The goal I set for myself was to create, modify, or discover a high-quality TRF (Tuned Radio Frequency) radio circuit that could perform at least as well as the MK484 - TA7642 AM radio chip, while using as few parts as possible. My aim was to learn about the constraints of early tube designs from the twenties (1920s).
Back then, designing a TRF radio presented numerous challenges. The tubes of that era had limited gain, necessitating the use of multiple stages, which, in turn, led to additional issues requiring filters and workarounds. Moreover, these designs lacked sensitivity, paving the way for Regenerative Radios and the Superhets. Tube gains hovered around 10, whereas modern transistors, like the 2n3904, range from 200 to 800. However, even with this improvement, the gain remained insufficient without regeneration (superheterodyne).
I tried all sorts of designs, using FET's, Darlingtons, etc. They were all pretty awful.
This led me to think of the existence of a transistor with a gain of 5000 to 10000, a concept nonexistent in the 20s. Eventually, I stumbled upon a straightforward design employing multiple MPSA13 transistors (Darlington). I streamlined it to the detector stage, it worked really well! Adding a preamp stage on the output and connecting it to my headphone amp yielded good sound quality. By adjusting the voltage to the collector, you can enhance the gain, and it even performs OK up to about 17 MHz (Needs an antenna of a few feet at least). The preamp section using the 2N3904 is a great audio preamp design you can use anywhere.
Utilizing a ferrite core antenna wound with litz wire, I achieved satisfactory results on the AM broadcast band without an antenna, but you can use anything at hand. I am able to pick up AM Broadcast stations 600 miles away. It will even work great with an aircore winding on a 1 inch diameter core of plastic or cradboard, but you will need an antenna of some sort, even a 2 foot length of wire. Earth ground is not required if using a ferrite antenna core
I'm planning to explore the MPSA14 Darlington, which boasts twice the gain of the MPSA13, to further enhance sensitivity.
Despite its success, there are a few drawbacks:
1. Driving it too hard with higher voltage can induce oscillations.
2. The extreme high selectivity makes it challenging to use poly variable capacitors effectively.
3. It whistles and oscillates around a station until it is tuned. Going forward, maybe I can figure out an AGC circuit.
This is an easy project to build. It Works.
There's a two transistor one I did some years ago, if that's any help. I ac coupled the two transistors and followed with a biased diode detector.
Follow the link below.
I built a similar shortwave regenerative receiver which used the two transistor's Base-Emitter intrinsic capacitors as a Colpitts oscillator. Performance is very good. Using 3 inch ferrite rod inductor in the tuned circuit I listen most evenings to 80 and 40meter SSB up and down the East coast of the U.S without the need for an external antenna or ground.
https://groups.io/g/TheNewRadioBoard/topic/strange_to_me_regen_cct/100532634?p=,,,20,0,0,0::recentpostdate/sticky,,,20,2,0,100532634,previd%3D1691466224508899524,nextid%3D1681129345133277562&previd=1691466224508899524&nextid=1681129345133277562
Forgot to mention, I used Google Gemini, Bing AI, and ChatGPT to get information on this transistor. Gemini seemed to help the best. I'm still fuzzy on how or why this circuit works. It sounds great when adjusted correctly. If you are interested, please let me know if this info below sounds correct. I took out all the info that did not pertain to my experience and kept in my actual observations. Going forward, if you use AI, it would be nice to isolate it to give people a heads up, since we are all people. The way I look at it, it's can be used as a really nice data sheet. ==================== AI Info ============================
1. The variable resistor connected to the collector and the 1MΩ resistor from base to collector could create a weak positive feedback loop, leading to some level of regeneration and signal amplification. This might explain why it works across the AM band without needing a separate oscillator circuit. I don't think this is happening. Any Thoughts on this ?
2. Direct detection: The 47pF capacitor and the MPSA13 base connection could be forming a simple diode detector circuit. The MPSA13 acts as a non-linear element, demodulating the AM signal from the tank circuit and producing an audio output at the collector. I think this and 3 below are most likely
3. Amplification: The MPSA13 with the collector resistor and bypass capacitor acts as an amplifier stage, boosting the weak detected signal from the tank circuit/diode detector before sending it to the output capacitor. Voltage and oscillation:
Increasing the voltage on the collector likely increases the gain of the MPSA13, pushing the circuit closer to oscillation. This can explain why it starts to oscillate at higher voltages.
Keeping the voltage below the oscillation threshold allows the circuit to function as an amplifier without generating unwanted noise or interference. =======================End of AI Info ===============================
Interesting - thank you guys, for the explanation on that, it sure does look like a colpitts when drawn that way.
Win W5JAG
Hi. The regeneration comes from the Darlington transistor. If you redraw it as two transistors the base emitter capacitance on each transistor form a capacitive tap for a Colpitts oscillator.
From point 3 it does seem you are getting a regenerative effect, though from where is quite difficult to say since there are no visible feedback loops. The 0.1 uf capacitor should do a good job getting rid of RF so it's not like the 2n3904 is ampifing some RF and leaking it back some way.
Hi Larry. Have you tried any of Sir Douglas Hall's circuits?
I had a lot of fun experimenting with very low current AM detection and using transistors at very low current. Eventually you figure out AM detection is much more efficient at higher currents. Then again you might want to use 2 directly connected transistors for AM detection (as with a Darlington pair) and you have to balance out the current settings.