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Extrarius

CMOS Nand/And

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Extrarius    1412
I'm in a digital logics class this semster, where the focus is essentially boolean algebra and using it to compute things (well, eventually, right now we're still doing truth tables and optimization of simple functions). No, this isn't homework, it is just something I thought of relating to the material in the class. We briefly went over the construction of NMOS, PMOS, and CMOS versions of the various gates, and, according to my book, Nand in CMOS is(excuse my lack of art skills): To get an And gate according to my book, you'd add a two-transistor CMOS inverter such that F (the output) would be inverted and that is how And gates are made. My question is: Wouldn't simply switching the 'PMOS Transitors' and 'NMOS Transistors' like below make an and gate? : Or is there some electronic reason that wouldn't work to make an And gate with the same transistor count as a Nand gate?

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Cosmic314    2002
Quote:
Or is there some electronic reason that wouldn't work to make an And gate with the same transistor count as a Nand gate?


Logically speaking you are correct. Electrically, however, pmos transistors transfer only vdd well, while nmos only transfer gnd well. Adding two nmos devices in series doubly diminishes its capacity to transfer vdd. Lookup information on threshold voltage (Vth) drops across these devices for more information, if you have a transistor theory book lying around.

The end effect is that the circuit may not transfer enough energy to the output to make it efficient or function correctly. Either way you will diminish yield on a part which = $$$.

You may wish to investigate transfer gates which use pmos and nmos in parallel as another form of static logic.

I stare at these devices all day at IBM, working on a, *ahem*, next generation gaming processor.

[Edited by - Cosmic314 on September 29, 2004 6:20:49 PM]

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grhodes_at_work    1385
Quote:
Original post by Extrarius
No, this isn't homework, it is just something I thought of relating to the material in the class.


I'll let you get away with it this time; however, if you read the Forum FAQ you'll see that, since its impossible to distinguish homework from other schoolwork, basically anything school related is off topic.

Also, you'll also see that the Forum FAQ asks that you show your own work before a post that is school or similar will even be considered to remain open. Fortunately, you created some images that satisfies this requirement!

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