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In the linked application note, I have come across a symbol on page 5, of which a captured image is embedded below. Unusual MOSFET symbol

I have never seen this symbol before, but based on context I am assuming that it is two drain connected NMOS MOSFETs.

This MOSFET is being used to switch the 20V on or off just as a mechanical switch.

Is this so? If not what is it?

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  • \$\begingroup\$ Sure, but reading the description of the figure in the app note makes it pretty clear that they are PFETs \$\endgroup\$ – W5VO Apr 21 '17 at 19:02
  • \$\begingroup\$ @W5VO I misread that PFET as power FET at first. My brain decided to take an off day. \$\endgroup\$ – sww1235 Apr 21 '17 at 19:04
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I believe that that is a P-MOSFET. I've seen that in a few diagrams, although I don't have a link to them at the moment. A quick google search reveals this:

https://image.slidesharecdn.com/3673mosfet-150528123937-lva1-app6891/95/3673-mosfet-22-638.jpg?cb=1432816813

which is fairly conclusive.

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  • \$\begingroup\$ Thank you. There is almost no information in the document about what it might be so this helps a lot. I had seen the similar diagrams, but had missed the negation bubble on the bottom. \$\endgroup\$ – sww1235 Apr 21 '17 at 19:01
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    \$\begingroup\$ The "simplified" versions are pretty common when digital guys start drawing FETs. \$\endgroup\$ – The Photon Apr 21 '17 at 19:01
  • \$\begingroup\$ @ThePhoton with the super simplified variants (farthest right) how to tell what is D vs S? \$\endgroup\$ – sww1235 Apr 21 '17 at 22:07
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    \$\begingroup\$ @sww1235, You have to figure it out from context. The PMOS source is the one connected to the positive rail, the NMOS drain is the one connected to the negative rail. Or if it's an IC design rather than discrete devices, then the body is connected to the appropriate rail and the difference between source and drain is irrelevant (for example, in a pass gate). If you need to make an explicit distinction between source and drain, you shouldn't use the simplified symbols. \$\endgroup\$ – The Photon Apr 21 '17 at 22:13
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The circle-on-gate symbol is industry-standard for PFET. Instead of having to search for the (often tiny) arrows showing the polarity of isolation implants, the designer just looks for the bubble (versus no bubble).

Thus in old much-photocopied schematics, the Pch and Nch are trivially obvious.

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