Biasing (electronics)
Encyclopedia
Biasing in electronics
is the method of establishing predetermined voltage
s and/or currents
at various points of an electronic circuit
to set an appropriate operating point. The operating point of a device, also known as bias point, quiescent point, or simply Q-point, is the point on the output characteristics that shows the direct current
(DC), collector-emitter voltage (VCE), and the collector current (IC) with no input signal applied. The term is normally used in connection with devices such as transistor
s.
, the term bias has the following meanings:
Most often, bias simply refers to a fixed DC voltage applied to the same point in a circuit
as an alternating current
(AC) signal, frequently to select the desired operating response of a semiconductor or other electronic component (forward or reverse bias). For example, a bias voltage
is applied to a transistor
in an electronic amplifier
to allow the transistor to operate in a particular region of its transconductance
curve. For vacuum tube
s, a (much higher) grid bias
voltage
is also often applied to the grid electrodes for precisely the same reason.
A hot bias can lower the tube life span, but a "cool" bias can induce crossover distortion
.
Bias is also the term used for a high-frequency signal added to the audio signal recorded on magnetic tape
. See tape bias
.
Bias is used in direct broadcast satellite
s such as DirecTV
and Dish Network
, the integrated receiver/decoder
(IRD) box actually powers the feedhorn or low-noise block converter
(LNB) receiver
mounted on the dish arm. This bias is changed from a lower voltage to a higher voltage to select the polarization of the LNB, so that it receives signals that are polarized either clockwise
or counterclockwise, thereby allowing it to receive twice as many channels
.
s typically require specific DC voltages and currents to operate correctly, which can be achieved using a biasing circuit. As an example of the need for careful biasing, consider a transistor amplifier. In linear amplifier
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 a transistor is nonlinear, the transistor amplifier only approximates linear operation. For low distortion
, 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 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
amplifier, although the terminology differs a little: the MOSFET must stay in the active mode (or saturation mode), and avoid cut-off or ohmic operation (or triode mode).
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 Q-point bias voltage, thereby either modulating
or switching the current, depending on the purpose of the circuit.
The quiescent point of operation is typically near the middle of DC load line
. The process of obtaining certain DC collector current at a certain DC collector voltage by setting up operating point is called biasing.
After establishing the operating point, when input signal is applied, the output signal should not move the transistor either to saturation or to cut-off. However, this unwanted shift still might occur, due to the following reasons:
To avoid a shift of Q-point, bias-stabilization is necessary. Various biasing circuits
can be used for this purpose.
elements typically include a junction field-effect transistor
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
interface which provides current to drive regular microphone cables over longer distances. Electret microphone bias is sometimes supplied on a separate conductor.
Electronics
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...
is the method of establishing predetermined voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
s and/or currents
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
at various points of an electronic circuit
Electronic circuit
An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow...
to set an appropriate operating point. The operating point of a device, also known as bias point, quiescent point, or simply Q-point, is the point on the output characteristics that shows the direct current
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
(DC), collector-emitter voltage (VCE), and the collector current (IC) with no input signal applied. The term is normally used in connection with devices such as transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s.
Overview
In electrical engineeringElectrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
, the term bias has the following meanings:
- A systematic deviation of a value from a reference value.
- The amount by which the average of a set of values departs from a reference value.
- Electrical, mechanical, magnetic, or other force (fieldVector fieldIn vector calculus, a vector field is an assignmentof a vector to each point in a subset of Euclidean space. A vector field in the plane for instance can be visualized as an arrow, with a given magnitude and direction, attached to each point in the plane...
) applied to a device to establish a reference level to operate the device. - In telegraph signaling systems, the development of a positive or negative DCDirect currentDirect current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
voltageVoltageVoltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
at a point on a lineTelephone lineA telephone line or telephone circuit is a single-user circuit on a telephone communication system...
that should remain at a specified reference level, such as zero.
- Note: A bias may be applied or produced by (i) the electrical characteristics of the line, (ii) the terminal equipmentTerminal equipmentIn telecommunication, the term terminal equipment has the following meanings:* Communications equipment at either end of a communications link, used to permit the stations involved to accomplish the mission for which the link was established....
, and (iii) the signaling scheme.
Most often, bias simply refers to a fixed DC voltage applied to the same point in a circuit
Electrical network
An electrical network is an interconnection of electrical elements such as resistors, inductors, capacitors, transmission lines, voltage sources, current sources and switches. An electrical circuit is a special type of network, one that has a closed loop giving a return path for the current...
as an alternating current
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
(AC) signal, frequently to select the desired operating response of a semiconductor or other electronic component (forward or reverse bias). For example, a bias voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
is applied to a transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
in an electronic amplifier
Electronic amplifier
An electronic amplifier is a device for increasing the power of a signal.It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude...
to allow the transistor to operate in a particular region of its transconductance
Transconductance
Transconductance, also known as mutual conductance, is a property of certain electronic components. Conductance is the reciprocal of resistance; transconductance, meanwhile, is the ratio of the current change at the output port to the voltage change at the input port. It is written as gm...
curve. For vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
s, a (much higher) grid bias
Grid bias
Grid bias is a DC voltage applied to electron tubes with three electrodes or more, such as triodes. The control grid of these devices is used to control the electron flow from the heated cathode to the positively charged anode...
voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
is also often applied to the grid electrodes for precisely the same reason.
A hot bias can lower the tube life span, but a "cool" bias can induce crossover distortion
Crossover distortion
Crossover distortion is a type of distortion which is caused by switching between devices driving a load, most often when the devices are matched...
.
Bias is also the term used for a high-frequency signal added to the audio signal recorded on magnetic tape
Magnetic tape
Magnetic tape is a medium for magnetic recording, made of a thin magnetizable coating on a long, narrow strip of plastic. It was developed in Germany, based on magnetic wire recording. Devices that record and play back audio and video using magnetic tape are tape recorders and video tape recorders...
. See tape bias
Tape bias
Tape bias is the term for two phenomena, DC bias and AC bias, that improve the fidelity of analogue magnetic tape sound recordings. DC bias is the addition of a direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal to the audio...
.
Bias is used in direct broadcast satellite
Direct broadcast satellite
Direct broadcast satellite is a term used to refer to satellite television broadcasts intended for home reception.A designation broader than DBS would be direct-to-home signals, or DTH. This has initially distinguished the transmissions directly intended for home viewers from cable television...
s such as DirecTV
DirecTV
DirecTV is an American direct broadcast satellite service provider and broadcaster based in El Segundo, California. Its satellite service, launched on June 17, 1994, transmits digital satellite television and audio to households in the United States, Latin America, and the Anglophone Caribbean. ...
and Dish Network
Dish Network
Dish Network Corporation is the second largest pay TV provider in the United States, providing direct broadcast satellite service—including satellite television, audio programming, and interactive television services—to 14.337 million commercial and residential customers in the United States. Dish...
, the integrated receiver/decoder
Integrated receiver/decoder
An integrated receiver/decoder is an electronic device used to pick-up a radio-frequency signal and convert digital information transmitted in it.-Consumer IRDs:...
(IRD) box actually powers the feedhorn or low-noise block converter
Low-noise block converter
A low-noise block downconverter is the receiving device of a parabolic satellite dish antenna of the type commonly used for satellite TV reception...
(LNB) receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
mounted on the dish arm. This bias is changed from a lower voltage to a higher voltage to select the polarization of the LNB, so that it receives signals that are polarized either clockwise
Clockwise
Circular motion can occur in two possible directions. A clockwise motion is one that proceeds in the same direction as a clock's hands: from the top to the right, then down and then to the left, and back to the top...
or counterclockwise, thereby allowing it to receive twice as many channels
Channel (communications)
In telecommunications and computer networking, a communication channel, or channel, refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel...
.
Importance in linear circuits
Linear circuits involving transistorTransistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s typically require specific DC voltages and currents to operate correctly, which can be achieved using a biasing circuit. As an example of the need for careful biasing, consider a transistor amplifier. In linear amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
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 a transistor is nonlinear, the transistor amplifier only approximates linear operation. For low distortion
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
, 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 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
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
amplifier, although the terminology differs a little: the MOSFET must stay in the active mode (or saturation mode), and avoid cut-off or ohmic operation (or triode mode).
Bipolar junction transistors
For bipolar junction transistorBipolar junction transistor
|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...
s the bias point is chosen to keep the transistor operating in the active mode, using a variety of circuit techniques
Bipolar transistor biasing
Bipolar transistor amplifiers must be properly biased to operate correctly. In circuits made with individual devices , biasing networks consisting of resistors are commonly employed...
, establishing the Q-point DC voltage and current. A small signal is then applied on top of the Q-point bias voltage, thereby either modulating
Modulation
In electronics and telecommunications, modulation is the process of varying one or more properties of a high-frequency periodic waveform, called the carrier signal, with a modulating signal which typically contains information to be transmitted...
or switching the current, depending on the purpose of the circuit.
The quiescent point of operation is typically near the middle of DC load line
Load line (electronics)
A load line is used in graphic analysis of circuits, representing the constraint other parts of the circuit place on a non-linear device, like a diode or transistor. A load line represents the response of a linear circuit connected to the nonlinear device in question...
. The process of obtaining certain DC collector current at a certain DC collector voltage by setting up operating point is called biasing.
After establishing the operating point, when input signal is applied, the output signal should not move the transistor either to saturation or to cut-off. However, this unwanted shift still might occur, due to the following reasons:
- Parameters of transistors depend on junction temperature. As junction temperature increases, leakage current due to minority charge carriers (ICBO) increases. As ICBO increases, ICEO also increases, causing an increase in collector current IC. This produces heat at the collector junction. This process repeats, and, finally, Q-point may shift into the saturation region. Sometimes, the excess heat produced at the junction may even burn the transistor. This is known as thermal runawayThermal runawayThermal runaway refers to a situation where an increase in temperature changes the conditions in a way that causes a further increase in temperature, often leading to a destructive result...
. - When a transistor is replaced by another of the same type, the Q-point may shift, due to changes in parameters of the transistor, such as current gain (
) which varies slightly for each unique transistor.
To avoid a shift of Q-point, bias-stabilization is necessary. Various biasing circuits
Bipolar transistor biasing
Bipolar transistor amplifiers must be properly biased to operate correctly. In circuits made with individual devices , biasing networks consisting of resistors are commonly employed...
can be used for this purpose.
Microphones
Electret microphoneElectret microphone
An electret microphone is a type of condenser microphone, which eliminates the need for a polarizing power supply by using a permanently charged material....
elements typically include a junction field-effect transistor
JFET
The junction gate field-effect transistor is the simplest type of field-effect transistor. It can be used as an electronically-controlled switch or as a voltage-controlled resistance. Electric charge flows through a semiconducting channel between "source" and "drain" terminals...
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
Phantom power, in the context of professional audio equipment, is a method for transmitting DC electric power through microphone cables to operate microphones that contain active electronic circuitry....
interface which provides current to drive regular microphone cables over longer distances. Electret microphone bias is sometimes supplied on a separate conductor.
See also
- Bipolar junction transistorBipolar junction transistor|- align = "center"| || PNP|- align = "center"| || NPNA bipolar transistor is a three-terminal electronic device constructed of doped semiconductor material and may be used in amplifying or switching applications. Bipolar transistors are so named because their operation involves both electrons...
- Bipolar transistor biasingBipolar transistor biasingBipolar transistor amplifiers must be properly biased to operate correctly. In circuits made with individual devices , biasing networks consisting of resistors are commonly employed...
- MOSFETMOSFETThe metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...
- Idling current
- Small signal modelSmall signal modelSmall-signal modeling is a common analysis technique in electrical engineering which is used to approximate the behavior of nonlinear devices with linear equations...
External links
- Bias - from Sci-Tech Encyclopedia