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biasing wikipedia jump to content main menu main menu move to sidebar hide navigation main page contents current events random article about wikipedia contact us contribute help learn to edit community portal recent changes upload file special pages search search appearance donate create account log in personal tools donate create account log in contents move to sidebar hide top 1 overview 2 importance in linear circuits 3 bipolar junction transistors 4 vacuum tubes thermionic valves 5 microphones 6 see also 7 references 8 further reading toggle the table of contents biasing 20 languages العربية català čeština dansk deutsch español فارسی français עברית हिन्दी 日本語 한국어 latviešu polski русский simple english српски srpski svenska ไทย 中文 edit links article talk english read edit view history tools tools move to sidebar hide actions read edit view history general what links here related changes upload file permanent link page information cite this page get shortened url switch to legacy parser print export download as pdf printable version in other projects wikimedia commons wikidata item appearance move to sidebar hide from wikipedia the free encyclopedia redirected from biasing electronics 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 edit a biasing example using a battery and resistor to overcome the forward voltage drop of a diode 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 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 importance in linear circuits edit 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 citation needed bipolar junction transistors edit 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 citation needed vacuum tubes thermionic valves edit 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 1 2 the required bias voltage is usually determined from published vacuum tube characteristic curves which show the relationship between plate current plate voltage and control grid voltage in a typical class a voltage amplifier and class a and ab 1 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 b 2 the bias voltage source must have low resistance and be able to supply the grid current 3 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 the selected bias determines the quiescent operating point of the tube on its characteristic curves and therefore the linearity gain and allowable signal swing of the amplifier stage 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 2 4 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 4 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 5 4 6 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 1 7 8 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 9 10 11 initial velocity bias is used only for small input signal voltages 11 microphones edit 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 12 electret microphone bias is sometimes supplied on a separate conductor 13 see also edit idling current small signal model references edit 1 2 veley victor f c 1987 the benchtop electronics reference manual 1st ed new york tab books pp 450 454 1 2 landee davis albrecht electronic designers handbook new york mcgraw hill 1957 p 2 27 landee et al 1957 p 4 19 1 2 3 orr william i ed 1962 the radio handbook 16th ed new augusta indiana editors and engineers ltd pp 266 267 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 everitt william littell 1937 communication engineering 2nd ed new york mcgraw hill pp 538 539 rca manufacturing co 1940 receiving tube manual rc 14 harrison nj rca p 38 ghirardi alfred a 1932 radio physics course 2nd ed new york rinehart books pp 505 770 771 giacoletto lawrence joseph 1977 electronics designers handbook new york mcgraw hill p 9 27 tomer robert b 1960 getting the most out of vacuum tubes indianapolis howard w sams co the bobbs merrill company p 28 1 2 landee et al 1957 p 2 28 phantom power and bias voltage is there a difference 2007 02 05 archived from the original on 2009 09 08 iec standard 61938 subscription required further reading edit 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 retrieved from https en wikipedia org w index php title biasing oldid 1372571594 categories electronic engineering vacuum tubes hidden categories articles with short description short description is different from wikidata all articles with unsourced statements articles with unsourced statements from december 2022 pages containing links to subscription only content this page was last edited on 1 september 2026 at 08 14 utc page was rendered with parsoid text is available under the creative commons attribution sharealike 4 0 license additional terms may apply by using this site you agree to the terms of use and privacy policy wikipedia is a registered 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