Thanks George! So it won't be on receivers sold at Best Buy anytime soon.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
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.
> 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
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
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Oh yeah those folks. I had forgotten all about them. Still no specs on noise, but perhaps they would tell you if you asked.
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
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.
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...
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...
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.
True, but the capacitance is high for anything approaching JFET performance at low frequencies.
The bugger is, there is continuing improvement in general of the surface/interface states, but with the rare exceptions (e.g., National Semi [now TI] CMOS opamps), hardly anybody specifies the noise. I measured a power MOSFET ages ago, and although it was relatively speaking, huge, it managed about 3nV/sq rt Hz at not-all-that low of frequency (don't recall the exact numbers). More recently I evaluated some 2N7000 parts to use in a cascode, and they weren't too bad, but definitely dominated by low frequencies for 20 - 20kHz.
No, you haven't missed anything --- a good design will be dominated strongly by the input stage contribution. If you are going for insanely low e sub n, you may start to notice the second stage, but that's a pretty extreme case.
I was referring to good quality imaging apparatus.
IR thermometers are very much cheaper. Measuring temp. with them can be tricky due to material's emissivity and field of view of the thermometer.
For diy IR experimentation:
1. Tweaking medical IR thermometers
2. Tweaking old cheap digital cameras.
Homemade IR Camera | eHow.com
how-to turn a digital camera into an IR-camera :: projects :: geek technique
George
PS. For temp. monitoring of transistors, I have filled/sanded the plastic encapsulation (TO-92) of an LM35. The 4-5 times reduced volume, reduced it’s thermal time constant considerably (but still 1-3 seconds depending on contact area)
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Gee, they're right around the corner
Well, drive on over and ask them!
Not being an expert in electronic applications at all I am wondering why a simple thermocouple device could not be used for checking temperature rise? I used these all the time in manufacturing in autoclave applications and also in temperature control circuits. Perhaps the rise time is just to slow to tell you what you are looking for, but it would seem to be more specific than a simple IR handheld device. How could you narrow the specific heat source with one of those devices? You would pick up all the surrounding heat sources and I have never seen a handheld IR that had that narrow of a field of view?
Actually we have one with only a few mils spot size. Omega does make some tiny thin surface conforming thermocouples, a much better idea.
I was referring to good quality imaging apparatus.
IR thermometers are very much cheaper. Measuring temp. with them can be tricky due to material's emissivity and field of view of the thermometer.
For diy IR experimentation:
1. Tweaking medical IR thermometers
2. Tweaking old cheap digital cameras.
Homemade IR Camera | eHow.com
how-to turn a digital camera into an IR-camera :: projects :: geek technique
George
PS. For temp. monitoring of transistors, I have filled/sanded the plastic encapsulation (TO-92) of an LM35. The 4-5 times reduced volume, reduced it’s thermal time constant considerably (but still 1-3 seconds depending on contact area)
Yes, emissivity is clearly important to take into account (I'm reminded of the rule-of-thumb at aluminum extrusion houses: assume any piece of extrusion is very hot until proven otherwise, as the emissivity of bare Al is so low!).
Another confounding variable for looking at a transistor with a short-wavelength detector: they are, to varying degrees, light emitters! In high-field regions especially. When Janesik et al. were developing CCDs for astronomy, there was a persistent excess of apparent dark current in the vicinity of the on-chip output amplifier. It turned out it was light emission from the MOS part, and iirc it got a good deal worse with cooling. They had to turn the amplifier off during long exposures.
Brad
