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electronics The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, biasing is the setting of DC (
direct current Direct current (DC) is one-directional flow of electric charge. An electrochemical cell is a prime example of DC power. Direct current may flow through a conductor such as a wire, but can also flow through semiconductors, insulators, or even ...
) operating conditions (current and voltage) of an active device in an amplifier. Many electronic devices, such as
diode A diode is a two-terminal electronic component that conducts current primarily in one direction (asymmetric conductance); it has low (ideally zero) resistance in one direction, and high (ideally infinite) resistance in the other. A diode ...
s,
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s and
vacuum tube A vacuum tube, electron tube, valve (British usage), or tube (North America), is a device that controls electric current flow in a high vacuum between electrodes to which an electric voltage, potential difference has been applied. The type kn ...
s, whose function is
processing Processing is a free graphical library and integrated development environment (IDE) built for the electronic arts, new media art, and visual design communities with the purpose of teaching non-programmers the fundamentals of computer programming ...
time-varying ( AC)
signals In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The ''IEEE Transactions on Signal Processing'' ...
, also require a steady (DC) current or voltage at their terminals to operate correctly. This current or voltage is called ''bias''. The AC signal applied to them is superposed on this DC bias current or voltage. The
operating point {{unreferenced, date=April 2013 The operating point is a specific point within the operation characteristic of a technical device. This point will be engaged because of the properties of the system and the outside influences and parameters. In el ...
of a device, also known as bias point, quiescent point, or Q-point, is the DC voltage or current at a specified terminal of an active device (a transistor or vacuum tube) with no input signal applied. A bias circuit is a portion of the device's circuit which supplies this steady current or voltage.


Overview

In electronics, 'biasing' usually refers to a fixed DC voltage or current applied to a terminal of an
electronic component An electronic component is any basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields. Electronic components are mostly industrial products, available in a singular form and are not ...
such as a diode, transistor or vacuum tube in a circuit in which AC signals are also present, in order to establish proper operating conditions for the component. For example, a bias voltage is applied to a transistor in an
electronic amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a Signal (information theory), signal (a time-varying voltage or Electric current, current). It may increase the power (physics ...
to allow the transistor to operate in a particular region of its
transconductance Transconductance (for transfer conductance), also infrequently called mutual conductance, is the electrical characteristic relating the current through the output of a device to the voltage across the input of a device. Conductance is the reciproca ...
curve. For vacuum tubes, a
grid bias In electronics, biasing is the setting of DC (direct current) operating conditions (current and voltage) of an active device in an amplifier. Many electronic devices, such as diodes, transistors and vacuum tubes, whose function is processing ...
voltage is often applied to the grid electrodes for the same reason. In
magnetic tape recording An audio tape recorder, also known as a tape deck, tape player or tape machine or simply a tape recorder, is a sound recording and reproduction device that records and plays back sounds usually using magnetic tape for storage. In its present- ...
, the term ''bias'' is also used for a high-frequency signal added to the
audio signal An audio signal is a representation of sound, typically using either a changing level of electrical voltage for analog signals, or a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of r ...
and applied to the
recording head A recording head is the physical interface between a recording apparatus and a moving recording medium. Recording heads are generally classified according to the physical principle that allows them to impress their data upon their medium. A record ...
, to improve the quality of the recording on the tape. This is called
tape bias Tape bias is the term for two techniques, AC bias and DC bias, that improve the fidelity of analogue tape recorders. DC bias is the addition of direct current to the audio signal that is being recorded. AC bias is the addition of an inaudi ...
.


Importance in linear circuits

Linear circuits involving
transistor upright=1.4, gate (G), body (B), source (S) and drain (D) terminals. The gate is separated from the body by an insulating layer (pink). A transistor is a semiconductor device used to Electronic amplifier, amplify or electronic switch, switch e ...
s typically require specific DC voltages and currents for correct operation, which can be achieved using a biasing circuit. As an example of the need for careful biasing, consider a
transistor amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
. In linear
amplifier An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It may increase the power significantly, or its main effect may be to boost the v ...
s, a small input signal gives larger output signal without any change in shape (low distortion): the input signal causes the output signal to vary up and down about the Q-point in a manner strictly proportional to the input. However, because the relationship between input and output for a transistor is not linear across its full operating range, the transistor amplifier only approximates linear operation. For low
distortion In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
, the transistor must be biased so the output signal swing does not drive the transistor into a region of extremely nonlinear operation. For a bipolar junction transistor amplifier, this requirement means that the transistor must stay in the active mode, and avoid cut-off or saturation. The same requirement applies to a
MOSFET The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
amplifier, although the terminology differs a little: the MOSFET must stay in the active mode, and avoid cutoff or ohmic operation.


Bipolar junction transistors

For
bipolar junction transistor A bipolar junction transistor (BJT) is a type of transistor that uses both electrons and electron holes as charge carriers. In contrast, a unipolar transistor, such as a field-effect transistor, uses only one kind of charge carrier. A bipolar ...
s the bias point is chosen to keep the transistor operating in the ''active'' mode, using a variety of circuit techniques, establishing the Q-point DC voltage and current. A small signal is then applied on top of the bias. The Q-point is typically near the middle of the DC
load line Load line may refer to: * Ship's load line, related to ship construction * Load line (electronics) In graphical analysis of nonlinear electronic circuits, a load line is a line drawn on the characteristic curve, a graph of the current vs. the v ...
, so as to obtain the maximum available peak-to-peak signal amplitude without distortion due to
clipping Clipping may refer to: Words * Clipping (morphology), the formation of a new word by shortening it, e.g. "ad" from "advertisement" * Clipping (phonetics), shortening the articulation of a speech sound, usually a vowel * Clipping (publications) ...
as the transistor reaches saturation or cut-off. The process of obtaining an appropriate DC collector current at a certain DC collector voltage by setting up the operating point is called biasing.


Vacuum tubes (thermionic valves)

Grid bias is the DC voltage provided at the control grid of a vacuum tube relative to the cathode for the purpose of establishing the zero input signal or steady state operating condition of the tube.Landee, Davis, Albrecht
''Electronic Designers' Handbook''
New York: McGraw-Hill, 1957, p. 2-27.
* In a typical Class A voltage amplifier, and class A and AB1 power stages of
audio power amplifier An audio power amplifier (or power amp) is an electronic amplifier that amplifies low-power electronic audio signals, such as the signal from a radio receiver or an electric guitar pickup (music technology), pickup, to a level that is high e ...
s, the DC bias voltage is negative relative to the cathode potential. The instantaneous grid voltage (sum of DC bias and AC input signal) does not reach the point where grid current begins. *
Class B amplifier In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active ...
s using general-purpose tubes are biased negatively to the projected plate current cutoff point. Class B vacuum tube amplifiers are usually operated with grid current (class B2). The bias voltage source must have low resistance and be able to supply the grid current.Landee et al., 1957
p. 4-19
When tubes designed for class B are employed, the bias can be as little as zero. *
Class C amplifier In electronics, power amplifier classes are letter symbols applied to different power amplifier types. The class gives a broad indication of an amplifier's characteristics and performance. The classes are related to the time period that the active ...
s are biased negatively at a point well beyond plate current cutoff. Grid current occurs during significantly less than 180 degrees of the input frequency cycle. There are many methods of achieving grid bias. Combinations of bias methods may be used on the same tube. * ''Fixed bias'': The DC grid potential is determined by connection of the grid to an appropriate impedance that will pass DC from an appropriate voltage source. * ''
Cathode bias In electronics, cathode bias (also known as self-bias, or automatic bias) is a technique used with vacuum tubes to make the direct current (dc) cathode voltage positive in relation to the negative side of the plate voltage supply by an amount equal ...
'' (''self-bias'', ''automatic bias'') - The voltage drop across a resistor in series with the cathode is utilized. The grid circuit DC return is connected to the other end of the resistor, causing the DC grid voltage to be negative relative to the cathode. * ''Grid leak bias'': When the grid is driven positive during part of the input frequency cycle, such as in class C operation, rectification in the grid circuit in conjunction with capacitive coupling of the input signal to the grid produces negative DC voltage at the grid. A resistor (the ''grid leak'') permits discharge of the coupling capacitor and passes the DC grid current. The resultant bias voltage is equal to the product of the DC grid current and the grid leak resistance. * ''Bleeder bias'': The voltage drop across a portion of a resistance across the plate voltage supply determines the grid bias. The cathode is connected to a tap on the resistance. The grid is connected to an appropriate impedance that provides a DC path either to the negative side of the plate voltage supply or to another tap on the same resistance. * ''Initial velocity bias'' (''contact bias''): Initial velocity grid current is passed through a grid-to-cathode resistor, usually in the range of 1 to 10 megohms, making the grid potential around one volt negative relative to the cathode.Landee et al., 1957
p. 2-28
Initial velocity bias is used only for small input signal voltages.


Microphones

Electret microphone An electret microphone is a type of electrostatic capacitor-based microphone, which eliminates the need for a polarizing power supply by using a permanently charged material. An ''electret'' is a stable dielectric material with a permanently em ...
elements typically include a
junction field-effect transistor The junction-gate field-effect transistor (JFET) is one of the simplest types of field-effect transistor. JFETs are three-terminal semiconductor devices that can be used as electronically controlled switches or resistors, or to build amplifiers ...
as an impedance converter to drive other electronics within a few meters of the microphone. The operating current of this JFET is typically 0.1 to 0.5 mA and is often referred to as bias, which is different from the
phantom power Phantom power, in the context of professional audio equipment, is DC electric power transmitted through microphone cables to operate microphones that contain active electronic circuitry. It is best known as a convenient power source for con ...
interface which supplies 48 volts to operate the backplate of a traditional condenser microphone. Electret microphone bias is sometimes supplied on a separate conductor.


See also

* Idling current *
Small signal model Small-signal modeling is a common analysis technique in electronics engineering used to approximate the behavior of electronic circuits containing nonlinear devices with linear equations. It is applicable to electronic circuits in which the AC si ...


References


Further reading

* * * {{cite book , author-last1=Sedra , author-first1=Adel , author-last2=Smith , author-first2=Kenneth , title=Microelectronic Circuits , publisher=Oxford University Press , date=2004 , isbn=0-19-514251-9 Electronic engineering Vacuum tubes