# What is Logic gates: Definition and 67 Discussions

A logic gate is an idealized model of computation or physical electronic device implementing a Boolean function, a logical operation performed on one or more binary inputs that produces a single binary output. Depending on the context, the term may refer to an ideal logic gate, one that has for instance zero rise time and unlimited fan-out, or it may refer to a non-ideal physical device (see Ideal and real op-amps for comparison).
Logic gates are primarily implemented using diodes or transistors acting as electronic switches, but can also be constructed using vacuum tubes, electromagnetic relays (relay logic), fluidic logic, pneumatic logic, optics, molecules, or even mechanical elements. With amplification, logic gates can be cascaded in the same way that Boolean functions can be composed, allowing the construction of a physical model of all of Boolean logic, and therefore, all of the algorithms and mathematics that can be described with Boolean logic.
Logic circuits include such devices as multiplexers, registers, arithmetic logic units (ALUs), and computer memory, all the way up through complete microprocessors, which may contain more than 100 million gates. In modern practice, most gates are made from MOSFETs (metal–oxide–semiconductor field-effect transistors).
Compound logic gates AND-OR-Invert (AOI) and OR-AND-Invert (OAI) are often employed in circuit design because their construction using MOSFETs is simpler and more efficient than the sum of the individual gates.In reversible logic, Toffoli gates are used.

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1. ### Interesting video on: Why are 2 logic gates faster than 1?

Just saw an interesting video on youtube explaining Logical Effort very nicely. https://en.wikipedia.org/wiki/Logical_effort
2. ### Engineering Combinational Logic Circuits using Logic Gates

Homework Statement Design a circuit with four inputs and two outputs which realizes the functions : f1 = Σm(0,2,4,5,6,14) f2 = Σm(0,1,4,6,12,14) Homework Equations - The Attempt at a Solution Here's my work : 2 levels (NAND / OR ) 10 gates 17 gate inputs 2 levels ( NAND / OR ) 7 gates 11...
3. ### Ohmic Resistance and Logic gates

Is ohmic resistance a "necessary evil" in order for transistors to be able to function as logic gates? I mean, I have seen some circuits for the NAND gate in RTL and TTL and they both seem to involve ohmic resistances. Can we make a super conducting NAND gate that will have total ohmic...
4. ### Logic gate question -- Tying both inputs of a gate together

Hello, I don't know much about logic gates and how they work. I was wondering if this (Image) can work. (Is something like this possible?) The idea is that if we have logic 1, AND gate gives 1 while NOR gives 0. And vice versa.
5. ### Engineering Circuit Realization of XOR function with Three inputs

Homework Statement This is from a past exam paper for logic design in my course. I have an exam coming up and would love to know how to solve this one. [/B] Develop a circuit realization of the XOR function with three inputs. You may use AND, OR and NOT-gates with not more than two inputs...
6. ### How to toggle and latch using logic gates?

We're implementing Dots and Boxes to logic circuits and the we would like to automate the process of changing turns. If you know the game, the game changes turn when one move doesn't score a point or form two or more boxes. I was able to create overlapping XOR gates that alternates from high to...
7. ### Logic gates, truth tables, circuits

Homework Statement (a) The NOR gate combination in Figure 1 acts as an AND gate. Copy and complete the truth table. (b) An aircraft door is locked by two bolts A and B each of which operates a sensor giving a logic 1 when the bolt is fully inserted. A further sensor C gives a logic 0...
8. ### Construct a truth table for the combination of logic gates

Homework Statement Construct a truth table for the combination of logic gates shown above. 2. The attempt at a solution First of all the combination of logic gates with added letters to the combination: The truth table: I think it should be correct. But just want to be sure.
9. ### What is the output of a logic gate when one input is not connected?

Hello, I'm currently studying a course in digital logic. The course is included in my IT program. And I have not taken a course in electronics yet; I will later. My question basically is: what is the output of a logic gate given that one input is neither connected to a high voltage nor...
10. ### ASIC Logic Gates NAND Homework: Solving 2-Input NAND Gate Circuit

Homework Statement Hi everyone, Very new to this forum so please bear with me! I have an assignment as part of my engineering course that I seem to be struggling with so I'm wondering if I can grab some help. Basically I have a water tank that feeds three separate processed (A, B, C). When...
11. ### Logic Gates Algebra: Simplify A/B + /BC + /AB

Homework Statement I am confused a little bit. I have been asked to simplify this function: A/B + /BC + /AB I seriously can't seem to know how to simplify this as for me its already simplified. Homework Equations N/A The Attempt at a Solution The best I could do is : (/A+/B) * (A+B+C) I did...
12. ### Logic Gates (NAND to AND Gates)

couldnt understand why the input is ( (XY)' (XY)' ) ' , IMO , it should be ((XY)' ) ' . because the 2nd logic gates receive (XY)' input from the first logic gates , and the second logic gates act as inverter for the first logic gates.
13. ### PN Junctions vs Transistors When Building Logic Gates

Is there any reason why logic gates are built using transistors instead of PN junctions? Wouldn't it be more cost-efficient to use PN junctions? I am referring to CMOS logic gates. Also, what can a transistor do that a PN junction can't?
14. ### Digital Electronics- logic gates problem

A bulb in a staircase has two switches , one at the ground floor and the other one at the first floor. The bulb can be turn ON and also OFF by any of the two switches irrespective of the state of the other switches, then what is the logic of switching of the bulbs
15. ### MHB What are the practical uses of logic gates in computing and electronics?

what are some applications of logic gates? (NOT, AND, OR, NAND, NOR, XOR) also what is a binary selective mask
16. ### Question Regarding IC pin numbering and logic gates

Homework Statement http://i.imgur.com/k8nq4HZ.jpg So I need to assign IC pin numbers to the input and output of the logic gates.Homework Equations None The Attempt at a Solution I only have limited knowledge on integrated circuits as a whole, so I'm pretty confused altogether. I know that I...
17. ### IC Testing for Logic Gate Errors

Hello,IS there any way to check IC [AND NOT OR gate] for errors ? Is there any method for it ?
18. ### Consecutive logic gates voltage loss

Hi, I am introducing myself to logic circuits and I want to build a network of logic gates (CMOS) that have push buttons as inputs and LEDs as ouputs. At most there are 4 gates between a button and a LED. However, at the inputs of the 4th gate, which is a AND gate, I measure 4 V and 4 V, but...
19. ### How can permutations help with logic gates and circuit design?

Hello, today in class we started a topic on permutations and combinations and I have come across a way in which it could be of use to me whilst working with 'logictutor' (a premade circuit board used to teach simple logic circuits). We have an experiment tomorrow where we will investigate...
20. ### Building Converters using Logical Gates for 4-bit Inputs

Homework Statement I have to build some converters (only for 4 bits) and i didn't understand how. How do i proceed? Which logical gates combined in such a way give me the y3y2y1y0 answer for x3x2x1x0? For example, build a coverter from EXCES3 to BCD, or from GRAY TO BCD, or C1 complement to C2...
21. ### Need help in simplifying circuits with logic gates

hi, I am new in basic electronics course. i m having some problem with simplifying the circuits using boolean algebra. can you suggest me any good book to practice this? and i also need a short chart of the laws.
22. ### 7 segment display + logic gates

Homework Statement I'm tasked with creating a logic network that can spell out a simple 4 letter word. It has 2 inputs A and B, and 7 outputs (for a 7 segment display). The 2 bits (A and B) give the 4 total possible inputs to the circuit. Write the truth table for the 7 output functions...
23. ### Compact Logic Gate Options for Efficient Circuit Design

Hi I am learning about logic gates and am having some fun with them but somtimes in my designs I only have use for maybe 1 or 2 gates, this is not convenient sense the chips I use have 4 or 6 gates in one chip, does anyone know where I can find NAND, AND, and inverter gates in 8pin dip or less...
24. ### Basic Logic Gates / Pulse Train Problem (Includes Solution)

Homework Statement The problem and its solution are attached. Homework Equations N/A The Attempt at a Solution I'm very confused about how output pulse trains work. I already checked online (including Wikipedia) so, could someone please give me an explanation of the absolute basics in...
25. ### Embarrassing electronics problem (logic gates)

Hi, I'm fairly new to and not very good at electronics (my A-level project was an LDR that turned on a light, LOL, and I got an A! {UK education for you}) and was wondering how the basic logic gates work. From hyperphysics this is an AND gate: Here's how I understand this...
26. ### Open Circuit Problem with Logic Gates

I wonder how can I make the logic gate read 0V when it is open? For instance, the AND logic gate. If one of its input pin has 5V supply, and the other don't have, the output signal given to me is 5V. How can I make it 0V? Well, theoretically, it should be 0V, but in reality, it appears that...
27. ### Understanding the Matrices of Quantum Logic Gates

(I am not sure if this is the right section for this). This question probably is extremely trivial and silly, but I haven't been able to find the answer to it despite going through quite a bit of The Internet. So, it appears that each Quantum Logic Gate corresponds to a matrix. Ones that...
28. ### Logic Gates/ Truth Tables Trouble

Hey guys I am having todo a resit paper for a maths module i failed and I've come up against a question on my paper where I am stomped and don't know if I am down the right road or not. Its truth tables from Boolean equations. Question was: K = ( X + Y) . (X . Y) I Came up with: K X...
29. ### NOT Gate: Inverting Inputs & Current Flow - Help Clear Confusion!

From what I learned at school today.. Afaik NOT gate acts as an inverter, right? Which means if input is 0, output will be 1 and vice versa. My question is, if input is 0 then output will be 1. From where does the gate gets its current flow? Since input is 0. Or, it cannot be used like...
30. ### MAking logic gates with 3-pin trasnistors

I'm trying to make logic gates using MOSFET transistors, for a lab. for the prep, my lab group designed logic gates based on 4-pin transistors, but the MOSFETs we were provided with only have 3 pins. Is it possible to make logic gates using 3-pin trasnistors, and if so how?
31. ### How to Draw Logic Gate Graphs for NOR and XOR Gates

Homework Statement Graphs A and B are entries to a NOR gate with 2 inputs. Draw the output C. Graphs X and are entries to a XOR gate. Draw the output Z. Homework Equations The Attempt at a Solution
32. ### Logic Gates with XOR gate need to verify solution

Homework Statement Basically and most importantly... I wanted to see if I got my F function correct. That's at clause 2. At clause 3 I'm told a = b = 1 c = d = 0 and to calculate At clause 4 I'm asked if the logic value of P is 1, if the logic value of Q matters... The...
33. ### What is the Simplification of a Boolean Function Using Karnaugh Maps?

Homework Statement Given the function: http://img26.imageshack.us/img26/9718/elel0.jpg A) Write the truth table of the function F (A, B, C, D) B) Present the function F (A, B, C, D) via Karnaugh Map C) Express the function F as the sum of multiplications with minimum literals...
34. ### Logic gates, inverting output without increasing transistor count

Homework Statement Hello all, I have the following logic gates: (A'.(B+C))' which I had reduced to A+B'.C'. I then want use a NAND gate connected to a NOR gate, then connected to an inverted. However, I have been informed that I need not to use ten transistors, and that 8 can get the job...
35. ### Logic Gates: Practice and Review

I want to practice the little I know about logic gates... Can you guys help me out? :) Thanks in advance and merry christmas! Nexor
36. ### Logic Gates and CPUs: Basic Design Structure of Computer Processors

I would like to know the basic design structure of computer processors. My concept of a processor is that it reads some binary data from memory, performs operations on it (according to a set of instructions, which it also reads in), and then writes the result to memory somewhere. (Is this...
37. ### Logic gates in excitonic single-quantum-dot qubits

I am searching for physical realizations of universal logic operations (phase rotation, CNOT, Hadamard) in single-dot excitionic qubits. Phase rotations are easy to implement with sinusoidal electric fields but my literature search for CNOT and Hadamard gates runs dry. I can find them in spin...
38. ### Logic gates in excitonic single quantum dot qubits

I am searching for physical realizations of universal logic operations (phase rotation, CNOT, Hadamard) in single-dot excitionic qubits. Phase rotations are easy to implement with sinusoidal electric fields but my literature search for CNOT and Hadamard gates runs dry. I can find them in spin...
39. ### Logic Gates (OR and AND) Wave Form Diagrams

If you take a look at the attachment that I have posted, you will see 4 wave diagrams labeled A, B, C, D, respectively. My objective to is to answer the questions after the A-D diagrams. The first one is a AND logic gate with A and B. So how would I combine A and B. Do I draw only the highs of...
40. ### Use of input current and quiescent supply current in logic gates

What is the meaning and use of input current in logic gates? Also meaning and use of quiescent supply current.
41. ### Looking for a Detailed Explanation of How Transistors and Logic Gates Compute

Hi, I've looked everywhere for an explanation of this, including this forum, and none of them seem to satisfy the level of detail I am looking for. More specifically, I am interested in how processing units used a fixed setup of logic gates and transistors to perform universal computation...
42. ### How can compilers be used to implement logic gates and IDEs?

I want to find out how computer language compilers works from hardware point of view. How logic gates are implemented using compilers? Can anyone suggest me a good reference book or website where I can find this information?
43. ### Solving Boolean Logic Expressions with And, Or and Not Gates

The three equations below represent three different logic expressions using and, or and not gates of two boolean variables A and B. A boolean variable can take only one of two possible values: True or False. Which of the two expressions are identical, meaning that for any combination of A...
44. ### Memory in logic gates for specified time intervals

I am a physics student new to the world of electronics and I have a question that may turn out to be very simple. What I am trying to design requires that a single output given some input be held in memory for a specified time despite any differing input being sent through during that time...
45. ### How Do BJTs Help Create Logic Gates in Circuits?

Hello all :smile: I am a mechanical engineering student taking an introductory circuits course. I find the things we are doing in lab quite interesting, but I really lack a good understanding of what we are doing. We are currently building logic gates using BJTs. I have some basic...
46. ### Quick questions about logic gates (Nand, nor, etc)

So recently just in my spare times I've found an interest in logic gates such as the nand gates and nor gates and such and although most of it's logic, there's one thing i just couldn't figure out. Let's say you have a nand gate configuration consisting of two pmos and two nmos transistors as...
47. ### Logic gates and boolean algebra

Hi I'm not a science or physics student - quite a layman. Therefore, please keep your replies simple and explanatory so that your guidance and help can be fully appreciated. Thanks. What are these logic gates - AND, NOR, NAND, etc? Are these some physical circuits? In Boolean algebra...
48. ### How Do Logic Gates and State Tables Interact in Digital Circuits?

Homework Statement I have been asked to create a logic diagram and state table and state diagram for two equations, with two inputs and one output. The inputs are, x and y. the output is z. Homework Equations The two equations are: D(sub a) = x'y + xA D(sub b) = xB + xA z = B x' =...
49. ### Designing Logic Gates for Multiple Inputs and Outputs in Homework Problems

Homework Statement ok there are two questions. one i was able to do so which is fairly easy, since it had only two variables and one output. Design a circuit that has two inputs (x and y) and one output (f) that functions in the following way: the function f is false (0) when x and y are...
50. ### Basic Questions over the function of Logic Gates

Hello, this is not homework. I am working on a personal project and had I think a fairly simple set of questions. They really are all the same, so answering one as true probably means the others are true as well, but if not please point out where and how I'm wrong. I have a very basic idea of...