The detector can operate with the source resistance approaching 1 M, even at extremely feeble currents it is still very sensitive. Still, give it a try, a single active device FM radio, pulling < 100 uA is mighty impressive! Some additional audio volume can be achieved by redesigning the quench circuit to utilize the piezo capacitance directly, but the source resistance has to be dropped quite a lot to achieve a viable quench frequency and the gain in sensitivity isn't as fantastic as one might hope. A largish resistor (~10 k) prevents the source circuit from seeing too much of the fairly large capacitance of the piezo element (about 14 nF) and pulling the quench well down into the audio range. The detector alone provides sufficient audio to drive a crystal ear piece in a very quiet room, giving a true "single transistor" FM receiver. The quench gets frequency-modulated somewhat by the drain current, so it varies with signal strength and the recovered modulation, this is typical for self-quenched circuits (simplicity has its price). The RFC to an RC source circuit to implement self-quenching is very traditional for super-regenerative detectors. By tapping down towards the cold-end of the coil the feedback isn't as critical as your usual source-drain capacitor feedback and it tends to be far less difficult to get to work across a broad range of frequencies. The drain resonator inductance is centre-tapped with feedback to the source through a small capacitance. The topology is your classic grounded-gate FET VHF Hartley oscillator. It's been quite a while since I built a super-regenerative receiver for the FM broadcast band and I've been meaning to play with a different source feedback topology, so this little radio was thrown together on the Saturday of the APEC long weekend.
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