How to make the Vth temperature independent?

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tommydidi

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Need to cancel the temperature drift of Vgs but don't know how. Any thoughts?

Thanks.
 

For any FET, there should be one "magic" point where
Vgs is relatively constant. This beats the subthreshold slope
change against the VT change for a net null. I would not expect
it to be fabulously consistent, but play around with a FET at a few
different current densities, across temp and you may see
what I'm talking about.

Then you only have the small problem of getting that accurate,
constant-vs-temp current instead. Heh.

Another option is to make the bias current decrease at a proper
rate w/ temp. If you had a bandgap reference you could do
this closed loop, perhaps.
 

dick_freebird said:
...Then you only have the small problem of getting that accurate,
constant-vs-temp current instead. Heh.

If you had a bandgap reference you could do this closed loop, perhaps.
Click to expand...
If there is a temperature-independent Vgs/Ids couple it's easier to keep Vgs constant, deduced from a good bandGap reference.

Below pls. find a paper on a similar topic (TC0 of Ids).
 

The Zero-TC operating point have a relative high spread. There are some approches for high-precision Zero-TC opamps w/o choppers in addition to offset correction also to trim the bias current to the Zero-TC-Point. In contrast to bipolar input stages where with offset voltage trimming trough the load resistor trim also the TC is trimmed.

I did not have the patent number but read it some times ago. The issue is that the current spread could be factor 3-4. That is difficult to achieve with offset trimming at the same time.

Using JFET lead to a much less spread in the Zero-TC current level.
 

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