Editing Logic gate (section)

=== Electronic gates ===
A [[functionally complete]] logic system may be composed of [[relay]]s, [[thermionic valve|valves]] (vacuum tubes), or [[transistor]]s.

Electronic logic gates differ significantly from their relay-and-switch equivalents. They are much faster, consume much less power, and are much smaller (all by a factor of a million or more in most cases). Also, there is a fundamental structural difference. The switch circuit creates a continuous metallic path for current to flow (in either direction) between its input and its output. The semiconductor logic gate, on the other hand, acts as a high-[[Gain (electronics)|gain]] [[voltage]] [[amplifier]], which sinks a tiny current at its input and produces a low-impedance voltage at its output. It is not possible for current to flow between the output and the input of a semiconductor logic gate.

[[File:TexasInstruments 7400 chip, view and element placement.jpg|thumb|180px|The 7400 chip, containing four NANDs. The two additional pins supply power (+5 V) and connect the ground.]]
For small-scale logic, designers now use prefabricated logic gates from families of devices such as the [[Transistor–transistor logic|TTL]] [[7400 series]] by [[Texas Instruments]], the [[CMOS]] [[4000 series]] by [[RCA]], and their more recent descendants. Increasingly, these fixed-function logic gates are being replaced by [[programmable logic device]]s, which allow designers to pack many mixed logic gates into a single integrated circuit. The field-programmable nature of [[programmable logic device]]s such as [[FPGA]]s has reduced the 'hard' property of hardware; it is now possible to change the logic design of a hardware system by reprogramming some of its components, thus allowing the features or function of a hardware implementation of a logic system to be changed.

An important advantage of standardized integrated circuit logic families, such as the 7400 and 4000 families, is that they can be cascaded. This means that the output of one gate can be wired to the inputs of one or several other gates, and so on. Systems with varying degrees of complexity can be built without great concern of the designer for the internal workings of the gates, provided the limitations of each integrated circuit are considered.

The output of one gate can only drive a finite number of inputs to other gates, a number called the '[[fan-out]] limit'. Also, there is always a delay, called the '[[propagation delay]]', from a change in input of a gate to the corresponding change in its output. When gates are cascaded, the total propagation delay is approximately the sum of the individual delays, an effect which can become a problem in high-speed [[synchronous circuit]]s. Additional delay can be caused when many inputs are connected to an output, due to the distributed [[capacitance]] of all the inputs and wiring and the finite amount of current that each output can provide.

==== Logic families ====
{{Main| Logic family}}
There are several [[logic families]] with different characteristics (power consumption, speed, cost, size) such as: [[diode logic|RDL]] (resistor–diode logic), [[resistor–transistor logic|RTL]] (resistor-transistor logic), [[DTL]] (diode–transistor logic), [[transistor–transistor logic|TTL]] (transistor–transistor logic) and CMOS. There are also sub-variants, e.g. standard CMOS logic vs. advanced types using still CMOS technology, but with some optimizations for avoiding loss of speed due to slower PMOS transistors.

The simplest family of logic gates uses [[bipolar transistors]], and is called [[resistor–transistor logic]] (RTL). Unlike simple diode logic gates (which do not have a gain element), RTL gates can be cascaded indefinitely to produce more complex logic functions. RTL gates were used in early [[integrated circuit]]s. For higher speed and better density, the resistors used in RTL were replaced by diodes resulting in [[diode–transistor logic]] (DTL). [[Transistor–transistor logic]] (TTL) then supplanted DTL.

[[File:CMOS inverter.svg|thumb|125px|[[CMOS]] diagram of a [[NOT gate]], also known as an inverter. [[MOSFET]]s are the most common way to make logic gates.]]
As integrated circuits became more complex, bipolar transistors were replaced with smaller [[field-effect transistor]]s ([[MOSFET]]s); see [[PMOS logic|PMOS]] and [[NMOS logic|NMOS]]. To reduce power consumption still further, most contemporary chip implementations of digital systems now use [[CMOS]] logic.  CMOS uses complementary (both n-channel and p-channel) MOSFET devices to achieve a high speed with low power dissipation.

Other types of logic gates include, but are not limited to:<ref>{{cite news |author-last=Rowe |author-first=Jim |title=Circuit Logic – Why and How |agency=Electronics Australia |issue=December 1966}}</ref>

{| class="wikitable"
|+ 
! Logic family !! Abbreviation !! Description
|-
| [[Diode logic]]|| DL || 
|-
| Tunnel diode logic || TDL || Exactly the same as diode logic but can perform at a higher speed.{{failed verification|reason=Tunnel diodes have gain and state|date=December 2017}}
|-
| Neon logic || NL || Uses neon bulbs or 3-element neon trigger tubes to perform logic.
|-
| Core diode logic || CDL || Performed by semiconductor diodes and small ferrite toroidal cores for moderate speed and moderate power level.
|-
| 4Layer Device Logic || 4LDL || Uses thyristors and SCRs to perform logic operations where high current and or high voltages are required.
|-
| [[Direct-coupled transistor logic]] || DCTL || Uses transistors switching between saturated and cutoff states to perform logic. The transistors require carefully controlled parameters. Economical because few other components are needed, but tends to be susceptible to noise because of the lower voltage levels employed. Often considered to be the father to modern TTL logic.
|-
| [[Metal–oxide–semiconductor]] logic || MOS || Uses [[MOSFET]]s (metal–oxide–semiconductor field-effect transistors), the basis for most modern logic gates. The MOS logic family includes [[PMOS logic]], [[NMOS logic]], [[complementary MOS]] (CMOS), and [[BiCMOS]] (bipolar CMOS).
|-
| [[Current-mode logic]] || CML || Uses transistors to perform logic but biasing is from constant current sources to prevent saturation and allow extremely fast switching. Has high noise immunity despite fairly low logic levels.
|-
| [[Quantum dot cellular automaton|Quantum-dot cellular automata]]
| QCA
| Uses tunnelable q-bits for synthesizing the binary logic bits. The electrostatic repulsive force in between two electrons in the quantum dots assigns the electron configurations (that defines state 1 or state 0) under the suitably driven polarizations. This is a transistorless, currentless, junctionless binary logic synthesis technique allowing it to have very fast operation speeds.
|-
| Ferroelectric FET || FeFET || FeFET transistors can retain their state to speed recovery in case of a power loss.<ref>{{cite web | url=https://semiengineering.com/tapping-into-non-volatile-logic/ | title=Tapping into Non-Volatile Logic | date=21 April 2021 }}</ref>
|}

==== Three-state logic gates ====
[[File:Tristate buffer.svg|thumb|320px|right|A three-state buffer can be thought of as a switch. If ''B'' is on, the switch is closed. If B is off, the switch is open.]]
{{Main|Three-state logic}}

A three-state logic gate is a type of logic gate that can have three different outputs: high (H), low (L) and high-impedance (Z). The high-impedance state plays no role in the logic, which is strictly binary. These devices are used on [[Bus (computing)|buses]] of the [[CPU]] to allow multiple chips to send data. A group of three-state outputs driving a line with a suitable control circuit is basically equivalent to a [[multiplexer]], which may be physically distributed over separate devices or plug-in cards.

In electronics, a high output would mean the output is sourcing current from the positive power terminal (positive voltage). A low output would mean the output is sinking current to the negative power terminal (zero voltage). High impedance would mean that the output is effectively disconnected from the circuit.