# logic gate

The logic normally performed is Boolean logic and is most commonly found in digital circuits. Logic gates are primarily implemented electronically …

It lists the value of the output for every possible combination of the inputs and can be used to simplify the number of logic gates and level of nesting …

Unlike diode logic gates, RTL gates can be cascaded indefinitely to produce more complex logic functions. These gates were used in early integrated circuits.

For …

http://en.wikipedia.org/wiki/Logic_gate

**The AND gate **is a digital logic gate that implements logical conjunction – it behaves according to the truth table to the right. A HIGH output (1) results

only if both the inputs to the AND gate are HIGH (1). If neither or only one input to the AND gate is HIGH, a LOW output results. In …

If no specific AND gates are available, one can be made from NAND or NOR gates, because NAND and NOR gates are considered the “universal gates,” [1] meaning

that they …

http://en.wikipedia.org/wiki/AND_gate

**Logic Gates**

Logic gates process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V represents false. Other terms

which are used for the true …

Note that logic gates are not always required because simple logic functions can be performed with switches or diodes: …

A truth table is a good way to show the function of a logic gate. It shows the output states for every possible combination of input states. The …

NOT – AND – NAND – OR

http://www.kpsec.freeuk.com/gates.htm

**Basic Logic Gates**

Truth tables are used to help show the function of a logic gate. If you are unsure about truth tables and need guidence on how go about drawning them …

Digital systems are said to be constructed by using logic gates. These gates are the AND, OR, NOT, NAND, NOR, EXOR and EXNOR gates. The …

The diagrams below show two ways that the NAND logic gate can be configured to produce a NOT gate. It can also be done using NOR logic gates in the same way.

…

http://www.ee.surrey.ac.uk/Projects/Labview/gatesfunc/index.html

**What is logic gate? ** .

A logic gate is an elementary building block of a digital circuit. Most logic gates have two inputs and one output. At any given moment, every terminal is in

one of the two binary conditions low …

The logic state of a terminal can, and generally does, change often, as the circuit processes data. In most logic gates, the low state is approximately zero

volts (0 V), while the high state is approximately five volts positive (+5 V). …

http://whatis.techtarget.com/definition/0,,sid9_gci213512,00.html

**Introdution to Logic Gates**

This web site provides a brief description of logic gates and defines a few of the common logic gates found in simple digital circuits. …

isweb.redwoods.cc.ca.us/Instruct/…/Diglogic/Index.Htm

Logic Gates – Digital Logic Circuits

LOGIC GATES has been developed by. Kevin R. Sullivan. Professor of. Automotive Technology Skyline College San Bruno, California. Images by …

http://http://www.autoshop101.com/trainmodules/logicgate/101.html

**Logic Gates. 3.1 Introduction.** This chapter concentrates on the design of combinational logic functions. The knowl-. edge gained in the last chapter on

fabrication is important …

first family of logic gate circuits we will consider are static, fully complementary. gates, which are the mainstay of CMOS design. We …

Logic Gates. study the delays through wires, which can be much longer than the delays through the. gates. 3.2 Combinational Logic Functions …

http://ptgmedia.pearsoncmg.com/images/chap3…/chap3_0130619701.pdf

Boolean Algebra and Logic Gates

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Boolean Algebra and Logic Gates. F Hamer, M Lavelle & D McMullan. The aim of this document is to provide a short, …

Logic Gates (Introduction). The package Truth Tables and Boolean Algebra set out the basic. principles of logic. Any Boolean algebra operation can be

associated …

The basic rules for simplifying and combining logic gates are called. Boolean algebra in honour of George Boole (1815–1864) who was a …

http://www.tech.plym.ac.uk/maths/resources/PDFLaTeX/boolean_alg2.pdf

**Inside Logic Gates**

I have received a number of requests, asking just what goes on inside logic gates to actually perform logic functions. So, by popular demand, here are the

internal schematics of various gates, as implemented by several different logic families. …

There are several different families of logic gates. Each family has its capabilities and limitations, its advantages and disadvantages. The following list

describes the main logic families and their …

http://www.play-hookey.com/digital/electronics/

While the binary numeration system is an interesting mathematical abstraction, we haven’t yet seen its practical application to electronics. This chapter is devoted to just that: practically applying the concept of binary bits to circuits. What makes binary numeration so important to the application of digital electronics is the ease in which bits may be represented in physical terms. Because a binary bit can only have one of two different values, either 0 or 1, any physical medium capable of switching between two saturated states may be used to represent a bit. Consequently, any physical system capable of representing binary bits is able to represent numerical quantities, and potentially has the ability to manipulate those numbers. This is the basic concept underlying digital computing.

Electronic circuits are physical systems that lend themselves well to the representation of binary numbers. Transistors, when operated at their bias limits, may be in one of two different states: either cutoff (no controlled current) or saturation (maximum controlled current). If a transistor circuit is designed to maximize the probability of falling into either one of these states (and not operating in the linear, or *active*, mode), it can serve as a physical representation of a binary bit. A voltage signal measured at the output of such a circuit may also serve as a representation of a single bit, a low voltage representing a binary “0” and a (relatively) high voltage representing a binary “1.” Note the following transistor circuit:

In this circuit, the transistor is in a state of saturation by virtue of the applied input voltage (5 volts) through the two-position switch. Because its saturated, the transistor drops very little voltage between collector and emitter, resulting in an output voltage of (practically) 0 volts. If we were using this circuit to represent binary bits, we would say that the input signal is a binary “1” and that the output signal is a binary “0.” Any voltage close to full supply voltage (measured in reference to ground, of course) is considered a “1” and a lack of voltage is considered a “0.” Alternative terms for these voltage levels are *high* (same as a binary “1”) and *low* (same as a binary “0”). A general term for the representation of a binary bit by a circuit voltage is *logic level*.

Moving the switch to the other position, we apply a binary “0” to the input and receive a binary “1” at the output:

What we’ve created here with a single transistor is a circuit generally known as a *logic gate*, or simply *gate*. A gate is a special type of amplifier circuit designed to accept and generate voltage signals corresponding to binary 1’s and 0’s. As such, gates are not intended to be used for amplifying analog signals (voltage signals *between* 0 and full voltage). Used together, multiple gates may be applied to the task of binary number storage (memory circuits) or manipulation (computing circuits), each gate’s output representing one bit of a multi-bit binary number. Just how this is done is a subject for a later chapter. Right now it is important to focus on the operation of individual gates.

The gate shown here with the single transistor is known as an *inverter*, or NOT gate, because it outputs the exact opposite digital signal as what is input. For convenience, gate circuits are generally represented by their own symbols rather than by their constituent transistors and resistors. The following is the symbol for an inverter:

An alternative symbol for an inverter is shown here:

Notice th triangular shape of the gate symbol, much like that of an operational amplifier. As was stated before, gate circuits actually are amplifiers. The small circle, or “bubble” shown on either the input or output terminal is standard for representing the inversion function. As you might suspect, if we were to remove the bubble from the gate symbol, leaving only a triangle, the resulting symbol would no longer indicate inversion, but merely direct amplification. Such a symbol and such a gate actually do exist, and it is called a *buffer*, the subject of the next section.

Like an operational amplifier symbol, input and output connections are shown as single wires, the implied reference point for each voltage signal being “ground.” In digital gate circuits, ground is almost always the negative connection of a single voltage source (power supply). Dual, or “split,” power supplies are seldom used in gate circuitry. Because gate circuits are amplifiers, they require a source of power to operate. Like operational amplifiers, the power supply connections for digital gates are often omitted from the symbol for simplicity’s sake. If we were to show *all* the necessary connections needed for operating this gate, the schematic would look something like this:

Power supply conductors are rarely shown in gate circuit schematics, even if the power supply connections at each gate are. Minimizing lines in our schematic, we get this:

“V_{cc}” stands for the constant voltage supplied to the collector of a bipolar junction transistor circuit, in reference to ground. Those points in a gate circuit marked by the label “V_{cc}” are all connected to the same point, and that point is the positive terminal of a DC voltage source, usually 5 volts.

As we will see in other sections of this chapter, there are quite a few different types of logic gates, most of which have multiple input terminals for accepting more than one signal. The output of any gate is dependent on the state of its input(s) and its logical function.

One common way to express the particular function of a gate circuit is called a *truth table*. Truth tables show all combinations of input conditions in terms of logic level states (either “high” or “low,” “1” or “0,” for each input terminal of the gate), along with the corresponding output logic level, either “high” or “low.” For the inverter, or NOT, circuit just illustrated, the truth table is very simple indeed:

Truth tables for more complex gates are, of course, larger than the one shown for the NOT gate. A gate’s truth table must have as many rows as there are possibilities for unique input combinations. For a single-input gate like the NOT gate, there are only two possibilities, 0 and 1. For a two input gate, there are *four* possibilities (00, 01, 10, and 11), and thus four rows to the corresponding truth table. For a three-input gate, there are *eight* possibilities (000, 001, 010, 011, 100, 101, 110, and 111), and thus a truth table with eight rows are needed. The mathematically inclined will realize that the number of truth table rows needed for a gate is equal to 2 raised to the power of the number of input terminals.

**REVIEW:**- In digital circuits, binary bit values of 0 and 1 are represented by voltage signals measured in reference to a common circuit point called
*ground*. An absence of voltage represents a binary “0” and the presence of full DC supply voltage represents a binary “1.” - A
*logic gate*, or simply*gate*, is a special form of amplifier circuit designed to input and output*logic level*voltages (voltages intended to represent binary bits). Gate circuits are most commonly represented in a schematic by their own unique symbols rather than by their constituent transistors and resistors. - Just as with operational amplifiers, th power supply connections to gates are often omitted in schematic diagrams for the sake of simplicity.
- A
*truth table*is a standard way of representing the input/output relationships of a gate circuit, listing all the possible input logic level combinations with their respective output logic levels.

COMMENTsongsA logic gate is an elementary building block of a digital circuit. Most logic gates have two inputs and one output. NAND and NOR logic gates are the two pillars of logic, in that all other types of Boolean logic gates (i.e., AND, OR, NOT, XOR, XNOR) can be created from a suitable network of just NAND or just NOR gate(s).

The simplest form of electronic logic is diode logic. This allows AND and OR gates to be built, but not inverters, and so is an incomplete form of logic. Further, without some kind of amplification it is not possible to have such basic logic operations cascaded as required for more complex logic functions. To build a complete logic system, valves (vacuum tubes) or transistors can be used. The simplest family of logic gates using bipolar transistors is called resistor-transistor logic, or RTL. Unlike diode logic gates, RTL gates can be cascaded indefinitely to produce more complex logic functions. These gates were used in early integrated circuits. For higher speed, the resistors used in RTL were replaced by diodes, leading to diode-transistor logic, or DTL. It was then discovered that one transistor could do the job of two diodes in the space of one diode even better, by more quickly switching off the following stage, so transistor-transistor logic, or TTL, was created. In virtually every type of contemporary chip implementation of digital systems, the bipolar transistors have been replaced by complementary field-effect transistors (MOSFETs) to reduce size and power consumption still further, thereby resulting in complementary metal-oxide-semiconductor (CMOS) logic.

gates process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V represents false.

Other terms which are used for the true and false states are shown in the table on the right. It is best to be familiar with them all.

Logic Gates are identified by their function: NOT, AND, NAND, OR, NOR, EX-OR and EX-NOR. Capital letters are normally used to make it clear that the term refers to a logic gate.

Note that logic gates are not always required because simple logic functions can be performed with switches or diodes:

* Switches in series (AND function)

* Switches in parallel (OR function)

* Combining IC outputs with diodes (OR function)

Logic gate symbols

There are two series of symbols for logic gates:

* The traditional symbols have distinctive shapes making them easy to recognise so they are widely used in industry and education.

The IEC (International Electrotechnical Commission) symbols are rectangles with a symbol inside to show the gate function. They are rarely used despite their official status, but you may need to know them for an examination.

Inputs and outputs:

Gates have two or more inputs, except a NOT gate which has only one input. All gates have only one output. Usually the letters A, B, C and so on are used to label inputs, and Q is used to label the output. On this page the inputs are shown on the left and the output on the right.

The inverting circle (o):

Some gate symbols have a circle on their output which means that their function includes inverting of the output. It is equivalent to feeding the output through a NOT gate. For example the NAND (Not AND) gate symbol shown on the right is the same as an AND gate symbol but with the addition of an inverting circle on the output.

Truth tables:

Input A Input B Output Q

0 0 0

0 1 0

1 0 0

1 1 1

A truth table is a good way to show the function of a logic gate. It shows the output states for every possible combination of input states. The symbols 0 (false) and 1 (true) are usually used in truth tables.

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