I had the need recently to measure BF862 Ig and had a Victoreen "High-Meg" 2Gohm R. I looked at drain current changes with either the 2G R or a much lower one, and I had to shield things a fair amount to prevent local E fields from driving enough voltage to screw up the measurement. I set it up to poke the wire with a long plastic rod so that the lower R was contacted.
Backing out the delta V via the transconductance, I got a result of about 4pA for a 2V Vds. Since the gate lead had only as much heat conduction as a #30 bit of wire, the chip temperature was probably a little higher than an in-use one.
Backing out the delta V via the transconductance, I got a result of about 4pA for a 2V Vds. Since the gate lead had only as much heat conduction as a #30 bit of wire, the chip temperature was probably a little higher than an in-use one.
When I was measuring low femptoamp fets, freon was your friend. As was a dry nitrogen cover.
Problem then was static electricity. What was best for the fet measurement was also best for killing em.
John
Sub-threshold behavior will interfere with these measurements. The curve fitting technique over the range of actually useful Id's is better.
EDIT - It would be a useful data point to compare the Vp from a 10%-90% Idss range curve fit and one from an arbitrarily low Id. Obviously the difference between your 60nA and the manu's 1u
A are substantial. In my opinion the Vp and Idss that, for instance, correctly predicts the Id with say a 50 Ohm source degeneration are more useful.
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Thirty years ago we used 10^^12 Ohm glass Victoreens in a fixture made of teflon standoffs and a machined solid billet of aluminum to measure 20fA repeatable to better than 5% or so.
I may have told the story of when Balu Balakrishnan (now long the CEO of Power Integrations) was working as a tech in my lab at UCLA while finishing up his Master's, and he ran across a teraohm Victoreen in a cabinet and was laughing. It was marked 1,000,000 Megohms, and he thought it was surely a mistake---that it was really simply a megohm. I informed him otherwise.
Back in the day.... We're getting old Scott, getting old....
I remember reviewing the test sheet printouts... 50 fA, 20 fA, 10 fA...
100 aA.. Huh???
I had to look up Atto...
John
You would need to separate collapse from actual voltage breakdown for externally polarized capsules. From Pashen's law you are probably getting collapse rather than breakdown.
The spacing is quite close. Many are designed for 200 volts as I suspect you know. However that is for around the world use. So in a dry climate you might get 300-350 volts but even that would be pushing it. The spacing is very close. In my experience you design for a breakdown voltage of 5,000 volts per inch but it really takes 10,000 volts. From what I recall of the spacing in the microphones they are closer to the 10,000 volt number to start.
Now I wonder if it would be safe to drive a capsule to, say, 400V in series with 2Gohms? That is, would it be nondestructive? Or would the point of contact still sustain damage? I guess it would come down to the energy in the capacitance and the effective contact area. And presumably the C is getting a bit larger as it comes close. Maybe 4uJ available in the ones I have is plausible.
I suspect since the diaphragm is sputtered gold limiting the peak current would suffice, there is little info out there but I assume an extreme SPL event would also crash the diaphragm into the backplate.
I expect in a measuring microphone the diaphragm is metal not sputtered plastic. That is why they are so stable with temperature and humidity.
Now if you want to play the trick is to get the capsules at a reasonable price. On eBay the capsules do sell for a bit, but the sound level meters with the 1" capsule on it sell for quite a bit less. Of course there is always some idiot who prices things way high.
For example:
Bruel Kjaer 2203 Sound Level Meter | eBay
Brüel KJÆR Bruel Kjaer 4132 1 inch Condenser Microphone Calibrated | eBay
Of course if you buy the meter you also get a connector for the capsule.
The article here on calibrating the B&K sound level meter might amuse those who read Dick Burwen's editorial on the effect of people and objects to the sound field. Some pretty dramatic pictures of the effects of a person standing near by. Brad - I assume you are playing with standard recording mics the metal diaphragms of standard measurement mics are not mechanically optimal for noise though B&K has an extreme example for special purposes that sacrifices the high frequencies for a rather stunning -4dB noise.
http://www.bksv.com/doc/TechnicalReview1969-2.pdf
http://www.bksv.com/doc/TechnicalReview1969-2.pdf
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That's a cool article! B&K are the preeminent experts on such things I think.
Yes, I'm dealing with recording mics (a new client). I had an objection raised about the proposed preamp, that it would add to the polarization voltage unduly. The objector however did not know what the capsule limitations were. I'd say that is a pretty important thing to know.
Getting back to Burwen, whom I respect in general, I thought that was a bit of a wacky piece in LA. But not because we can't measure significant effects from changes in the room environment, but because the human ears/brain has such a remarkable ability to adapt. Toole points out that the standard approach of using a single microphone for soundfield characterization, as all of the room correction boxes do that I know of, is fundamentally unsound (except for possibly mapping the bass response).
and another disappointment
All these years I believed the Toshiba datasheet for the 2SA1015, in which they allege that the parts have an rbb' of 30 ohms. The SM equivalent, the 2SA1162, no longer lists this important number. But H&H 3rd tabulates noise data for a flock of bipolars, and to my chagrin lists the 1162 at 10mA as 120 ohms!
Guess what? That is apparently correct! In fact I measure one sample (a 2SA1015GR) as 148 ohms. How disappointing!
Now I am almost afraid to measure a 2SA1316.
I guess the Diodes Inc./Zetex parts are the way to go.
All these years I believed the Toshiba datasheet for the 2SA1015, in which they allege that the parts have an rbb' of 30 ohms. The SM equivalent, the 2SA1162, no longer lists this important number. But H&H 3rd tabulates noise data for a flock of bipolars, and to my chagrin lists the 1162 at 10mA as 120 ohms!
Guess what? That is apparently correct! In fact I measure one sample (a 2SA1015GR) as 148 ohms. How disappointing!
Now I am almost afraid to measure a 2SA1316.
I guess the Diodes Inc./Zetex parts are the way to go.
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