# How do logic gates operate? [closed]

I know what the major logic gates are, I know that each gives a different output depending on the input. However, that's all I've been able to find. Every time I look it up, its a list of gates and truth tables, with diagrams of the flow of information, and the gates themselves are just stuffed into a shape in the diagram. This isn't about Boolean logic, I'd like to know how it physically happens. What does a logic gate do with electrical signal(s) to produce output?

## closed as too broad by Fizz, PeterJ, Daniel Grillo, jippie, The PhotonNov 5 '15 at 21:47

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• Are you asking for schematic diagrams for different gates, or are you asking how the transistors used to build logic gates work? – Caleb Oct 31 '15 at 15:27
• I'd like to know how they're actually built, yes. As mentioned in the question, everything I've seen trying to find an answer is only schematic diagrams, which l already understand. – Kevin Milanowski Oct 31 '15 at 15:29
• You can look up CMOS gate for the one most commonly used technologies today. Of course if you don't understand what a MOSFET is and does, that won't help you much. So depending on your background you may have a fair bit of reading to do. – Fizz Oct 31 '15 at 16:13

If you go to Texas instruments' website and search for the datasheets for gates like the SN7400, SN7404, SN7402, SN7408... you'll find schematics of the gates showing them down to transistor level and then you can figure out how the magic happens. :-)

Here, for instance, is the schematic and a circuit description for one of the four identical 2-input NANDS in an SN7400

A and B are the inputs, and notice that if nothing in the external world is connected to them they'll float high because the diodes are cutting them off from ground.

But wait! If you look carefully, and if you know that bipolar transistors can have their emitters and collectors swapped with some reduction in gain, you'll see that there's something very clever going on here; namely that while A and B are idling, the input transistor's collector is posing as an emitter and is being used to supply base current into the second transistor, turning it (the second transistor) ON enough to pull the base of the upper output totem pole transistor low enough to turn it OFF, while close enough to Vcc to turn the lower totem-pole transistor ON, resulting in a current sink at Y strong enough to call it a logic zero, with A and B floating high,

Now, assuming that instead of floating, A and B are both hard-wired to Vcc, (which is the same as a logic "1") nothing will have changed and "Y" will still be close enough to ground (zero volts) to be a logic low.

Pull either A or B or both of them to ground, though, and the current which was formerly flowing from the collector of the first transistor into the base of the second will be sucked to ground by the first transistor.

That'll put the base of the second transistor pretty close to ground, which will turn it off.

Then, with no current through the second transistor there'll be no voltage dropped across the 1k resistor and the bottom totem-pole transistor will be turned off.

At the same time, however, the current through the 1.6k resistor will now be steered into the base of the top totem-pole transistor and will emerge from 'Y' as a voltage close enough to Vcc to be called a logic '1'.

So, only if 'A' and 'B' are both ones will 'Y' will be a zero, which defines a NAND.

• I'm a bit too new to understand this. :( Help would be very welcome! – Kevin Milanowski Oct 31 '15 at 16:27

Actually i don't know if this answer would help. But i think that logic gates actually negate the flow of current if it has true input on both it's terminal in case of a NOT gate and basically produces a current when two terminals have a true input for an AND gate.

• I know that's what logic gates do, I want to know how that happens. – Kevin Milanowski Oct 31 '15 at 16:02

"The Art of Electronics" by Horowitz and Hill has a reasonable explanation.
3rd Edition is in Chapter 10 "Digital Logic", starting at page 703
2nd Edition is in Chapter 8 "Digital Logic" starting at page 471

They include schematic diagrams of gates made with discrete transistors, along with explanations of how the circuits work.

It is only about 20 pages, so it wouldn't take long to photograph or copy.

The advantage of either of those books is the earlier chapters cover more than enough on discrete components to fill in the details, if you feel the need.

The 3rd Edition came out recently, so you might be able to pick up the 2nd Ed cheaply; a friend bought one 'pre-loved' in a 'charity shop' for about 1/3rd price of a new one.

• If you have a proper library system, you might be able to order it. I think the whole chapter might be interesting if you are 'into' this sort of stuff. A friend said he found a lump of the 2nd Ed on line, scanned, in a pdf, a couple of years ago. – gbulmer Oct 31 '15 at 16:30

Read this Article about Creating Logic Gates with Transistors. This should give you the explanation you look for. I highly recommend building some logic gates with transistors and resistors. It is a great learning experience. You will probably learn a lot about the way gates are built, and you will probably learn about the transistors themselves too.

Cheers!