Zero Threshold MOSFETs…

Published on 2009-10-24 by Mark VandeWettering

I don’t know why this percolated to the top of my consciousness today, but I thought I’d dump a link here so I can find it again. I recalled reading an article about the use of a particular special MOSFET device for use in crystal radios. It took me a tiny bit of time to find it, but here is a link to the Bob Cutler’s QST article A High Sensitivity Crystal Set. In conventional FET devices, the FET doesn’t conduct unless the gate voltage exceeds the threshold voltage. For a common power MOSFET like the IRF510, the threshold voltage might be 2 volts. For devices like the ALD110900A (datasheet here) the threshold is essentially zero. This makes a great variety of very lower power oscillators and amplifiers possible.

Cutler’s crystal set design is innovative because it doesn’t require an outdoor antenna or ground to function. It’s a zero power radio with enhanced performance provided by this MOSFET device. I’m not the only one who thinks so either: no less a radio luminary than Wes Hayward, W7ZOI found Cutler’s ideas thought provoking and interesting.

Addendum: G3XBM was looking at the same zero threshold devices. His page links to an application note for a 32 uw oscillator that can operate on a mere 0.5v supply voltage.

Addendum²: Ben Tongue has an interesting article on extracing energy received from a strong radio station to charge a supercapacitor, and then power a very low power amplifier that can be used to enhance the operation of his crystal set. It seems to rely on a micropower operational amplifier manufactured by Texas Instruments.