The diodes you tried and the circuit with diodes you found severely distort the audio with clipping, they do not provide automatic gain control.
The horrible regenerative "radio" circuit has very low levels for weak distant signals but has very high levels for strong local signals. A real ordinary radio has AGC for its radio signal, not clipping of its audio signal then a weak signal sounds at the same volume as a strong signal.
I have not tried this but I can't see a reason why it won't work: use the circuit in the link but instead of the LED/LDR, use a photo-mosfet. I'm thinking of something like the PVT412 from International Rectifier, it is designed as an isolated SSR but it must have a linear region where the mosfet resistance could be used instead of the LDR. Dark resistance is around 1,000M and light resistance is <10 Ohms. They are already encased in lightproof resin and the response speed is magnitudes faster than an LDR.
Brian
Incidentally, your comments on simple receivers are quite right and their performance can easily exceed more advanced designs if you are prepared to sacrifice user friendliness and their dynamic range. I have started to work on a rather novel design of my own with limited success but it has a long way to go before being unleashed to the World. It is quite simple and under manual control it is very sensitive but the final product will have automatic regeneration level control and maybe (it is theoretically possible) a digital frequency display and tuning by rotary control or keypad. I have never seen any design using the same method so it is highly experimental. It should have a good dynamic range and the ability to use real RF AGC to a pre-amplifier stage. The only drawback I've found so far is it stops working when tuned above about 6MHz but I'm sure that can be improved up to VHF with better than 'junk box' components I have used so far.
Brian.
Years ago I tried an H11 opto-fet as the AGC for a Wien bridge audio oscillator but its distortion was too high so I added a second one to cancel the distortion of the main one and it worked not too badly. They say it is "distortion-free" but of course it produces distortion since a fet is non-linear.
The symbol for the MPF102 is shown as P-channel but it is an N-channel device and the AGC seems to work in reverse.
What if I reverse the two germanium diodes and the smoothing capacitor, so that negative bias is applied to the fet?That schematic seems wrong to me. The symbol for the MPF102 is shown as P-channel but it is an N-channel device and the AGC seems to work in reverse. Highest gain is achieved when the FET is conducting so the AGC has to cut the conduction off to reduce the gain. In that schematic the gain is at maximum with no signal but instead of reducing the gain it forward biases the G-S pins and risks damaging it. The suggested alternative 2N3819 is no better.
Brian.
Of course the circuit makes only sense if the rectified DC voltage reverse biases the JFET. So it would either use a PJFET (in contradiction to the type indication) or simply reverse the rectifier polarity. You don't damage a JFET with a few mA forward current, by the way, but the circuit won't work.
The circuit at post #12 did not work for me.That schematic seems wrong to me. The symbol for the MPF102 is shown as P-channel but it is an N-channel device and the AGC seems to work in reverse. Highest gain is achieved when the FET is conducting so the AGC has to cut the conduction off to reduce the gain. In that schematic the gain is at maximum with no signal but instead of reducing the gain it forward biases the G-S pins and risks damaging it. The suggested alternative 2N3819 is no better.
Brian.
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