Electronics > Projects, Designs, and Technical Stuff
Matched transistors for analog experiments
David Hess:
--- Quote from: magic on February 21, 2024, 12:50:14 pm ---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.
--- End quote ---
Bipolar parts, especially with an all NPN process, should be much easier. I remember one guy who was making tunnel diodes in his garage since they are easier yet because of high doping.
magic:
I have heard that silicon epitaxy is not easy at all (temperature, pressure, fun chemicals like silane) and without epitaxy you are limited to quite primitive discrete devices.
mawyatt:
--- Quote from: RoGeorge on February 21, 2024, 10:12:53 am ---
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.
Took a search to double check that, it was George Erdi:
--- Quote ---There are some circuit subtleties belied by the schematic's simplicity, but yet important.
Q1 and Q2 are actually a quad set (dual pairs), with the paralleled pairs straddling the
chip's axis of thermal symmetry. The idea behind this was that thermal changes due to
output stage dissipation would be seen as equal thermally induced offsets by the two
input stage halves, and thus be rejected. This principle, first established in the 725 design,
has since become a basic precision design principle (see Reference 15, again, and within
Reference 23, the Fig. 2 chip photograph).
--- End quote ---
Source: page 61 of 970 http://www.miedema.dyndns.org/co/2018/Op_Amp_Applications_Handbook-Walt-Jung_2005.pdf
Well, not that I need any of such near perfect matching performance. :P
--- End quote ---
Eric's cross-coupled quad was a very valuable concept that everyone used, Fairchild really is the origin of all the greatest early semi folks!!
At the IC level other things can creep in that even Erdi's quad can't eliminate. We utilized very complex thermal models for the SiGe BiCMOS processes we were involved with, these included not one but two non-linear device thermal time constants and took into account surrounding circuity and layout of such. These thermal models scaled non-linearly with device type, size, and layout orientation, even location of wire bonds had an effect as they became a thermal sink.
In the end if one required extreme matching, it's always best to use differential techniques, Erdi's quad, symmetrical layout within and around the critical pair. Recall one integrated VCO design where we located a "matched pair" inside a large open center inductor where they would "see" an equal and matched thermal and electrical local environment.
Lots of effort went into thermal modeling even at the device level, including thermal details inside the device, where even doping type, concentration and profiles have effects at the time.
BTW your idea of using a high gain op-amp based diff amp front end for the DUT as the diff amp devices is likely the best means for precise matching., especially if one can reasonably match the desired electrical bias conditions. This is exactly what we did back in ~70 to check selected matched NPN pairs in TO-99 cans that National provided.
Best,
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