you what I see and then we'll show it to you in a bit here's the input and I'm seeing about two volts about twenty volts peak-to-peak so it's it's making some gain that means it's amplifying that's all it means right now we don't have bias optimized we don't have drive optimized don't have anything like that but those are steps to take also you'll note this has no input pad it's a good idea with with any drive to have some pad there make mainly to make sure you got a good 50 ohm match I've tested this it's pretty good it's it's only it's better than two to one across the HF spectrum on the input with nothing so as long as you don't overdrive it it's fine the output obviously doesn't have any filtering on it or anything that's just a basic amplifier so this wouldn't be good to put on the air yet but next steps is to start playing with it okay now I'm showing the scope I'm just gonna see here here's the input from my test oscillator about two volts got put and it's closer to 20 volts if you move over here to the spectrum analyzer we can see fundamental as seven megahertz the second harmonic fourteen megahertz is much lower the third is a little higher than that and the fourth is low again and that's is indicative of a push-pull amplifier and the even harmonics will be much reduced from the odds so finally I'm going to add this PNP transistor switch here to take 12 volts into the regulator switching it with the same RF decoder RF signals coming in from over here that'll complete the whole thing I've put the little switching transistor down here it's a PNP it's looking at the ground that comes from this RF sensor circuit that we built earlier and it's providing 12 volts DC then over to the regulator that regulates down to 5 volts for the 2 biased supplies this circuit allows it then to switch the bias on and off with transmit so it's not on all the time like it was in my test case I also added this little piece of coax that connects the receive signal through around the amplifier on the normally closed contacts of these relays and that completes that build now I did some interesting tests and I know this is hard to read but these three curves represent the power output the efficiency and the gain in decimals and different scales over here but what I was measuring against interestingly enough was differences in the quiescent drain current in other words the bias settings if you will so this is 20 40 60 milliamps per device in the earlier part of the video here I said to set it for 10 milliamps or 20 milliamps per device that was a good starting point what I found out is the efficiency gain and the power out this is with one watt drive on 40 meters don't know what difference would make everywhere else right away but that's what I did one watt on 40 that all three of those items converged at a peak right here when I was set at 30 milliamps per device so that's where I reset it to and I think that's a good good spot for CW some of the stuff I read said you got to go way up to I tested up 250 milliamps per vice but you need to go up and maybe even 200 milliamps per device to get into Class A on these I don't know that I haven't tested it but that's it is capable in your operation this particular design okay now I want to show how to make a low-pass filter for this radio I'm going to use a very small piece of copper clad board that's about 3/4 of an inch by 2 inches then I drilled a couple of those islands on it and I mounted the first coil this will be a seven element low-pass filter so I'll have three toroid sand four capacitors I'm just mounted the first one I wanted fairly big wire didn't have it so used two strands of about a twenty six gauge to be good enough that seems to work now I've gone ahead and added the two other inductors and these RCA jacks on the ends I like using these they work fine through HF I've tested them on the spectrum analyzer and they look good but if you're a purist one being C's or anything else go for it doesn't matter what you use this is homebrew all F do now is add capacitors I'll be adding capacitors from here to ground and from here ground there the ground was the completed filter it's only for capacitances but I took a parallel pair of capacitors on both ends because I didn't have the right values these in the middle or 680 680 Pico farad and I needed about 260 or 270 on the ends I have a 220 and a 40 and parallel on each end for that so there it is and now we're going to test it and here it is on the spectrum analyzer running the tracking generator through it back to the input of the analyzers would come out of here through the filter and back in closely on the marker there 7.06 megahertz has got about a half a DB of insertion loss here it is with the low-pass filter in the circuit something and now looking at the spectrum analyzer here goes to the 1 watt input again you can see all the way out to 50 megahertz that it's clean this little amplifier will make about 20 watts on 20 meters and this filter here with an adapter goes right on the output it works on 20 and 30 meters currently 40 meter filter goes on 40 meters this thing will put out about 30 watts you know 80 meters it'll make a belt almost 40 Watts about 37 months so works works pretty good and that's with one watt of Drive it does that and that that's completes this video and I just want to say one more thing about legalities here you are responsible for making sure you have the proper reduction of harmonics in my case I built three separate low-pass filters to plug on externally to change bands that way I just change the filters in the u.s. here the FCC requires that those harmonics be suppressed by 43 DB below the carrier so you need to do that make sure it's done make sure done right if you don't have the test equipment to verify it and make sure you know someone who does just to be sure and that is it thank you for watching i'm wa7 GI Oh

Channel: Ron Taylor

Views: 18,022

Rating: 4.9583335 out of 5

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Id: 7W26HpNryhY

Length: 30min 23sec (1823 seconds)

Published: Sat Apr 04 2020