CW Superhet Receiver Project PART-3

RF Pre Amp

So I am back feeling a little better and it is time to move this project alone a little. Some say that for a 40m band receiver an RF Preamp is overkill. In some places it probably is, but where I live most of my contacts are over 1000km away, are weak at the best of times so I think having a little gain ahead of the mixer is a good thing, but it needs to be variable.

My initial thought was to replace an emitter resistor with a potentiometer but that would mean having rf running though cables to the front panel and the chances of things going bonkers increases when you do things like that. Emitter degeneration was still going to be the way to do this, but by using a second transistor as current controller, control the current and control the gain and that can be done with voltage and a pot. So no RF being shipped about the place.

So now for the actual circuit, this was one given to me by Pete N6QW homebrewing legend and all round good guy. Its one of his GOTO circuits, nothing fancy just a BJT amp optimized for bandwidth and as an IF Amp. So I took that and modified it slightly to fit my needs as a 7mhz rf preamp. And like batman off to the batcave, we hit the simulator in LT Spice to confirm that things are close to spec and things will work well within our design parameters.

The circuit is rather straight forward. No rocket surgery required to follow whats going on there.

Ultimate gain is about 9db, and with 3v on the base of Q2 the gain is like 3db or there abouts. Should be good enough for what I require. Now I just have to build it and see if the simulation meets up with reality. Will post an update when I have it built.

Now i also should mention that I was going to compare and evaluate a number of designs for each stage. I did pull some designs out of different schematics on the web and put them through their paces, mostly they were crap. There were some that were better, but they were not popcorn enough for this project. A very notable mention is the front end of the simple ceiver with its dual gate mosfet made from a pair of J310. While i have J310 here in the crap box I am saving them for another project. If I can keep this one to BJT only i will be happy. Anyways, time to melt some solder and see if it works.


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