Recently I've been playing around with JFET transistors, something new for me if I must be honest. Originally I was intending to build a grid (or gate in this case) leak detector using a JFET in place of a tube, but I found that I couldn't get it to work. So after looking at schematics online I instead decided to try using the JFET as an RF amplifier and then feed the output into a crystal detector. That worked surprisingly well, once I figured out how to bias the thing properly! Of course I had to use a piezo earphone to hear anything, but it was much louder than the unamplified crystal sets I had built previously. I then decided to build an audio amp so I could use low impedance headphones in place of the piezo buzzer. The RF amplifier is a 2N5457 JFET and the AF amp is a 2N3906 PNP. I used a 1N34A for the detector diode.
Performance wise it works alright, I'm able to pick up three local stations during the day, including one I can't really hear with an unamplified crystal radio. This is still experimental so I'm using a loopstick I bought at a hamfest (so I'm not really sure what it was originally intended for) with a 200 pF varicap for the tank circut. As a result the band coverage is limited. Now that I know it works though I'm going to make a better tapped coil and use a different varicap with a higher value. Unfortunately my antenna situation is terrible (patch cords hooked up to the metal vent hood over the stove in my apartment) so my reception is limited anyway.
Anyway, I know it's not much, but it's the first amplified radio I've gotten to work well so I'm proud of it. If anyone has any suggestions for improved performance please let me know.
PS, Here is the schematic:
Thanks for inserting my schematic. Rich
Thank you for all your input!
I've been a little busy with work this past week and haven't been able to do much with the set. I found that if I placed a 100 ohm resistor and a 10 mF cap in parallel from the collector on the 2N3906 to ground I get a noticeable bump in volume. I think I am going to try DrM's regenerative suggestion as I'm crunching the numbers for winding a new coil to replace the loopstick, and figure adding a tickler coil to the assembly shouldn't be that much more work.
I'm still not exactly sure how to control the regeneration. Looking at old tube schematics and trying to work forward it looks like I'd put a variable resistor across the source to ground, though I don't really know what I'm doing so that might be the wrong way to go about it. Maybe it would be better to replace the 1 k drain resistor with a variable resistor in parallel with the tickler coil.
A lot to experiment with!
Also, LarryDee, I was wondering if the OPA2134 is available in a dip package or if it's only available in surface mount? I ask because I'm building this thing wire wrapped on a plank of wood, using copper plated weather stripping nails as my binding posts. For such a rudimentary method, I've found it works really well!
Don't know how to display this image? Rich
About 20 yrs ago, I bought a "one IC radio kit" from Dick Smith Electronics while I was in Australia. It turned out to be a crystal set using a CMOS IC CD4007, as a chain of audio amplifiers. I'll try to find a circuit diagram. Worked surprisingly well even up to 5 MHz shortwave. Rich
You can transform this receiver into a small superhet by removing the source decoupling resistor of 10 nF, injecting the LO signal from a Local Oscillator onto the source of the FET and replace the Drain resistor of 1 k by an LC circuit tuned at an IF of 455 kHz. Another option is to connect a Q-Multiplier stage to the areal tank circuit, transforming this TRF receiver into a regenerative one.
If you use Litz wire, a ferrite rod antenna, and a 0 to 365 variable cap, I bet your circuit would perk up.
Wire and Cap here https://www.mikeselectronicparts.com/
"The Type 61 Ferrite rod NiZn R40C1- 200x10mm for High Q Amateur & Crystal Radio Coils, AM SW(151884949745) " Search Ebay
If you keep the rod full size , it will receive great. You can easily cut it down by scoring it and breaking it at the score. Be interesting to see how it works out.
I also cheat and use an OPA2134 as a preamp. It's awesome. You can buy TI stuff directly from TI . It looks hard, but it's actually easy to do. The performance is off the charts, and you can feed it directly with your circuit. You'll be amazed at what you are missing due to low amplification.
If you make the R4 10k feedback resistor a 20 k potentiometer, you can adjust the gain. I use the small, blue precision pots from Amazon. I also use a DIP8 socket so I can swap out IC's, with the gain adjustment, that works out well
Look into Manhattan style construction.
Parts called MePads here:
Hope that helps.