Zero Threshold MOSFETs…

October 23, 2009 | Amateur Radio | 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.

Comments

Comment from KJ4PD Kevin
Time 4/4/2010 at 11:41 am

Almost three months ago I started on making a crystal radio when I quickly found these same articles on the zero threshold MOSFET devices. So, one of goals is to try and exploit the potential benefits that the three terminal MOSFET may have over two terminal diode detectors.

My experiments are inconclusive (and far from over) at this point, but so far the most sensitive detector I have used is a HSMS-285Y “zero bias” schottky diode from Avago (formerly Agilent, HP and Avantek).

This whole business of zero bias is a convenience for us self powered radio enthusiasts.

Coincidentally the same company that makes the zero threshold mosfet also makes an energy harvesting module for collecting energy from sources and delivering it to loads. I have one and plan to use it with the micro powered op amps that Ben Tongue used in his experiments.

So keep the thought around. It is a good experimenter project.
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