I have built a few amp stages, most of them either worked or worked and were total crap. This is the first time I have designed and built a multi stage amp and have things actually work how they are meant to. I wanted this amp to be very linear and to keep harmonics to an absolute minimum. And it is clean, each stage plays nice with the next stage, there is no loading up one stage, no distortion. Obviously this is not a huge power out monster, but there is 40db of gain in there so far with more to come from a fet final stage. So far I am pretty happy with how things are going.
The complete mess 🙂
About 4 watts out.
Harmonics are -45db down or better.
Schematic for amplifier chain.
So after a little bit of tinkering, its now making 5w out. After 10mins keydown it hardly gets warm, only 6 deg C above ambient measured on the case of the FET.
Its funny, I needed a 10db gain amplifier that used a 2n2222 and being lazy and not wanting to MATH i typed into google 2n2222 10db preamplifier and low and behold, it came up with an answer that was what i actually wanted. Sorry to the OM i stole this from, i will edit this later with a link back to your site.
So anyway, I simulated in LT Spice and it was spot on, -6db input and +4db output, thats 10db and just what the doctor ordered. Who needs math when google can elmer like the best of them.
Well this one has been a long time coming. From the very first amp i built till now has been close on 5 years. Those first amps were rather scary contraptions dead bug on scrap pcb and had more than a few fets explode on me. And after the debacle that the 2×4 amp was, this time around i decided to cut it in half and make sure it is all working right and to have the mechanical aspects of the build right also, so if something does break and lets be honest something always breaks, that its easy to fix. I have not really started to push this yet, but atm with conservative biasing, drive and voltage i am getting a respectable 36 watts out. Its been key down for the last 15mins and other than the heatsink needing to be bigger to dissipate the heat its going well.
Here is the schematic.
Here is the PCB as laid out in my ecad of choice.
So rather than try and go the home made SMD route, i just went old school with the mechanical design. What i did was mount the fets loosely to the heat sink, then i snug them up, bent the legs with the pcb and then tightened them all down. It meant i could slide the PCB on and off the legs really easily until i was ready to solder them on permanently. If i fet dies, it should make replacing it a lot easier.
Setup for testing. I kind of just snuck up on it with voltage and drive. 13.8v and 200mw drive and I was getting about 7 watts out. Slowly and surely i brought the voltage up and the drive as well. In the end i was giving it 1 watt of drive at 24v and was getting 36w out. Its got way more to go, but its nice and stable.
A step in the chain, was about 25w here.
60v peak is 36 watts. Now i need to put some 200v caps in the low pass filter and make that look like a sine wave LOL
PDF of the schematic : irf510-2×2-amp
Gerber files for the pcb : REMOVED found an error when I ran 10A through it.
Here is the next part of my home brew CW transmitter. An Arduino and AD9850 are providing the DDS VFO signal. Q1 is a buffer to isolate the AD9850. I am not sure that it is needed, but its there to stop the gain stage from loading it up. Q3 is an amplifier that takes the 400mV P-P DDS signal and amplifies it to 8/9V P-P or there abouts into a 50ohm load. Thats about 200mW, enough drive to supply something like the QRP Labs 10w Linear. My plan is to design an amp to follow on from this that will provide 30 to 50w. More on that to come. Q2 is a PNP switch, pulling the base to ground turns lets power flow to Q3 thus keying the transmitter on and off. Simple but works.
Here is the schematic.
The PCB as laid out in Ecad.
Here is the board as built, if you are not colour blind and can read the colours on R5 you will notice i stuffed up here HAHAHA. Also Q1 silk screen is backwards.
Signal output into 50 ohm load at J2. This jumps up to about 10V P-P into the QRP Labs Linear amp. Not sure why yet.
Schematic PDF: CW-Buffer-Amp
Board Gerber Files: DDS-CW-Buffer-Amp
Ok, I have still been working at building stuff. Here is the latest piece of what will become a CW transmitter. These boards are 7th order Chebychev low pass filters.
This is the PCB as laid out in ecad. 4 boards per PCB and you can get 5 PCB from JLCpcb for $2 so that is 20 low pass filter boards. A life times supply.
My current project is a 40m CW transmitter, here is the LPF sitting on top of the TX/RX switch.
Filter response plotted with the NanoVNA.
If you want to have your own LPF boards made, here are the gerber files: LPF_Singles
So a number of what we all call SDR receivers are capable of transmitting. The output power in the case of the HackRF one is about 250mV p-p, in other words a very small signal, 0.15mW. This is not much good for transmitting further than the end of the street, but with some amplification you can get that up some. I added the QRPLabs 10w TX to it and was getting close to 0.8W and was being heard over 800km away.
Anyway, my plan for a while has been to use the HackRF as a receiver and make a couple of low power CW transmitters to go with it. By low power i am talking in the 10w kind of range. But this has gotten me thinking, with some VHF and UHF amplification, this might be a really cheap and easy way into SSB and CW on those bands for field day here in VK. It would require a 2nd SDR to act as the receiver, because the HackRF is only half duplex and there is no software out there that can so the switching currently, and it can be locked to a reference signal so it might be somewhat useful.