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Using Arduino => General Electronics => Topic started by: db2db on Nov 14, 2014, 05:20 am

Title: suggest an NPN not afffected by temperature?
Post by: db2db on Nov 14, 2014, 05:20 am
I'm looking for a NPN BJT (SOT23) that has a pretty stable VBE as the temperature changes.
I was told that the Fairchild 2N5088BU is good, but I see no supporting data.

Any suggestions??
Title: Re: suggest an NPN not afffected by temperature?
Post by: weedpharma on Nov 14, 2014, 06:15 am
What temperature range are you looking for?
Title: Re: suggest an NPN not afffected by temperature?
Post by: Henry_Best on Nov 14, 2014, 06:55 am
What temperature range are you looking for?
Over about 300oC, ALL transistors are affected.   :smiley-mr-green:
Title: Re: suggest an NPN not afffected by temperature?
Post by: weedpharma on Nov 14, 2014, 09:34 am
I had considered a range from 0 deg K as starting point. :smiley-mr-green:

Weedpharma
Title: Re: suggest an NPN not afffected by temperature?
Post by: JimboZA on Nov 14, 2014, 09:56 am
0 deg K
Bzzzzzzt.

Temperatures on the Kelvin scale are not named "degrees", they're just "Kelvin", as in "0 Kelvin".
Title: Re: suggest an NPN not afffected by temperature?
Post by: weedpharma on Nov 14, 2014, 12:16 pm
Bzzzzzzt.

Temperatures on the Kelvin scale are not named "degrees", they're just "Kelvin", as in "0 Kelvin".
Mea culpa
Title: Re: suggest an NPN not afffected by temperature?
Post by: TomGeorge on Nov 14, 2014, 12:18 pm
Hi, what is the application that you need a transistor with this characteristic, there may be another way of accomplishing what you need.

Tom......... :)
Title: Re: suggest an NPN not afffected by temperature?
Post by: MarkT on Nov 14, 2014, 06:36 pm
All silicon transistors have the same dependency on temperature, there's nothing you
can do by choosing a better device.

What you can do is use a cancelling circuit to subtract the temperature dependence by
using two matched transistors kept at the same temperature.  You keep a constant
current through one and then the Vbe difference cancels out.

You can also choose a semiconductor with a much higher bandgap voltage, as the
thermal term subtracts from this, so the larger the bandgap the less the thermal
term proportionally.  That means gallium arsenide, silicon carbide or other exotic/expensive
devices.

But unless you say what you are trying to achieve its hard to know how to help.
The cancellation circuit is useful for making analog logarithmic amplifiers and
multipliers, for instance.

It might be that you need feedback in your circuit to linearise it, there are many ways
round a problem if you know the actual goal.
Title: Re: suggest an NPN not afffected by temperature?
Post by: cjdelphi on Nov 15, 2014, 02:08 am
It's possible to turn a transistor or even a diode into a temperature sensor based on it's forward voltage drop..

How's Germanium hold out over silicon?  Better  / worse regarding temperature?
Title: Re: suggest an NPN not afffected by temperature?
Post by: db2db on Nov 15, 2014, 06:34 am

Thanks for the answers. I didn't realize all equally temp sensitive.
Title: Re: suggest an NPN not afffected by temperature?
Post by: TomGeorge on Nov 15, 2014, 07:39 am
Hi,please read post #7, what is the application, what are you going to do with a transistor if you could  find one?

Tom.. trying to help! ! ! ..... :)
Title: What is the real question here?
Post by: Paul__B on Nov 15, 2014, 08:49 am
As always - you should explain what your actual problem is that you need to address, not what you have imagined as the solution to the problem.
Title: Re: suggest an NPN not afffected by temperature?
Post by: Grumpy_Mike on Nov 15, 2014, 09:15 am
XY problem again

http://xyproblem.info/ (http://xyproblem.info/)
Title: Re: suggest an NPN not afffected by temperature?
Post by: db2db on Nov 17, 2014, 07:25 am

I am using it in a feedback circuit for a constant current driver. I have some other solutions in mind, so don't need more info.. yet.

Thanks.
Title: Re: suggest an NPN not afffected by temperature?
Post by: Paul__B on Nov 17, 2014, 11:39 am
That's good, because unless you describe it in more detail - in fact a lot more detail, explanation and specifics including current and proposed circuitry - you are not going to get much useful assistance.  :smiley-grin: