Electronics > Projects, Designs, and Technical Stuff

Matched transistors for analog experiments

<< < (7/8) > >>

magic:

--- Quote from: RoGeorge on February 21, 2024, 10:12:53 am ---Don't know for others, but I'm a little disappointed.  We still can not 3D print at home our own ICs after so many years.  Don't want the finest sub nanometer tech, but something from 50+ years ago should be desktop technology today, one might expect.

--- End quote ---
The problem is not nanometers but chemistry. It was ugly 50 years ago, it is ugly today, it will stay ugly forever.
You can't 3D print doped semiconductors.

A few people managed to fab basic MOSFETs in home labs.

iMo:
This guy is doing DIY transistors for many years already  :D
Many vids on the equipment in his basement as well..

magic:
I wonder how good bad >:D his Vgs matching is.

Zero999:
What about the CD4007UB for MOSFETS? I expect it won't be as well matched as specially designed matched pairs, but it's likely to be better than two discrete MOSFETs. There's also the slightly newer HEF4007UB, which might be better.
https://assets.nexperia.com/documents/data-sheet/HEF4007UB.pdf

David Hess:

--- Quote from: RoGeorge on February 21, 2024, 10:12:53 am ---For manually matching I would probably make a differential pair with sockets for the 2 transistors, and select the pairs with the best common mode input rejection.  No matter how careful they are matched initially, the slightest temperature difference between the two transistors will affect the performance.  Adding a common radiator for the individual transistors helps, but there will still be some thermal resistance that might keep the transistors at slightly different temperatures.
--- End quote ---

Differential pairs can be further thermally balanced by adding resistors in series with the collectors to halve the collector voltages.  Then when the pair is unbalanced, the power dissipation stays the same on both sides.  High frequency performance is preserved by capacitively bypassing the resistors.


--- Quote ---The advantage with factory matched transistors is that they are in very close proximity to each other, often on the same die, which will keep them at a much closer running temperature.  Even so, that will work for pairs only, while sometimes there is a need to match more than 2 transistors.

Ideally will be to be able to make custom ICs.  :)

Rant aside, even inside the same IC, I've read there are situations where thermal pairing must be treated specially, i.e. precision opamps have a thermal axis of symmetry, where the input transistor are placed in such a way that the heat from the output stage/transistors will spread to the input transistors equally, to not ruin the matching.  IIRC first introduced for uA725.
--- End quote ---

Monolithic matching is not a panacea.  Parasitic coupling between the transistors will spoil things like settling time and will spoil performance in high frequency applications, so there is still a place for matched dual parts which provide thermal isolation.  In the past precision fast settling designs used either discrete or hybrid construction.  Monolithic fast settling time parts do not compete and have relaxed accuracy from low open loop gain.

Navigation

[0] Message Index

[#] Next page

[*] Previous page