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I've seen schematic diagrams with transistors that have a little circle right where the gate connects with the rest of the transistor. From the context it seems like the behavior is that the resistance between the source and drain goes way down if the gate voltage is LOW, not high. Does such a transistor exist? If it does, how can I create a schematic diagram with it, using the tools on this forum? I couldn't find a transistor with such a circle among the items I can choose from.

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    \$\begingroup\$ Do you mean "the base", which only BJTs have, or "the gate", which FETs have? \$\endgroup\$
    – Ben Voigt
    Commented May 4, 2019 at 4:43

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That circle is used to show a P-channel device, as opposed to an N-channel. The idea is the that the gate behavior is inverted.

It isn't very common* to draw it that way, but it means the same thing as a more typical P-FET symbol. The schematic tool on this site doesn't use it. Just use the P-MOS or P-FET symbols, they mean the same thing.

*As Hearth points out in the comments, it is pretty typical to find these on CMOS topology diagrams.

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    \$\begingroup\$ I wouldn't say it isn't very common; as I understand it, it's very common in digital logic. \$\endgroup\$
    – Hearth
    Commented May 3, 2019 at 22:15
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    \$\begingroup\$ The use of circle to mean inversion or NOT is very common, yes, but not on transistor symbols themselves. I only rarely encounter it. \$\endgroup\$ Commented May 3, 2019 at 22:16
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    \$\begingroup\$ I see it all the time whenever I have occasion to look up schematics of CMOS topologies. \$\endgroup\$
    – Hearth
    Commented May 3, 2019 at 22:17
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    \$\begingroup\$ Oh, yes, you bring up a good point! That is the one place I see it routinely. I was thinking of circuit diagrams, good catch. \$\endgroup\$ Commented May 3, 2019 at 22:18
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Using the circle (bubble) to indicate a PMOS transistor is crucial to easily-read IC schematics.

One or two generations of photo-copies later, the non-bubble schematics with only those little arrows ----- become useless.

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    \$\begingroup\$ what are photo-copies? \$\endgroup\$
    – Indraneel
    Commented May 22, 2019 at 6:29

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