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

Discussion in 'Electronic Design' started by catty wu, Oct 16, 2012.

1. ### catty wuGuest

I'm working on a school project to detect the weak e-field(<1000V/m).
I want to use the FET. But I just don't know what kind of FETshould I
choose. Can anyone help me? Thanks a lot!

2. ### Guest

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

http://www.fairchildsemi.com/ds/MM/MMBF4117.pdf

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

3. ### Guest

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

4. ### George HeroldGuest

-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?)

5. ### Guest

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.

6. ### George HeroldGuest

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.

7. ### JamieGuest

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

8. ### JamieGuest

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

9. ### ArtemusGuest

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

11. ### John SGuest

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

12. ### John SGuest

That is curious to me. I would like to understand why there are so many
warnings about floating gates if there is not much worry about that.

Can you help me with that?

13. ### Tim WilliamsGuest

The instant you connect gate to drain (a mechanical circuit closes in
under 1ns, by the way, so it does make a reasonable impulse), current
flows into the gate capacitance. Some will flow out of the drain's
(assuming the transistor was off first), as well as the source (which in
this example is a harmless resistor and LED). As the gate charges, drain
voltage falls. Within a few ns, charge is equalized and Vg = Vd ==
Vgs(th).

There might be some hazard running such a circuit from a supply over 40V,
where the drain charge may be sufficient to cause dangerous gate
transients before the entire gate charges (a process limited by gate
spreading resistance). Note I say this with some confidence while
recognizing Vgs(max) is only 20V (or is it 30V on the 2N7002?).

What you don't want to do is touch the gate after scuffing your feet on
the carpet. Which John never recommended, and I certainly don't either!

As for general purposes, uncommitted pins are generally a hazard, so you
don't want to leave things floating. Sometimes this has real consequences
(floating CMOS inputs are constrained by the input diodes, but otherwise
free inbetween; around the midpoint, the input stage goes class A and
supply current increases dramatically). Other times, it doesn't so much
(John's FET-resistor-LED will stay where it was more or less, and drift is
simply part of the experiment). But general advice is to avoid undefined
voltages anywhere, be they in circuit, on components (like unused gates)
or PCBs (I hate it when copper pours add those damn floating patches
without checking for vias to them!).

Tim

14. ### George HeroldGuest

"Cheeeese Gromet"

(Shaking hands)

George H.

15. ### George HeroldGuest

Ouch, I'm never exactly right.
(Grin :^)
My daughter is taking some science class (8th grade)
She told me her teacher was interested in weather science,
and I said we should build a field mill.
So we've recently been looking at web pages.
I've been to the second link.
If I can carve out some time to work on it, maybe I can hit you up for
ideas on how to improve the front end.

George H.

16. ### Tim WilliamsGuest

That's quite fine -- generalities are meant to have specific exceptions.

Would be more accurate to say: "avoid unintended open circuits", since in
low-leakage stuff, like this circuit, the signal of interest is
more-or-less indistinguishable from an "open circuit". But it's intended,
which makes it all okay. ;-)

Tim