How to make the Vth temperature independent?

[tommydidi]

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

Thanks.

 

[dick_freebird]

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.

 

[erikl]

...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.

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).

 

[rfsystem]

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.