76 votes

Why are NAND gates used to make AND gates in computers?

In order to get non-inverting operation for logic (i.e., AND or OR vs. NAND or NOR), you need to operate the transistors in common-drain mode, also known as "source follower" mode. Among the problems ...
Dave Tweed's user avatar
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66 votes
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What is the purpose of a buffer gate?

Buffers are used whenever you need... well... a buffer. As in the literal meaning of the word. They're used when you need to buffer the input from the output. There are countless ways to use a buffer. ...
metacollin's user avatar
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53 votes

Why would an AND gate need six transistors?

Your classmate is wrongly treating the transistors in your circuit as magical devices whose behaviour is completely controlled by something that appears at the gate and only the gate. They are failing ...
DKNguyen's user avatar
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37 votes
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What is the purpose of chaining series of NOT gates?

It's an old trick to delay a pulse a certain amount of time, for ex. 100 nanoseconds. It's used to ensure that different circuits (or different parts of the same circuit) get the same digital logic ...
Hoogault's user avatar
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36 votes
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Why does a single AND gate need 60 transistors?

There can be several reasons why more than the minimum 6 MOSFETs (4 for an NAND + 2 for an inverter) are used in this IC: As stated in the datasheet: The internal circuit is composed of multiple ...
Bimpelrekkie's user avatar
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35 votes

How to compensate noise at the output of logic gates?

They are not noisy. They are responding to the signals you have delayed through U9 to U12. Combinatorial logic responds after a short delay to any change on its input. If closely spaced changes occur, ...
Neil_UK's user avatar
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34 votes
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Why would an AND gate need six transistors?

In the logic gate level digital design abstraction, inputs are assumed to switch from logic HIGH to logic LOW and vice-versa instantaneously. This is done to simplify logic design. However, in the ...
Shashank V M's user avatar
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34 votes
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Why do CMOS NOT gate designs differ from BJT NOT gate designs?

However, I've always been perplexed as to why people do not use the former method for chip manufacturing. In chip design, resistors take up more area than transistors, and area is one of the main ...
The Photon's user avatar
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31 votes
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What will happen if the output from a NOT-gate injected- BACK to its OWN input?

What happens is usually cases 3. or 5. You have not defined case 5 :-) The joined input-output will sit at some voltage near the middle of the supply. 74HC14: When a Schmitt triggered gate is ...
Russell McMahon's user avatar
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30 votes
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Why aren't many classical logic gates reversible?

That is because a logic gate have one or more inputs, and only the inputs determine the output. If you have two inputs on a logic gate, and you only know the output, you can't determine what the ...
Justme's user avatar
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29 votes
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Why do two reverse diodes represent the logic gate AND?

Imagine A and B are both high. Then there is no current that flows out of A nor is there ...
slightlynybbled's user avatar
28 votes
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Why do we need to use transistors when building an OR gate?

What you describe is called a wired OR connection. It is possible in some logic families, particularly ECL (emitter coupled logic), but not in the most common ones (TTL and CMOS). In CMOS it isn't ...
The Photon's user avatar
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27 votes
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What is the purpose of an AND gate with the same signal on both inputs?

This is a buffer. Two gates means twice the output current. But why use AND gates instead of buffers, you might ask? I was originally going to say that they probably used one or two AND gates ...
alex.forencich's user avatar
26 votes

Do MOSFET logic gates necessarily need an N channel MOSFET?

Couldn't it be replaced with a simple resistor into ground like this? Yes it can and is has been done: PMOS logic there's also the complementary version: NMOS logic We've been there and done that! So ...
Bimpelrekkie's user avatar
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25 votes
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Digital Logic circuit - exam question

I wrote an algorithm in C# that tries every possible combination of those Nor 3->1 Xor 2->1 ...
Ido Kessler's user avatar
25 votes
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What is the process of going from machine code to electrically controlling individual transistors?

Ah, you're missing the STATE MACHINE concept. That's where we can "write code" made out of TTL hardware chips: data-selectors, 4-bit counters, gangs of parallel flipflops. (But all those are the ...
wbeaty's user avatar
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25 votes
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Why do logic gates need multiple MOSFETs?

The ‘C’ in ‘CMOS’ means ‘complementary’. CMOS uses both P and N FET types that work with opposite polarity. A CMOS inverter (P source to +V, N source to GND) works much like a single pole double throw ...
hacktastical's user avatar
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24 votes

Why are NAND gates used to make AND gates in computers?

A reason I don't think anyone mentioned yet: Technology constraints: Resistors on chip are massive compared to transistors. To get a decent value, we are talking orders-of-magnitude bigger than the ...
Joren Vaes's user avatar
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24 votes
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How does electricty flow in a flip-flop circuit? I can't understand how memory bits work with NAND gates

As a software guy, I had the same kinds of problems with HDL/Verilog... the hardware does not run in in any order, everything happens continuously, all at the same time. Your idea that "logic ...
MarkU's user avatar
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23 votes

What will happen if the output from a NOT-gate injected- BACK to its OWN input?

what you are describing is called a Ring oscillator Your output will oscilate with a certain frequency depending of the gate delay of your NOT gate. A perfect NOT Gate would oscillate with an ...
KarlKarlsom's user avatar
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23 votes
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Help in the closing of a logic circuit

This is an English language problem. You've been given a specification in natural language, and are struggling to turn it into explicit logic. Switches A and B are both on. You've used an AND gate ...
Neil_UK's user avatar
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23 votes
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Why do we need NMOS transistors for NAND gate?

if X or Y on both PMOS transistors is 0 , then both transistors will be open and the output will be 0 because there will be no flow of the current. The output will not be '0', it will be floating. ...
Transistor's user avatar
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23 votes

Why do CMOS NOT gate designs differ from BJT NOT gate designs?

It's possible to build this circuit with 2 BJT's or one MOSFET You're reading too much into it. The difference isn't the technology of the transistor, it's method of circuit operation In the first ...
Kyle B's user avatar
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22 votes

What is a logic gate?

You can define the term "logic gate" to be whatever you want, and no one will force you to change your mind. Each person is free to define the term as best suits their needs. As a CMOS VLSI designer ...
Elliot Alderson's user avatar
22 votes
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Why are the outputs of these gates tied together in the Apollo Guidance Computer?

RTL logic is open-collector, which allows the wire-OR connection (in this case, more precisely, wire-NOR.) The characteristic of wire-OR is that gates only drive low, so even if more than one is ...
hacktastical's user avatar
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22 votes
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What does a chip's propagation delay vary on?

The propagation delay can vary with manufacturing conditions as well as operating conditions. Temperature probably will play an effect. However, the point is, you can't control the propagation delay. ...
Math Keeps Me Busy's user avatar
21 votes
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Why does the race hazard theorem work?

As others have pointed out, mathematically the statements are exactly the same, and the additional term is "redundant". It would also be "redundant" for me to copy their mathematical proofs here. ...
jbord39's user avatar
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21 votes
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Why are OR gates using transistors different from OR gates using diodes?

"There are two ways to make an OR gate" and they are both crap unless combined with other (voltage amplifying) stages. In both circuits the output voltage will be 0.6V lower than the input voltage, ...
Wouter van Ooijen's user avatar
21 votes

What is a logic gate?

Wikipedia says that it is a idealized or physical device that implements the Boolean function. The mistake you make is assuming that the converse is true. Something that may implement a Boolean ...
Andy aka's user avatar
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