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Old 11th January 2008, 02:02 AM   #1
maxpou is offline maxpou  Canada
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Question Zero temco?

Hi guys,
i read in many post zero tempco on jfet, V mosfet and lateral mosfet. i have a question for you: What the zero tempco and how do i measure? Thank You! Maxpou
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Old 11th January 2008, 02:24 AM   #2
KSTR is offline KSTR  Germany
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It means that the parameter in question -- typically drain current -- does not change with temperature, zero temperature coefficient. With many FETs (not all) there is one single useful operating point, strongly specific to the actual device, where this happens.

Example:
http://www.nxp.com/acrobat_download/...245A-B-C_2.pdf
Look ak fig.9. At Vgs=-1V there is little variation with temperature, the zero tempco point (flat straight curve) seems to be at -1.1V or so.

Example, different way of plotting the relationship:
http://www.fairchildsemi.com/ds/2N%2F2N5457.pdf
See first two figures, there is a point where the three curves meet, at that point the drain current does not vary with temperature (but transconductance, normally corresponding to gain, does). If they don't meet, then there is no operating point with stable characteristics over temperature, then this is to be compensated for externally with additional circuitry (always true for bipolar transistors).

From the plots you directly derive how it is measured.

- Klaus
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Old 11th January 2008, 07:54 PM   #3
maxpou is offline maxpou  Canada
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Hi KSTR,
thank you for explanations but Why it is so important to take this factor into account? thank you Maxpou
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Old 12th January 2008, 01:34 PM   #4
AndrewT is offline AndrewT  Scotland
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most devices are driven by voltage.
If the V vs I curve varies with temperature then the Current will change with Temperature.
This makes setting up more difficult.
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regards Andrew T.
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Old 12th January 2008, 03:36 PM   #5
maxpou is offline maxpou  Canada
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Hi,
It is what can cause thermal distortion and it's why the vertical mosfet have a thermal stability problem?

But if i don't have a good figure for found the zero tempco how can i do this? For 2SK170 2SK246, 2SJ74 and 2SJ109

Thank you! Maxpou
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Old 12th January 2008, 04:14 PM   #6
roender is offline roender  Romania
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Quote:
Originally posted by maxpou
Hi,
It is what can cause thermal distortion and it's why the vertical mosfet have a thermal stability problem?

But if i don't have a good figure for found the zero tempco how can i do this? For 2SK170 2SK246, 2SJ74 and 2SJ109

Thank you! Maxpou
2sk170 has zero TC at 13-13.5mA, only the V version are suitable for ZTC.
2sk246 has ZTC at 0.32 ... 0.35mV.
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Old 12th January 2008, 05:45 PM   #7
maxpou is offline maxpou  Canada
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Hi,
where are you found the information of zero tempco for K170 and K246? I look in datasheet but i don't found. thank you! Maxpou
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Old 12th January 2008, 05:53 PM   #8
KSTR is offline KSTR  Germany
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Quote:
Originally posted by maxpou
Hi,
It is what can cause thermal distortion and it's why the vertical mosfet have a thermal stability problem?
There a two things going on. While with many devices you can find an operating point where the device current has zero tempco, the gain of the device (transconductance) will still vary. This change also can give thermal distortion, albeit in rather less important amounts, compared to shifted bias problems in class A/B outputs.

With VMOS, these devices often have a zero tempco current point, but unfortunately at very high currents that one cannot make practical use of it. Therefore one needs external bias stability circuitry to keep standing current under control, same problem as with bipolar transistors

Klaus
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Old 12th January 2008, 06:02 PM   #9
KSTR is offline KSTR  Germany
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Quote:
Originally posted by maxpou
Hi,
where are you found the information of zero tempco for K170 and K246? I look in datasheet but i don't found. thank you! Maxpou
There is no information on this in the datasheet, at least in the ones I have for the K170 (but other toshiba JFET datasheets have curves for it, e.g. K330). It must be measured.

Klaus
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Old 12th January 2008, 06:08 PM   #10
maxpou is offline maxpou  Canada
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Hi Klaus,
thank you for your answer you confirm to me what i was thought. How can i mesure the zero tempco? thank you! Maxpou
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