# Cut-off (electronics)

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{{Short description|State of negligible conduction when a control condition is met}}
{{Other uses|Cutoff voltage}}
{{mi|
{{Unreferenced|date=October 2015}}
{{notability|date=March 2026}}
}}
In electronics, '''cut-off''' is a state of negligible [conduction](/source/electrical_conduction) that is a property of several types of [electronic component](/source/electronic_component)s when a control parameter (that usually is a well-defined [voltage](/source/voltage) or [electric current](/source/electric_current), but could also be an incident [light intensity](/source/Intensity_(physics)) or a [magnetic field](/source/magnetic_field)), is lowered or increased past a value (the conduction threshold). The transition from [normal conduction](/source/electrical_conduction) to cut-off can be more or less sharp, depending on the type of [device](/source/Electronic_component) considered, and also the [speed](/source/speed) of this transition varies considerably.

==Cutoff values==
===Diodes===
* [Copper oxide diode](/source/Copper_oxide_diode): Usually between germanium and silicon diodes (0.2-0.5V)
* [Diac](/source/Diac): Depends on configuration.
* [Germanium diode](/source/Germanium_diode):apx 0.3 V; varying with temperature.
* [Schottky diode](/source/Schottky_diode):0.10–0.45, varying with temperature.
* [Selenium diode](/source/Selenium_diode):Depends on age and current. Usually higher than silicon diodes.
* [Silicon diode](/source/Silicon_diode): cutoff occurs when Vf falls below apx 0.7 V. The exact voltage varies with temperature.
* [Thermionic diode](/source/Thermionic_diode): cutoff voltage depends on device design. Much higher than for silicon devices.
* [Zener diode](/source/Zener_diode): reverse cutoff defined by diode voltage rating. Forward cutoff apx 0.6 V.

===Transistors===
* [BJT](/source/BJT): Depends on the configuration.
* [Germanium transistor](/source/Germanium_transistor): apx 0.2 V, varying with temperature.
* [MOSFET](/source/MOSFET): Depends on the configuration.
* [Silicon transistor](/source/Silicon_transistor): apx 0.6 V, varying with temperature.
* [TRIAC](/source/TRIAC): Also depends on the configuration.

==={{anchor|Tubes}}Valves===
* [Triodes](/source/Triodes): triodes cut off when applied grid bias is too low. This will be a negative voltage under ordinary conditions.
* [Tetrode](/source/Tetrode), [pentode](/source/pentode) etc.: There is some degree of interaction between the grids, and values will vary from one device to another. Anode voltage also affects cutoff voltage.
Prolonged periods in cut-off leads to [''cathode poisoning''](/source/Hot_cathode).

====Remote cutoff====
A [vacuum tube](/source/vacuum_tube) (such as a [pentode](/source/pentode), but also sometimes [triode](/source/triode)s, [hexodes, heptodes and so on](/source/pentagrid_converter)) with its [control grid](/source/control_grid) given a [helix with a variable pitch](/source/variable_mu) can be made to operate with more negative grid voltages, with reduced [amplification](/source/Amplifier), before it is completely cut off (i.e. yielding no significant output). This ability to vary the amplification (sometimes called mu) and also the [transconductance](/source/transconductance), is useful in [Automatic Gain Control](/source/Automatic_Gain_Control) (AGC) stages of [radio receivers](/source/radio_receivers).  Devices with this characteristic are called '''remote-cutoff''' or '''variable-mu''' or '''super-control''' types.

====Sharp cutoff====
With a normal [control grid](/source/control_grid) arrangement, a [vacuum tube](/source/vacuum_tube) will have close to a [square-law](/source/Power-law) relationship between input (grid) voltage and output (anode/plate) current, with the latter falling sharply to roughly zero. This characteristic is normally required for linear [RF](/source/RF) and audio uses. Examples; [EF86](/source/EF86) and [6AK5](/source/6AK5).

====Semi-remote cutoff====
A '''semi-remote cutoff''' device has characteristics somewhere between a '''remote-cutoff''' device and a '''sharp-cutoff''' one.

== See also ==
* [Diode](/source/Diode)
* [Electrical conduction](/source/Electrical_conduction)
* [Electronic component](/source/Electronic_component)
* [Field effect transistor](/source/Field_effect_transistor) in [JFET](/source/JFET) and [MOSFET](/source/MOSFET) form
* [Transistor](/source/Transistor)
* [Vacuum tube](/source/Vacuum_tube)

== References ==
<references/>

==External links==
* [https://web.archive.org/web/20071214125242/http://www.tpub.com/content/neets/14178/css/14178_75.htm Explanation of sharp-cutoff control grids in vacuum tubes.]
* [https://web.archive.org/web/20080919000814/http://www.tpub.com/content/neets/14178/css/14178_76.htm Explanation of remote-cutoff control grids in vacuum tubes.]

Category:Electrical parameters

{{Electronics-stub}}

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Adapted from the Wikipedia article [Cut-off (electronics)](https://en.wikipedia.org/wiki/Cut-off_(electronics)) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Cut-off_(electronics)?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.