J308 for really-low voltage oscillator

BF862, a jfet, might work.

A phemt might work better than a jfet. Transconductance can be really high, 10x a jfet maybe.

Ok thanks! About $0.54 each at Mouser.

Ooh, Digikey is out (non-stock). What is a Phemt?

And, if I'm trying to run at really low voltages (basically, wringing out all the energy out of dead AA batteries) what kind of characteristics am I looking for in a JFET?

Thanks,

Michael

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Oh, never mind, I found it! It really is a thing!

Michael

octoparts?

George H.

Another way to wring more energy out is to put 'em in series. Then you need an efficient way to cut down the voltage during the early part of the discharge curve.

Back in the Olden Days when Heavy Duty (carbob-zinc) batteries were cheaper (and lighter) per watt-hour than alkalines, I used 4 of them to drive a 3-volt bulb for a caving headlamp, with PWM to control the power. Turned out to be trickier than I had anticipated, since (of course) the power is proportional to V^2.

Best regards,

Bob Masta DAQARTA v9.00 Data AcQuisition And Real-Time Analysis

High transconductance and fairly high Idss, mostly.

You want to modulate a bunch of drain current with a small gate swing, but how much gate swing is available depends on the circuit.

A nearly dead (low voltage) battery won't have much energy left.

As I remember (my databooks are buried too deep to dig it out), the J308, J309, and J310 are on the same datasheet and are basically the same thing with slightly different specs. I don't remember if the specs get tighter as the number goes up. Try looking for a J310.

Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)

t all the energy out of dead AA batteries) what kind of characteristics am I looking for in a JFET?

That works, until one battery decides to become a traffic jam and prevent a ny further current from passing :)

Reminds me of a science day at my son's school. His group was playing with maybe 10 9V batteries in series to get impressive sparks. Then the sparks got weaker. So I suggested that they test the batteries, remove the weake st one, and try again. They found a dead one, removed it and re-assembled the pile. Huge sparks again!

Thanks,

Michael

Oh, cool! I remember the days when my dad would collect Radio Shack free c arbon-zinc battery club cards, and we'd always show up and get a few C or D cells :)

Thanks!

Michael

Oh. My. Goodness. THANKS! :D

Michael

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Okie doke. Thanks!

Michael

pHEMTs are amazing parts for some things, but I'm pretty sure that John's pulling your leg about using one in a Joule Thief. The slowest pHEMTs are about 10 GHz iirc. They also have seriously crummy Early voltages, so they might not work as well as a JFET when V_DS gets down to the hundreds of millivolts.

The J308/309/310 are pretty well the same FET with different I_DSS limits, iirc. You can still get J309s with no worries.

(My fave pHEMT circuit fragment is a SiGe:C BJT as a combination cascode and bootstrap on the pHEMT's drain. I've talked about that in SED quite a bit in the last couple of years.)

Cheers

Phil Hobbs

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Oh ok, thanks!

Michael

Putting a 1-10 farad supercap across the battery might be the best way to get most power out of the battery. The cap gets a trickle charge from the battery, and can then for some minutes supply low loss power.

I was serious. Rds-on is about 6 ohms for that NEC part at zero gate voltage, and transconductance is outrageous compared to a jfet or a depletion mosfet. Just what you want to power some oscillator thing from a thermocouple or some such millivolt source.

Oh ok! The pHEMT is $1.50 or so from Mouser. I'll try both it and the J308-215.

Mouser part numbers:

Thanks guys!

Michael

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"Conclusion

This patent circuit is absolutely fabulous. I would use the J310 in applica tions that require a small current from an ultra low voltage source, like c harging a supercap slowly. If an input voltage of 70mV more can be tolerate d and if the source can provide enough current, I would definitely use the J108, as it is able to provide much more current. The LED really shines lik e the Joule thief circuit, but starting reliably at a much lower voltage!"

Thanks,

Michael

Okay, my mistake. (That "lunatic fringe" thing again.) ;)

The transconductance is nice and high at fixed bias, true. However, the Avago parts have the wimpiest drain impedance I've ever seen--something like 160 ohms. You can't even use them as source followers unless you bootstrap the drain. The Skyworks parts (SKY65050) are quite a bit better, but still not even as good as a JFET. I haven't tried the NEC parts.

It would be entertaining to try using a 14-GHz part on a white plastic breadboard. OTOH they're surprisingly stable for such fast and quiet devices--a lot more stable than the SiGe parts!

Cheers

Phil Hobbs

Phemts have some potential as low-frequency amps and switches. I don't know what is the record for making an oscillator powered by millivolts, but a phemt is a candidate to do that mostly useless thing. I've seen the claim of 5 mV, with paralleled jfets.

They are very stable as compared to bipolars. Drain-gate capacitances are absurdly low, and the substrate is the source. A hacked copperclad breadboard usually works fine.

I have some DC measurements of the NE3508 if anybody's interested.

Neato. Is that what boosts the mV from the thermocouple in a gas water heater, to control the temperature?

"... very stable as compared to bipolars." That would make a funny mental health joke. :)

Sure!

Michael