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htforever
Joined: 01 Jul 2003
Posts: 74
Location: China
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 04 Jul 2003 2:50 Can anyone explain passive and active biasing? |
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WA
Joined: 27 Mar 2002
Posts: 75
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Location: wherever I go - there I am!
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| 04 Jul 2003 3:07 |
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I would guess (by the names)
That passive uses something like voltage divider networks to fix bias.
Active probably has something like a feedback circuit to control gain.
Maybe more knowlegable ppl can "enlighten" us. But, I will look up and try to find answer.
WA
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flatulent
Joined: 19 Jul 2002
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Location: Middle Earth
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| 04 Jul 2003 4:52 right |
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The above discussion is close. Passive bias uses the device's own characteristics. One example would be putting a source rssitor in a depletion mode fet. The old traditional method of bjt bias uses a bypassed emitter resistor and base voltage derived from the power supply or collector voltage.
Active bias meaures the drain current and uses an external transistor or op amp to adjust the transistor gate/base voltage.
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jennifer
Joined: 13 Oct 2003
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| 14 Oct 2003 17:44 Re: right |
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| flatulent wrote: |
The old traditional method of bjt bias uses a bypassed emitter resistor and base voltage derived from the power supply or collector voltage.
Active bias meaures the drain current and uses an external transistor or op amp to adjust the transistor gate/base voltage. |
Is there any books talk about bjt biasing topics? 
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qiang
Joined: 18 Sep 2003
Posts: 52
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| 15 Oct 2003 2:45 Re: Can anyone explain passive and active biasing? |
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the active biasing has the ability to make the working point more ability but it need more space .
i have find some about it in a place ,but now i'v forget where
but you can search with google i think you can find some about it
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Borber
Joined: 01 Jan 1970
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| 15 Oct 2003 3:26 Re: Can anyone explain passive and active biasing? |
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| Active biasing is often used in rf power amplifiers where emitter or source is grounded. Such amplifiers are usually dissipating heat. To improve temperature stability negative dc feedback is used to keep average collector (drain) current constant. Feedback is usually made with PNP transistor.
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godz
Joined: 22 Sep 2003
Posts: 34
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| 15 Oct 2003 10:27 Re: Can anyone explain passive and active biasing? |
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Hi,
Passive biasing is still used widely in biasing of discrete circuits and transistors, it depends on the voltage division using resistors potential divider, its main advantage is the simplicity of biasing, yet the main disad is the high sensitvity to supply variation, as it directly depends on the supply for obtainig the desired voltage drop.
Active biasing is dominant in IC design, tolerances in monolithic resistors up to 20% make it un-practical for passive basing on-chip.
Active biasing is composed of a bias cell core(PTAT,constant-gm,band gap) which generates a reference current, and current mirroring transistors to mirror current from bias cell to the the desired circuit with a well defined ratio from the refernce current.
Active biasing topics can be found in many literature, Design of Analog Integrated circuits, by P. Gary and Meyer is a good place to read about it.
regards,
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