Biasing

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Biasing in electronics means establishing predetermined voltages or currents at various points of an electronic circuit for the purpose of establishing proper operating conditions in electronic components. Many electronic devices such as diodes, transistors and vacuum tubes, whose function is processing time-varying (AC) signals also require a steady (DC) current or voltage to operate correctly — a bias. The AC signal applied to them is superposed on this DC bias current or voltage. The operating point 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.

Electronics physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter

Electronics comprises the physics, engineering, technology and applications that deal with the emission, flow and control of electrons in vacuum and matter. The identification of the electron in 1897, along with the invention of the vacuum tube, which could amplify and rectify small electrical signals, inaugurated the field of electronics and the electron age.

Voltage difference in the electric potential between two points in space

Voltage, electric potential difference, electric pressure or electric tension is the difference in electric potential between two points. The difference in electric potential between two points in a static electric field is defined as the work needed per unit of charge to move a test charge between the two points. In the International System of Units, the derived unit for voltage is named volt. In SI units, work per unit charge is expressed as joules per coulomb, where 1 volt = 1 joule per 1 coulomb. The official SI definition for volt uses power and current, where 1 volt = 1 watt per 1 ampere. This definition is equivalent to the more commonly used 'joules per coulomb'. Voltage or electric potential difference is denoted symbolically by V, but more often simply as V, for instance in the context of Ohm's or Kirchhoff's circuit laws.

Electric current flow of electric charge

An electric current is a flow of electric charge. In electric circuits this charge is often carried by electrons moving through a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in an ionized gas (plasma).

Contents

Overview

In electronics, bias usually refers to a fixed DC voltage or current applied to a terminal of an electronic component such as a diode, transistor or vacuum tube in a circuit in which alternating current (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 to allow the transistor to operate in a particular region of its transconductance curve. For vacuum tubes, a grid bias voltage is often applied to the grid electrodes for the same reason.

Electronic component basic discrete device or physical entity in an electronic system used to affect electrons or their associated fields

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 to be confused with electrical elements, which are conceptual abstractions representing idealized electronic components.

Diode electronic component

A diode is a two-terminal electronic component that conducts current primarily in one direction ; it has low resistance in one direction, and high resistance in the other. A diode vacuum tube or thermionic diode is a vacuum tube with two electrodes, a heated cathode and a plate, in which electrons can flow in only one direction, from cathode to plate. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material with a p–n junction connected to two electrical terminals. Semiconductor diodes were the first semiconductor electronic devices. The discovery of asymmetric electrical conduction across the contact between a crystalline mineral and a metal was made by German physicist Ferdinand Braun in 1874. Today, most diodes are made of silicon, but other materials such as gallium arsenide and germanium are used.

Transistor semiconductor device used to amplify and switch electronic signals and electrical power

A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power. It is composed of semiconductor material usually with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals controls the current through another pair of terminals. Because the controlled (output) power can be higher than the controlling (input) power, a transistor can amplify a signal. Today, some transistors are packaged individually, but many more are found embedded in integrated circuits.

In electronic engineering, the term bias has the following meanings:[ citation needed ]

Electronic engineering electrical engineering discipline which utilizes nonlinear and active electrical components to design electronic circuits, devices, and their systems

Electronic engineering is an electrical engineering discipline which utilizes nonlinear and active electrical components to design electronic circuits, devices, VLSI devices and their systems. The discipline typically also designs passive electrical components, usually based on printed circuit boards.

  1. A systematic deviation of a value from a reference value
  2. The amount by which the average of a set of values departs from a reference value
  3. Electrical, mechanical, magnetic, or other force applied to a device to establish a reference level to operate the device
  4. In telegraph signaling systems, the development of a positive or negative DC voltage at a point on a telephone line that should remain at a specified reference level, e.g. zero.[ citation needed ]

In magnetic tape recording, the term bias is also used for a high-frequency signal added to the audio signal and applied to the recording head, to improve the quality of the recording on the tape. This is called tape bias.

An audio signal is a representation of sound, typically using a level of electrical voltage for analog signals, and a series of binary numbers for digital signals. Audio signals have frequencies in the audio frequency range of roughly 20 to 20,000 Hz, which corresponds to the upper and lower limits of human hearing. Audio signals may be synthesized directly, or may originate at a transducer such as a microphone, musical instrument pickup, phonograph cartridge, or tape head. Loudspeakers or headphones convert an electrical audio signal back into sound.

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 recording head is often mechanically paired with a playback head, which, though proximal to, is often discrete from the record head.

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 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 signal. Most contemporary tape recorders use AC bias.

Importance in linear circuits

Linear circuits involving transistors 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. In linear amplifiers, 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, 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 amplifier, although the terminology differs a little: the MOSFET must stay in the active mode, and avoid cutoff or ohmic operation.

Amplifier electronic device that can increase the power of a signal

An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the power of a signal. It is a two-port electronic circuit that uses electric power from a power supply to increase the amplitude of a signal applied to its input terminals, producing a proportionally greater amplitude signal at its output. The amount of amplification provided by an amplifier is measured by its gain: the ratio of output voltage, current, or power to input. An amplifier is a circuit that has a power gain greater than one.

Distortion is the alteration of the original shape of something. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal representing sound or a video signal representing images, in an electronic device or communication channel.

Bipolar junction transistors

For bipolar junction transistors 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, so as to obtain the maximum available peak-to-peak signal amplitude without distortion due to clipping 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.

Bipolar junction transistor transistor that uses both electron and hole charge carriers.In contrast,unipolar transistors such as field-effect transistors,only use one kind of charge carrier.For their operation,BJTs use 2 junctions between 2 semiconductor types,n-type and p-type

A bipolar junction transistor is a type of transistor that uses both electron and hole charge carriers. In contrast, unipolar transistors, such as field-effect transistors, only use one kind of charge carrier. For their operation, BJTs use two junctions between two semiconductor types, n-type and p-type.

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 voltage in a nonlinear device like a diode or transistor. It represents the constraint put on the voltage and current in the nonlinear device by the external circuit. The load line, usually a straight line, represents the response of the linear part of the circuit, connected to the nonlinear device in question. The points where the characteristic curve and the load line intersect are the possible operating point(s) of the circuit; at these points the current and voltage parameters of both parts of the circuit match.

Clipping (signal processing) form of distortion that limits a signal in processing

Clipping is a form of distortion that limits a signal once it exceeds a threshold. Clipping may occur when a signal is recorded by a sensor that has constraints on the range of data it can measure, it can occur when a signal is digitized, or it can occur any other time an analog or digital signal is transformed, particularly in the presence of gain or overshoot and undershoot.

Vacuum tubes (thermionic valves)

Grid bias is the DC voltage provided at the control grid of an electron tube relative to the cathode for the purpose of establishing the zero input signal or steady state operating condition of the tube. [1] [2]

Methods of obtaining grid bias:

Combinations of bias methods may be used on the same tube.

Microphones

Electret microphone 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, [12] which is different from the phantom power interface which supplies 48 volts to operate the backplate of a traditional condenser microphone. Electret microphone bias is sometimes supplied on a separate conductor. [13]

See also

Related Research Articles

Triode electronic device having three active electrodes; the term most commonly applies to a single-grid amplifying vacuum tube

A triode is an electronic amplifying vacuum tube consisting of three electrodes inside an evacuated glass envelope: a heated filament or cathode, a grid, and a plate (anode). Developed from Lee De Forest's 1906 Audion, a partial vacuum tube that added a grid electrode to the thermionic diode, the triode was the first practical electronic amplifier and the ancestor of other types of vacuum tubes such as the tetrode and pentode. Its invention founded the electronics age, making possible amplified radio technology and long-distance telephony. Triodes were widely used in consumer electronics devices such as radios and televisions until the 1970s, when transistors replaced them. Today, their main remaining use is in high-power RF amplifiers in radio transmitters and industrial RF heating devices. In recent years there has been a resurgence in demand for low power triodes due to renewed interest in tube-type audio systems by audiophiles who prefer the sound of tube-based electronics.

Vacuum tube Device that controls electric current between electrodes in an evacuated container

In electronics, a vacuum tube, an electron tube, or valve or, colloquially, a tube, is a device that controls electric current flow in a high vacuum between electrodes to which an electric potential difference has been applied.

Negative resistance

In electronics, negative resistance (NR) is a property of some electrical circuits and devices in which an increase in voltage across the device's terminals results in a decrease in electric current through it.

Valve amplifier type of electronic amplifier

A valve amplifier or tube amplifier is a type of electronic amplifier that uses vacuum tubes to increase the amplitude or power of a signal. Low to medium power valve amplifiers for frequencies below the microwaves were largely replaced by solid state amplifiers during the 1960s and 1970s. Valve amplifiers are used for applications such as guitar amplifiers, satellite transponders such as DirecTV and GPS, audiophile stereo amplifiers, military applications and very high power radio and UHF television transmitters.

Differential amplifier

A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs and and one output in which the output is ideally proportional to the difference between the two voltages

Buffer amplifier

A buffer amplifier is one that provides electrical impedance transformation from one circuit to another, with the aim of preventing the signal source from being affected by whatever currents that the load may produce. The signal is 'buffered from' load currents. Two main types of buffer exist: the voltage buffer and the current buffer.

Common emitter

In electronics, a common-emitter amplifier is one of three basic single-stage bipolar-junction-transistor (BJT) amplifier topologies, typically used as the voltage amplifier.

Transconductance, 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 reciprocal of resistance.

Small-signal modeling is a common analysis technique in electronics engineering which is used to approximate the behavior of electronic circuits containing nonlinear devices with linear equations. It is applicable to electronic circuits in which the AC signals, the time-varying currents and voltages in the circuit, have a small magnitude compared to the DC bias currents and voltages. A small-signal model is an AC equivalent circuit in which the nonlinear circuit elements are replaced by linear elements whose values are given by the first-order (linear) approximation of their characteristic curve near the bias point.

When describing a periodic function in the time domain, the DC bias, DC component, DC offset, or DC coefficient is the mean amplitude of the waveform. If the mean amplitude is zero, there is no DC bias. A waveform with no DC bias is known as a DC balanced or DC free waveform.

Pentode electronic device having five active electrodes; the term most commonly applies to a three-grid amplifying vacuum tube

A pentode is an electronic device having five active electrodes. The term most commonly applies to a three-grid amplifying vacuum tube, which was invented by Gilles Holst and Bernhard D.H. Tellegen in 1926. The pentode consists of an evacuated glass envelope containing five electrodes in this order: a cathode heated by a filament, a control grid, a screen grid, a suppressor grid, and a plate (anode). The pentode was developed from the tetrode tube by the addition of a third grid, the suppressor grid. This served to prevent secondary emission electrons emitted by the plate from reaching the screen grid, which caused instability and parasitic oscillations in the tetrode. The pentode is closely related to the beam tetrode. Pentodes were widely used in industrial and consumer electronic equipment such as radios and televisions until the 1960s, when they were replaced by transistors. Their main use now is in high power industrial applications such as radio transmitters. The obsolete consumer tubes are still used in a few legacy and specialty vacuum tube audio devices.

Grid-leak detector

A grid leak detector is an electronic circuit that demodulates an amplitude modulated alternating current and amplifies the recovered modulating voltage. The circuit utilizes the non-linear cathode to control grid conduction characteristic and the amplification factor of a vacuum tube. Invented by Lee De Forest around 1912, it was used as the detector (demodulator) in the first vacuum tube radio receivers until the 1930s.

In electronics, high impedance means that a point in a circuit allows a relatively small amount of current through, per unit of applied voltage at that point. High impedance circuits are low current and potentially high voltage, whereas low impedance circuits are the opposite. Numerical definitions of "high impedance" vary by application.

In electronics, cathode 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 to the magnitude of the desired grid bias voltage.

Valve RF amplifier

A valve RF amplifier or tube amplifier (U.S.), is a device for electrically amplifying the power of an electrical radio frequency signal.

Technical specifications and detailed information on the valve audio amplifier, including its development history.

Plate detector (radio)

In electronics, a plate detector is a vacuum tube circuit in which an amplifying tube having a control grid is operated in a non-linear region of its grid voltage versus plate current transfer characteristic near plate current cutoff in order to demodulate an amplitude modulated carrier signal. This differs from the grid leak detector, which utilizes non-linearity of the grid voltage versus grid current characteristic for demodulation. It also differs from the diode detector, which is a two terminal device.

References

  1. 1 2 Veley, Victor F. C. (1994). The Benchtop Electronics Reference Manual (3rd ed.). New York: Tab Books. pp. 362–365.
  2. 1 2 Landee, Davis, Albrecht, Electronic Designers' Handbook, New York: McGraw-Hill, 1957, p. 2-27
  3. Landee et al, 1957, p. 4-19
  4. 1 2 3 Orr, William I., ed. (1962). The Radio Handbook (16th ed.). New Augusta Indiana: Editors and Engineers, LTD. pp. 266–267.
  5. Headquarters, Department of the Army (1952). C-W and A-M Radio Transmitters and Receivers. Washington DC: USGPO. p. 97. TM 11-665.
  6. Everitt, William Littell (1937). Communication Engineering (2nd ed.). New York: McGraw-Hill. pp. 538–539.
  7. RCA Manufacturing Co. (1940). Receiving Tube Manual RC-14. Harrison, NJ: RCA. p. 38.
  8. Ghirardi, Alfred A. (1932). Radio Physics Course (2nd ed.). New York: Rinehart Books. pp. 505, 770–771.
  9. Giacoletto, Lawrence Joseph (1977). Electronics Designers' Handbook. New York: McGraw-Hill. p. 9-27.
  10. Tomer, Robert B. (1960). Getting the Most Out of Vacuum Tubes. Indianapolis: Howard W. Sams & Co., Inc. / The Bobbs-Merrill Company, Inc. p. 28. Archived from the original on 2009.
  11. Landee et al, 1957, p. 2-28
  12. "Phantom Power and Bias Voltage: Is There A Difference?". 2007-02-05. Archived from the original on 2009-09-08.
  13. IEC Standard 61938

Further reading