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.
Hi, Rob, Some questions for your design. I assume your design is based on the march 1999 qst article of wa2eby. Also in the book of EI9GQ one is described with RD16HHF1 Mosfets, I am trying to build such one. But why do you have 1uH in the feedback circuit? Also sometimes a small resisitance in series with the gate is used to prevent oscillation (like tubes) or act as fuses. But my main comment is on the input and output transformers. I measured them but mine performed poor when the frequency increased above 5 MHZ or so. I also measured transmission line transformers. These have quite a wide range up to 25Mhz (where my signal-gen stops) they had little loss. Any way. I now also check my design for high currents if the tracks are wide enough. 73 Paul
The feedback circuit is to neutralise miller capacitance, which gives greater bandwidth to the circuit. The output and input of an active device interact to create capacitance that increases with frequency. Its not really essential for my circuit as i am only really using it to 14mhz, but, rather than the gain decreasing as frequency increases, the gain should remain quite flat to 30mhz with this feedback circuit.
If you fire up LTspice and simulate a simple amp circuit with and without the feedback using AC Analysis you can see the frequency response, playing with the values will change that response also. I found in part by simulation and in part by following the example of others, that those values give good bandwidth with acceptable levels of gain.
If your input transformer is poor above 5meg, its most likely the ferrite material you are using. Type 43 gives good response at HF.
Hello
Rob Powell
I hope you’re doing well. first of all what a delightful website. second thank you for sharing this circuits. It help beginners to start with something big. I have a question about this circuit. I tried so much to analyze the circuit in Cadense Orcad Pspice. Can you help me to analyze it?
thanks anyway.
Sure i can help some, but i do not have Orcad.
What software you use in order to analyze your circuits?
LTspice, mostly i just do thing experimentally. Built it, measure it and then tweak it.
Hi
For me I always try to stimulate the circuit in softwares then I try to make the PCB. In your schematics I don’t see the capacitors 5,6,7,8, but the PCB is right. I think the top left part is not right. If you’ve analyzed it in any stimulation softwares, could you please share it? If no, Can you just share the bias points of your circuit??
thanks.
I do not have any simulations for it, i just built it and tested it. There are other blog posts about it. Bias it for how its intended to be used. Very little bias for class C and a fair amount for class AB. I found it quite difficult to find a balance with it. I think, without I had 2V bias.
My recommendation is to not use this design. It works, yes, it makes 50w yes, but its difficult to get working right. I should mention that this blog is not a HOW TO or GUIDE to do anything, its just a diary of what I have been doing. I am not an expert, I am not an engineer, I am a mail man who also enjoys learning, ham radio and electronics.
Hi rob please share T1,T2 and T3 winding details
2 of them are on the schematic, the center tap is not critical, just enough turns each side to block FT going back to the power supply and radiating. 5:5, 10:10 dont matter really.