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.
More to come on this.
