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Can anyone tell me what this symbol is?

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2 Answers 2

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It is shorthand notation for a PMOS Enhancement-Mode Device. Note that for this notation the PMOS has a circle on the input, like that of a digital logic’s notation where inversion is indicated with a circle. While a PMOS does not invert a signal, its operation is, in a way, inverted from that of an NMOS, whose notation does not include a circle on the input.

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It's a symbol for a P type mosfet, typically seen in the world of IC design. It is not generally an appropriate symbol to use in discrete designs.

A mosfet fundamentally has four connections, source, drain, gate and body (or bulk). At least in simple mosfet designs, drain and source are interchangeable. However, in discrete mosfets, source is nearly always connected to body inside the device. This makes the mosfet asymmetric, thanks to the presence of the "body diode" it can only block conduction in one direction.

In IC design on the other hand, the body is normally connected to the power rail and is often omitted on schematics to avoid clutter. That creates a problem though, the part of the regular mosfet symbol indicating the body connection, is also the part of the mosfet symbol that indicates whether the mosfet is N type or P type.

So some IC designers use the logical inversion symbol as a way to distinguish P type mosfets (which are turned on by a logic low) from N type mosfets (which are turned on by a logic high).

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