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Old 27th August 2012, 07:05 PM   #361
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Quote:
Originally Posted by dirkwright View Post
HAHAHA! make an opamp with that ONE chip. That's a challenge.
Problem being the resistors are only 2K
(didn't notice it at first)
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Old 27th August 2012, 07:12 PM   #362
bcarso is offline bcarso  United States
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Quote:
Originally Posted by gpapag View Post
Time constant close to 10ms for uncooled microbolometer sensors , 7.5-12um spectral range, detector pitch 17-25um. Typical modern dedicated IR camera (sensor, optics, electronics). ~5000-7000 $

George
Thanks George! So it won't be on receivers sold at Best Buy anytime soon.

I do wonder though how practical the (non-imaging!) dichroic-filter two-band ratiometric approach might be. There are photodiodes for fiber comm wavelengths that have not-terrible performance at warm ambients --- but would the change in slope of the blackbody curve as a function of temp be enough? I suspect simple silicon is way too short-wavelength responding (about transparent at 1.1um) to do any good.
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Old 27th August 2012, 07:14 PM   #363
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> Unfortunately, clearly switching-use characterized.

As are most fets these days. Even the ones Pass uses.

> DMOS does not usually have a zero- or negative-tempco
> of drain current except at impractically high currents,
> the exception being the ancient parts with rather low
> transconductance (see the long tangential discussions
> by tvrgeek in the Sound Quality versus Measurements
> thread about his mods to a Hafler amp, for example).

Sorta like bi-polars.
Maybe this one ? :

> Note that there are no curves showing temperature effects.
> However they do mention the tempco of gate-source threshold
> voltage at 1mA as -4.5mV/degree C
> (about twice as bad as a typical bipolar Vbe).
> With some devices, the datasheet will show a family of curves
> and a point where, at three different temperatures, the drain
> current is about the same. But looking at the power dissipation
> at that point is discouraging for most devices.

Seems like degeneration could handle that.
How hot does a preamp have to run ?


> There is also no mention of noise, and MOS is notoriously
> bad at low frequencies, if better today than years ago.
> Not recommended for the front end.

Besides that the resistances are too small.
Maybe this one ? :
http://www.aldinc.com/pdf/ALD1103.pdf

Last edited by hitsware; 27th August 2012 at 07:25 PM. Reason: add a new device
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Old 27th August 2012, 07:37 PM   #364
bcarso is offline bcarso  United States
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Originally Posted by hitsware View Post
Oh yeah those folks. I had forgotten all about them. Still no specs on noise, but perhaps they would tell you if you asked.
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Old 27th August 2012, 08:35 PM   #365
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Quote:
Originally Posted by EUVL View Post
John,

That means a passive resistive load would be preferred for low noise design ?
How about the output stage cascode ?

Patrick
Patrick,

There was an article in Linear Audio, I believe Vol 1 or 2, by Ovidiu Popa.
He showed a fet current source load gives the equivalent noise of the resistor used to set the current. So there need not per se be a noise penalty for using a current course load.

jan
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Old 27th August 2012, 08:58 PM   #366
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Quote:
Originally Posted by bcarso View Post
Oh yeah those folks. I had forgotten all about them. Still no specs on noise, but perhaps they would tell you if you asked.
Gee, they're right around the corner . 12V max. and talking about MOSFET current gain is rather bizzare . I'll bet the noise is not great.
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Old 27th August 2012, 09:07 PM   #367
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> talking about MOSFET current gain is rather bizzare

~ infinite
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Old 27th August 2012, 09:43 PM   #368
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Yupp, typically the noise figures of MosFets for switching applications are not great.
One year back I played with the NDC7002N and NDC7003P for use in an input stage and found noise figures in the category of 55...60nV/sqrt(Hz) euqivalent input voltage noise.

But from my understanding the resulting equivalent input noise of an OPamp is dominated by the noise of the input gain stage.
Consequently noisy MosFets in the following stages are not likely to become a show stopper, as long as we have nice 2SK170 in the input stage - or do I miss something? The 2SK170 or 2SK370 are not that hard to get, different situation with the complementary P channels...
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Old 27th August 2012, 09:55 PM   #369
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The small signal mosfets may be noisy,
but something everyone seems to miss
with the larger devices is that they are
quieted by statistical distribution of the
noise of the separate parallel cells they
are comprised of.
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Old 27th August 2012, 09:56 PM   #370
bcarso is offline bcarso  United States
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Originally Posted by janneman View Post
Patrick,

There was an article in Linear Audio, I believe Vol 1 or 2, by Ovidiu Popa.
He showed a fet current source load gives the equivalent noise of the resistor used to set the current. So there need not per se be a noise penalty for using a current course load.

jan
Except that the resistor will always need be smaller than the one that would substitute for said current source, so there is a penalty, unless you can make the voltages in the circuit appropriately larger.
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