Misplaced Pages

Biasing: Difference between revisions

Article snapshot taken from Wikipedia with creative commons attribution-sharealike license. Give it a read and then ask your questions in the chat. We can research this topic together.
Browse history interactively← Previous editContent deleted Content addedVisualWikitext
Revision as of 11:15, 5 July 2007 edit195.78.226.10 (talk) →Fixed bias← Previous edit Latest revision as of 20:49, 16 July 2024 edit undoKvng (talk | contribs)Extended confirmed users, New page reviewers108,308 editsm unpiped links using script. link first occ. 
(294 intermediate revisions by more than 100 users not shown)
Line 1: Line 1:
{{Short description|Background operating conditions for electronics}}
{{redirect|Biasing}}
{{about|biasing in '''electronics'''||Biasing (disambiguation)}}
{{Redirect|Bias point|the financial term|Basis point}}
{{Redirect|Bleeder bias|the safety discharge resistor|Bleeder resistor}}


]
'''Biasing''' in electronics is the method of establishing predetermined voltages and/or currents at various points of a circuit to set the appropriate operating point.


In ], '''biasing''' is the setting of DC (]) operating conditions (current and voltage) of an ] that processes time-varying ]s. Many electronic devices, such as ]s, ]s and ]s, whose function is ] time-varying (]) signals, 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 ] on this DC bias current or voltage.
== Requirement ==
In ], a '''bias point''', also known an ''operating point'', ''quiescent point'' or ''Q-point'', is a ] ] which, when applied to a device, causes it to operate in a certain desired fashion. The term is normally used in connection with devices such as ] and ] which are used in ] or ].


The ] 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 that supplies this steady current or voltage.
Linear circuits involving ]s typically require a specific ] voltage to operate correctly, which can be achieved using a biasing circuit. The method of keeping a device to operate in the active region is also referred to as biasing of the circuit. In ]s, a small input signal gives larger output signal without any change in its general shape. Before applying an AC signal, proper biasing of the transistor is necessary.


==Overview==
For example, for ]s the bias point would keep the transistor operating in the active mode, drawing a ]. A small signal is then applied on top of this bias voltage, thereby either ] or switching the current, depending on the design of the circuit. The input dc voltage is chosen to satisfy the required large signal parameters.
In electronics, 'biasing' usually refers to a fixed DC voltage or current applied to a terminal of an ] 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 ] to allow the transistor to operate in a particular region of its ] curve. For vacuum tubes, a ] voltage is often applied to the grid electrodes for the same reason.{{citation needed|date=December 2022}}


In ], the term ''bias'' is also used for a high-frequency signal added to the ] and applied to the ], to improve the quality of the recording on the tape. This is called ].{{citation needed|date=December 2022}}
The quiescent point of operation is typically near the middle of the dc ''load line''. <!-- reasons to be inserted here later --> The process of obtaining certain dc collector current at a certain dc collector voltage by setting up operating point is called biasing.


==Importance in linear circuits==
'''Quiescent current''' is the current that flows in an ] when no load is present. This term is commonly used in ] of ] and voltage regulator circuits. It is basically the current which flows through a component/circuit without actually contributing in any way to the load and usually of the order of ] to microamperes.
Linear circuits involving ]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 ]. In linear ]s, a small input signal gives a 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 ], 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 ], and avoid cut-off or saturation. The same requirement applies to a ] amplifier, although the terminology differs a little: the MOSFET must stay in the ], and avoid cutoff or ohmic operation.{{citation needed|date=December 2022}}


==Bipolar junction transistors==
After establishing the operating point, when input signal is applied, the Q-pt should not move either to saturation or cut-off region. However this unwanted shift might occur due to various reasons.
{{main|Bipolar transistor biasing}}
For ]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 ], so as to obtain the maximum available peak-to-peak signal amplitude without distortion due to ] 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.{{citation needed|date=December 2022}}


{{anchor|Grid bias|Fixed bias|Cathode bias|Grid leak bias|Bleeder bias|Contact bias}}
=== Reasons for Shift of Q-pt ===


== Vacuum tubes (thermionic valves) ==
The shifting of operating point is due to two major reasons -
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.<ref name="veley01"/><ref name="Landee">Landee, Davis, Albrecht, , New York: McGraw-Hill, 1957, p. 2-27.</ref>


* In a typical ] ], and class A and AB<sub>1</sub> power stages of ]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.
1. Parameters of transistor depend on temperature. As it increases, leakage current due to minority charge carriers (I<sub>CBO</sub>) increases. As I<sub>CBO</sub> increases, I<sub>CEO</sub> also increases, causing increase in collector current I<sub>C</sub>. This produces heat at the collector junction. This process repeat, and finally Q-pt may shift into saturation region.
* ]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 B<sub>2</sub>). The bias voltage source must have low resistance and be able to supply the grid current.<ref>Landee et al., 1957, .</ref> When tubes designed for class B are employed, the bias can be as little as zero.
Sometimes the excess heat produced at the junction may even burn the transistor. This is known as '''thermal runaway'''.
* ]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.
2. When a transistor is replaced by another of the same type, the Q-pt may shift, due to change in parameters of transistor such as ''current gain'' (<math>\beta </math>) which changes from unit to unit.


* ''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.<ref name="Landee"/><ref name="Orr_1962"/>
To avoid shift of Q-pt, bias-stabilization is necessary. Various biasing circuits can be used for this purpose.
* '']'' (''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.<ref name="Orr_1962"/>
* ''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.<ref name="Radio Transmitters"/><ref name="Orr_1962"/><ref name="Everitt_1937"/>
* ''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.<ref name="veley01"/><ref name="RCA_1940"/><ref name="Ghirardi_1932"/>
* ''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.<ref name="Giacoletto_1977"/><ref name="Tomer_1960"/><ref name="Landee03">Landee et al., 1957, .</ref> Initial velocity bias is used only for small input signal voltages.<ref name="Landee03"/>


==Microphones==
==BJT Transistor biasing==
] elements typically include a ] 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&nbsp;mA and is often referred to as bias, which is different from the ] interface which supplies 48 volts to operate the backplate of a traditional condenser microphone.<ref name="Phantom"/> Electret microphone bias is sometimes supplied on a separate conductor.<ref name="IEC_61938"/>
=== Requirements of Biasing Circuit ===
# Q-pt is established in center of active region of transistor characteristic. It should not shift to saturation region or cut-off region, when input is applied.
# Q-pt should be independent of transistor parameters ie. should not shift if transistor is replaced by another of the same type.
# Collector current should be stabilized against changes in temperature.
# The circuit must be practical in its implementation, and cost-effective.


=== Types === == See also ==
* ]
There are five main types of biasing circuits used with Bipolar transistors.
* ]
# Fixed bias
# Collector-to-base bias
# Fixed bias with emitter resistor
# Voltage divider bias
# Emitter bias


==References==
<!--
{{reflist|refs=
<ref name="veley01">{{cite book |author-first= Victor F. C. |author-last=Veley |title=The Benchtop Electronics Reference Manual |edition=1st |location=New York |publisher=Tab Books |date=1987 |pages=450–454 |url=https://archive.org/details/benchtopelectron00vele }}</ref>
<ref name="Ghirardi_1932">{{cite book |author-first=Alfred A. |author-last=Ghirardi |title=Radio Physics Course |edition=2nd |location=New York |publisher=Rinehart Books |date=1932 |pages=505, 770–771}}</ref>
<ref name="Orr_1962">{{cite book |editor-first= William I. |editor-last=Orr |title=The Radio Handbook |edition=16th |location=New Augusta Indiana |publisher=Editors and Engineers, LTD. |date=1962 |pages=266–267}}</ref>
<ref name="Radio Transmitters">{{cite book |author=Headquarters, Department of the Army |title=C-W and A-M Radio Transmitters and Receivers |id=TM 11-665 |location=Washington, D.C. |publisher=United States Government Publishing Office |date=1952 |page=97 |url=https://archive.org/stream/TM11-665#page/n119/mode/2up}}</ref>
<ref name="Everitt_1937">{{cite book |author-first=William Littell |author-last=Everitt |title=Communication Engineering |url=https://archive.org/details/communicationeng00ever |url-access=registration |edition=2nd |location=New York |publisher=McGraw-Hill |date=1937 |pages=}}</ref>
<ref name="RCA_1940">{{cite book |author=RCA Manufacturing Co. |title=Receiving Tube Manual RC-14 |location=Harrison, NJ |publisher=] |date=1940 |page=38}}</ref>
<ref name="Giacoletto_1977">{{cite book |author-first=Lawrence Joseph |author-last=Giacoletto |title=Electronics Designers' Handbook |location=New York |publisher=McGraw-Hill |date=1977 |page=9-27}}</ref>
<ref name="Tomer_1960">{{cite book |author-first= Robert B. |author-last=Tomer |title=Getting the Most Out of Vacuum Tubes |location=Indianapolis |publisher=Howard W. Sams & Co./The Bobbs-Merrill Company |date=1960 |page=28 |url=https://archive.org/details/GettingTheMostOutOfVacuumTubes }}</ref>
<ref name="Phantom">{{cite web |url=http://www.shure.com/ProAudio/Products/us_pro_ea_phantom |title=Phantom Power and Bias Voltage: Is There A Difference? |date=2007-02-05 |url-status=dead |archive-url=https://web.archive.org/web/20090908021543/http://www.shure.com/ProAudio/Products/us_pro_ea_phantom |archive-date=2009-09-08}}</ref>
<ref name="IEC_61938">] {{subscription required}}</ref>
}}


== Further reading ==
*under construction*
* {{cite book |author-last1=Boylestad |author-first1=Robert L. |author-first2=Louis |author-last2=Nashelsky |title=Electronic Devices and Circuit Theory |publisher=Prentice-Hall Career & Technology |date=2005}}
* {{cite book |author-last1=Patil |author-first1=P. K. |author-first2=M. M. |author-last2=Chitnis |title=Basic Electricity and Semiconductor Devices |publisher=Phadke Prakashan |date=2005}}
* {{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}}


Each of these is discussed in detail below, along with circuit diagram, explanation, derivation of Q-pt, merits and demerits of particular biasing circuit.

-->

====Fixed bias====
]
This form of biasing is also called. In the example image on the right, the single power source (ie. battery) is used for both collector and base of transistor, although separate batteries can also be used.

In the given circuit,

V<sub>CC</sub> = I<sub>B</sub>R<sub>B</sub> + V<sub>be</sub>

Therefore,

I<sub>B</sub> = (V<sub>CC</sub> - V<sub>be</sub>)/R<sub>B</sub>

For a given transistor, V<sub>be</sub> does not vary significantly during use. As V<sub>CC</sub> is of fixed value, on selection of R<sub>B</sub>, the base current I<sub>B</sub> is fixed. Therefore this type is called ''fixed bias'' type of circuit.

Also for given circuit,

V<sub>CC</sub> = I<sub>C</sub>R<sub>C</sub> + V<sub>ce</sub>

Therefore,

V<sub>ce</sub> = V<sub>CC</sub> - I<sub>C</sub>R<sub>C</sub>

From this equation we can obtain V<sub>ce</sub>. Since I<sub>C</sub> = βI<sub>B</sub>, we can obtain I<sub>C</sub> as well. In this manner, operating point given as (V<sub>CE</sub>,I<sub>C</sub>) can be set for given transistor.

'''Merits:'''

*It is simple to shift the operating point anywhere in the active region by merely changing the base resistor (R<sub>B</sub>).
*Very few number of components are required.

'''Demerits:'''
*The collector current does not remain constant with variation in temperature. Therefore the operating point is unstable.
*When the transistor is replaced with another one, slight (if not large) change in the value of β can be expected. Due to this the operating point will shift.

'''Usage:'''

Due to the above inherent drawbacks, fixed bias is rarely used in linear circuits, ie. those circuits which use the transistor as a current source. Instead it is often used in circuits where transistor is used as a switch.

====Collector-to-Base bias====
]
In this form of biasing, the base resistor R<sub>B</sub> is connected to the collector instead of connecting it to the battery V<sub>CC</sub>.

Similar to above,

V<sub>ce</sub> = V<sub>CC</sub> - I<sub>C</sub>R<sub>C</sub> (Since I<sub>B</sub> << I<sub>C</sub>)

In case of increase in temperature, collector current tends to increase, causing the voltage drop across resistor R<sub>C</sub> to increase. Hence V<sub>ce</sub> decreases. Therefore base current reduces, thereby compensating for the increase in collector current.

It can be noted that for the given circuit,

I<sub>B</sub> = (V<sub>CC</sub>)/(R<sub>B</sub>+βR<sub>C</sub>)

'''Merits:'''
*Circuit has a tendency to stabilize the operating point against variations in temperature and β (ie. replacement of transistor)

'''Demerits:'''
*The resistor R<sub>B</sub> causes an ac feedback, reducing the voltage gain of the amplifier. This is a mostly undesirable effect.

'''Usage:'''

Due to the major drawback of feedback, this biasing form is rarely used.

====Fixed bias with emitter resistor====
]

The fixed bias circuit is modified by attaching an external resistor to the emitter. Since V<sub>be</sub> is very small, we get

I<sub>b</sub> = (V<sub>CC</sub> - I<sub>E</sub>R<sub>E</sub>)/R<sub>B</sub>

When the temperature increases, the leakage current increases. Therefore there is increase in I<sub>C</sub> and I<sub>E</sub>. This increases the emitter voltage, which reduces the voltage across the base resistor. This reduces the base current which results in less collector current. Thus collector current is not allowed to increase, and operating point is kept stable.

Similarly, if the transistor is replaced by another, there may be a change in I<sub>C</sub> corresponding to change in β-value. By similar process as above, the change is negated and operating point kept stable.

V<sub>ce</sub> = V<sub>CC</sub> - (R<sub>C</sub>+R<sub>E</sub>)I<sub>C</sub> (since I<sub>C</sub> roughly equals I<sub>E</sub> as I<sub>B</sub> is very small)

'''Merits:'''

The circuit has the tendency to stabilize operating point against changes in temperature and β-value.

'''Demerits:'''

In this circuit, for proper functioning, the following condition must be met:

R<sub>E</sub> >> R<sub>B</sub>/β

As β-value is fixed for a given transistor, this relation can be satisfied either by keeping R<sub>E</sub> very large, or making R<sub>B</sub> very low.
*If R<sub>E</sub> is of large value, high V<sub>CC</sub> is necessary. This increases cost as well as precautions necessary while handling.
*If R<sub>B</sub> is low, a separate low voltage supply should be used in the base circuit. Using two supplies of different voltages is impractical.

In addition to the above, R<sub>E</sub> causes ac feedback which reduces the voltage gain of the amplifier.

'''Usage:'''

Due to the above disadvantages, this type of biasing circuit is generally not used.

====Voltage divider bias====
]

The voltage divider is formed using external resistors R<sub>1</sub> and R<sub>2</sub>. The voltage across R<sub>2</sub> forward biases the emitter junction. By proper selection of resistors R<sub>1</sub> and R<sub>2</sub>, the operating point of the transistor can be made independent of β.

In this circuit we get,

V<sub>B</sub> = V across R<sub>2</sub> = (R<sub>2</sub>*V<sub>CC</sub>)/(R<sub>1</sub>+R<sub>2</sub>)

Also V<sub>B</sub> = V<sub>be</sub> + I<sub>E</sub>R<sub>E</sub>

When temperature increases, I<sub>C</sub> increases. As I<sub>C</sub> makes up the majority of I<sub>E</sub>, I<sub>E</sub> also increases. When I<sub>E</sub> increases, V<sub>be</sub> decreases. Therefore I<sub>C</sub> decreases and the operating point remains stable.

Also, V<sub>C</sub> = V<sub>CC</sub> - I<sub>C</sub>R<sub>C</sub>

Since I<sub>C</sub> is roughly equal to I<sub>E</sub>,

V<sub>ce</sub> = V<sub>C</sub> - (R<sub>C</sub>+R<sub>E</sub>)I<sub>C</sub>

We note that β is absent from all the above equations. Therefore, if the transistor is replaced by another having a different value of β, the operating point is largely unaffected.

'''Merits:'''
*Unlike above circuit, only one dc supply is neccessary.
*Operating point is almost independent of β variation.
*Operating point stabilized against shift in temperature.

'''Demerits:'''
*Ac feedback is caused by R<sub>E</sub>, which reduces the voltage gain of the amplifier. A method to avoid this is discussed below.

'''Usage:'''

The circuit's stability and merits as above make it the most widely for linear circuits.

=====Voltage divider with capacitor=====
]

The standard voltage divider circuit discussed above faces one critical drawback - ac feedback caused by resistor R<sub>E</sub>. This can be avoided using a capacitor (C<sub>E</sub>) in parallel with R<sub>E</sub>, as shown in circuit diagram.

The impedence of the capacitor(X<sub>C</sub>) is given by the equation,

X<sub>C</sub> = 1/(2*π*F*C)

where-
*''F'' is the frequency of input signal
*''C'' the value of capacitance.
*''π'' is ]

Therefore, the capacitor offers low impedence to ac input as ac signal has high frequency. Thus the emitter is placed at ground potential for ac input. Only dc feedback is provided for stabilization of operating point.

====Emitter bias====
]

When a split supply (dual power supply) is available, this biasing circuit is the most effective. The negative supply V<sub>EE</sub> is used to forward-bias the emitter junction through R<sub>E</sub>. The positive supply V<sub>CC</sub> is used to reverse-bias the collector junction. Only three resistors are neccessary.

We know that,

V<sub>B</sub> - V<sub>E</sub> = V<sub>be</sub>

If R<sub>B</sub> is small enough, base voltage will be approximately zero. Therefore emitter current is,

I<sub>E</sub> = (V<sub>EE</sub> - V<sub>be</sub>)/R<sub>E</sub>

The operating point is independent of β if R<sub>E</sub> >> R<sub>B</sub>/β

'''Merit:'''

Good stability of operating point similar to voltage divider bias.

'''Demerit:'''

This type can only be used when split (dual) power supply is available.

==Sources==
*{{cite book | author=Sedra, Adel; Smith, Kenneth | title=Microelectronic Circuits | publisher=Oxford University Press | year=2004 | id=ISBN 0-19-514251-9}}
*{{cite book| author=P.K. Patil;M.M. Chitnis|title=Basic Electricity and Semiconductor Devices|publisher=Phadke Prakashan|year=2005}}

==See also==
*]
*]
*]
]
] ]
]

Latest revision as of 20:49, 16 July 2024

Background operating conditions for electronics This article is about biasing in electronics. For other uses, see Biasing (disambiguation). "Bias point" redirects here. For the financial term, see Basis point. "Bleeder bias" redirects here. For the safety discharge resistor, see Bleeder resistor.
A graphical representation of the current and voltage properties of a transistor; the bias is selected so that the operating point permits maximum signal amplitude without distortion.

In electronics, biasing is the setting of DC (direct current) operating conditions (current and voltage) of an electronic component that processes time-varying signals. 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 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 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 that 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 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 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.

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.

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 a 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.

Bipolar junction transistors

Main article: Bipolar transistor biasing

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.

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.

  • In a typical Class A voltage amplifier, and class A and AB1 power stages of audio power amplifiers, 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 amplifiers 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. When tubes designed for class B are employed, the bias can be as little as zero.
  • Class C amplifiers 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 (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. Initial velocity bias is used only for small input signal voltages.

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, 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.

See also

References

  1. ^ Veley, Victor F. C. (1987). The Benchtop Electronics Reference Manual (1st ed.). New York: Tab Books. pp. 450–454.
  2. ^ Landee, Davis, Albrecht, Electronic Designers' Handbook, New York: McGraw-Hill, 1957, p. 2-27.
  3. Landee et al., 1957, p. 4-19.
  4. ^ 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, D.C.: United States Government Publishing Office. 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./The Bobbs-Merrill Company. p. 28.
  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(subscription required)

Further reading

  • Boylestad, Robert L.; Nashelsky, Louis (2005). Electronic Devices and Circuit Theory. Prentice-Hall Career & Technology.
  • Patil, P. K.; Chitnis, M. M. (2005). Basic Electricity and Semiconductor Devices. Phadke Prakashan.
  • Sedra, Adel; Smith, Kenneth (2004). Microelectronic Circuits. Oxford University Press. ISBN 0-19-514251-9.
Categories: