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how to choose the best FET to detect weak e-field?

I'm working on a school project to detect the weak e-field(

Reply to
catty wu
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I couldn't tell for sure, but I'd shop around for the one with the smallest gate capacitance.

--
Tim Wescott 
Control system and signal processing consulting 
www.wescottdesign.com
Reply to
Tim Wescott

A 2N7000 is amazing. I posted about some tricks you can do with one... you might search google groups for "larkin" and "2n7000" (or maybe it was 2N7002?)

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

Yes, that, and bootstrap it. An electrometer fet, like the pn4117a might be good.

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An r.f. FET might be better; this was the best I noticed.

--
Cheers, 
James Arthur
Reply to
dagmargoodboat

The 2n7002 is the SMT part.

It's a very neat part, but the input capacitance is an order of magnitude higher than a small-geometry JFET. The input's effectively capacitively coupled to the world, so Cin attenuates the input voltage accordingly.

___ \ / * | C(ant) |-- '---||--+--->| | |-- --- --- Cin | ===

James

Reply to
dagmargoodboat

-what everyone else said-.

But what are you trying to measure? 1000V/m is not that small. I've been dreaming about making an electric field mill to measure the Earth's electric field (~100V/m). And when I last pushed the numbers around I thought I might be able to do it with a nice FET input opamp. If you have a largish collector plate (maybe 10 - 100 cm^2) you can get 10^-11 or 10^-10 coulombs of charge. If you're doing something like that you might try searching under electric field mill. I know there are circuits posted on the web.

George H.

(As a side question, say I've got a 10cm diameter plate pointing up into the sky, what's its capacitance?)

Reply to
George Herold

Sure, but that's no big problem if you're hunting macro amounts of charge.

In my little fet-LED setup, you could use a pencil or a tiny insulated screwdriver to transfer charge packets into/out of the gate, in visible steps.

Or use a source follower config to bootstrap the g-s capacitance. The cool thing about these parts is that they cost a few cents and the gate leaks electrons per second. For a reasonable e-field measurement, you'd probably want to swamp the Cgate with an external capacitor.

--

John Larkin         Highland Technology, Inc 

jlarkin at highlandtechnology dot com 
http://www.highlandtechnology.com 

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom laser drivers and controllers 
Photonics and fiberoptic TTL data links 
VME thermocouple, LVDT, synchro   acquisition and simulation
Reply to
John Larkin

Around 30 years ago I did some circuits for LLP (Lightning Location & Protection), a company with E & H sensors at non-radar-equipped private aircraft airfields. They were all networked together and were used to notify pilots of storm locations. There was a flat-plate E-field antenna and a shielded coil for magnetic field, with some analog signature analysis to separate lightning from man-made noise.

This was all pre-CAD, so it's on paper somewhere in my archives. When I get down that way I'll see if I can find anything interesting.

In the mean-time I suggest patent searching on Lightning Location & Protection (UofA, Tucson). ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

High Cin gives a modicum of overvoltage protection too. Cin matters less as a d.c. measurement rather than a.c., I s'pose--there are some neat VLF circuits that use a voltage probe antenna, bootstrap a FET's capacitance to nothingness with transformers, then go to town.

I made an e-field demo for some kids with Radio Shack parts. With an MPF102, a 4" wire antenna to the gate, and an LED in the drain, you could turn the LED full on and off by scuffing your feet from a good 5 meters away. Low Cin was key, obviously.

--
Cheers, 
James Arthur
Reply to
dagmargoodboat

You might want to take a look at electrometer-grade JFETs, such as the

2N/PN4117/4118/4119. Common-source input capacitance runs around 3 pF. Very low gate leakage.

These used to be used in smoke detectors, and are still used by hobbyists who put together (or refurbish) ionization-chamber radiation detectors.

I believe they've reached end-of-life and are no longer being manufactured, but new-old-stock parts are still available if you hunt around a bit (i.e. try searching eBay).

--
Dave Platt                                    AE6EO 
Friends of Jade Warrior home page:  http://www.radagast.org/jade-warrior 
  I do _not_ wish to receive unsolicited commercial email, and I will 
     boycott any company which has the gall to send me such ads!
Reply to
Dave Platt

Scientific American ran an article on building one of these some years ago. There was nothing at all exotic in the components selected, as I recall.

Jon

Reply to
Jon Elson

e
    ...Jim Thompson
   |    mens     |
  |     et      |
 |
      |

ide quoted text -

Thanks Jim, the web has circuits too. If I ever get around to actually building one, I'll be sure to ask here.

George H.

Reply to
George Herold

Yes they are, I just wish the last batch I got were better performing like all the prior batches I got in the past..

THe last batch it seems the body diode starts producing a forward of around 1.8v when it gets down to ~ 1 degree C. they are used in a sensing circuit that gets cold. We have a cluster of these circuits spread out to form an array of sensors so that each area can be monitored.

I've found that not only does the body diode forward increases more than they should, the Vgs(th) does too, that gets up around 3.5

All other batches prior to this were just fine in this temperature operation and the body diode forward never exceeded 1 volt.

So what I have done is to add an external diode instead of using the body diode and the Vgs(th) increasing I can live with. But that body diode plays an important part of the circuit, I was just trying to save on component count..

Jamie

Reply to
Jamie

they maybe cheap but with my older eyes, they are becoming harder to work with.

I have a 47x boom scope, although I don't use that sitting, am now thinking of getting a head set USB scope. The hands still work and I find it easy these days for PCB houses to do the boards.

Jamie

Reply to
Jamie

IIRC the research that led to LLP was done by Phil Krider's group in the UA Physics dept back in the '70's. Any patents will likely have his name on them. Art

Reply to
Artemus

2N7000 is a TO-92, which is easy to work with.

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This is a 2N7000, a resistor, a battery, and an LED. The gate is floating, and the fet is halfway on [1]. You can do cute tricks, like turning the LED on or off with your fingers, transferring charge with small objects, or calculating gate leakage over hours or days.

One added RC and a pushbutton makes a three-state on/bright/dim night light.

[1] briefly connect drain to gate to get halfway on. 
--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

message

Correct. I had forgotten Dr. Krider. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     | 
| Analog Innovations, Inc.                         |     et      | 
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    | 
| Phoenix, Arizona  85048    Skype: Contacts Only  |             | 
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  | 
| E-mail Icon at http://www.analog-innovations.com |    1962     | 
              
I love to cook with wine.     Sometimes I even put it in the food.
Reply to
Jim Thompson

ve

Yes, of course, George is exactly right--that's the instrument needed.

Here's the SciAm article, and another version of same:

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--
Cheers, 
James Arthur
Reply to
dagmargoodboat

Seems to me that it would be easy to blow the gate.

Reply to
John S

I never have.

--

John Larkin                  Highland Technology Inc 
www.highlandtechnology.com   jlarkin at highlandtechnology dot com    

Precision electronic instrumentation 
Picosecond-resolution Digital Delay and Pulse generators 
Custom timing and laser controllers 
Photonics and fiberoptic TTL data links 
VME  analog, thermocouple, LVDT, synchro, tachometer 
Multichannel arbitrary waveform generators
Reply to
John Larkin

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