Don’t you love it when a plan comes together? Well, if you are going to copy the work of a master, you cannot go past copying the work of Wez Hayward. Having built this amp design before on some copper board with through hole parts and done a whole lot of measurements on it, I knew that it was a design that was not going to disappoint. What I was not ready for though, was just how much better it is when using somewhat better design principles, SMD parts and an average PCB layout.
Just how good is it? Well, flat gain from 1Mhz to 50Mhz, without using any fancy transistors, just plain old vanilla popcorn 2n3904’s.
The dip between 5 and 7Mhz is an artifact from the NanoVNA and is not actually there, i looked with the oscilloscope and function generator just to be sure. While the NanoVNA is good, its not perfect, but as AVE likes to tell us, its good enough for the kinds of girls I go out with. The gain, in dB is actually also not right.
The input is 100mV and the output is 1V, by my math that is 20dB gain, so its exactly as designed. All that is left is to populate the attenuators and I have an variable gain amp to use for testing purposes or a receiver front end, which is the ultimate end use for this.
For a while now I have been thinking of receiver front ends. Yeah i know in many situations, home brew receivers do not need RF preamps, but hey, if i can build it, why not?
I have actually tried a number of times to make something that resembles variable gain, some kind of worked, others were a dismal failure. I think the best of of the ones off the internet was the dual gate mosfet front end with J310’s out of the simpleceiver. It worked well enough but was not really want I wanted and it did not have the bandwidth i desired.
So what do i actually want? Well, i kind of figure that I want a broadband amp to cover all of HF and maybe even all the way to 2 meters, with at least 20dB of gain and also -20dB of attenuation. After a lot of thought, I came up with the following solution.
What i did was take the TIA amp as designed by Wez Hayward, because its very broad banded, even with rubbish 2n3904’s I was getting flat gain from 80m to 6m. So with transistor with a better fT i should be able to get to 2m quite easy here. With a fixed gain of 20dB it has more than enough gain.
The amp is then followed by a series of 4 attenuators that can be switched in and out using RF relays suitable for use into the VHF bands. This means I have variable gain from -23dB to +20dB in 3dB steps. More than goof enough for the kinds of girls I go out with.
So I knocked up a test board to put it all to the test. Rather than using the expensive RF relays, I put in some DPDT switches I have here in a box. The boards are back from China and are with the post office for delivery tomorrow. I hope to build up a couple of these with different transistors I have here to see if it will do exactly what I want.
So between the first iteration and the 2nd, I have changed from using Sprint Layout to using Diptrace. I have tried a few different programs from Kicad to the free version of Altium and I just could never get the hang of how to make components and footprints in either of them
Diptrace on the other hand I find to be much more intuative and straight forward to use and I can make components and footprints in it with easy, as will be seen later when i get the latest batched of board back from china and write a blog post. So many new parts all created by me in the one library so i can find all my parts easily.
Anyway, to the board at hand. Its still got a couple of minor mistakes on it, the cap footprints were in 2.54mm not 5mm which the caps I use mostly are, the 5v regulator and caps are in the way of the right angled SMA connector and I am not sure that the variable RF preamp is actually varying. Circuit wise it should work, but the spectrum analyzer was not showing any gain change.
Anyway, the design will not be changing now, its built, tested and it works. And i can stay like that :
So the RF preamp has about 5x gain. On 40m its probably not really needed unless there is a lot of loss in the filters. Which in this case might actually be the case as the RF path from the antenna goes through the LPF, into the Relay and then into the BPF. So a little gain ahead of the mixer might be a good thing.
One thing that I left out for now were the back to back clipping diodes. I am only building this as a receiver pretty much to begin with and I am not sure clipping diodes are really necessary. Next I will be working on the LPF and BPF. Need to do some LTSpice here to make sure my values are correct.