why FET is not popularity on power amplifier?

Discussion in 'General Electronics Chat' started by voduychau, Jan 28, 2007.

  1. voduychau

    Thread Starter Member

    Jan 3, 2007
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    i see many power amplifer circuits,there're some good,there're some bad...

    but just a little circuits used FET as a basic component

    why?

    (sorry for bad English,i'm Vietnammese :) )
     
  2. mrmeval

    Distinguished Member

    Jun 30, 2006
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  3. voduychau

    Thread Starter Member

    Jan 3, 2007
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    thanks for your reply!

    but you don't understand me.Follow this link:
    http://sound.westhost.com/project101.htm

    Q1 to Q7,all of them is Bipolar Transistor,only Q8 and Q9 is power Mosfet,and i believe that many other circuits similar itself

    So i want to ask you: why they not used Fet-Mosfet for Q1 to Q7 ?

    i think Fet-Mosfet have good perfomance than BJT,and bias of Fet-Mosfet is more simple than BJT,why we can't choose it ?
     
  4. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    One would suspect it is more to do with "better the devil you know". BJTs excel in situations of ruggerised environments (potentially the realm of power amplifiers) and tend to be popular across many discrete circuit designs because of the wide range of established and well understood BJT devices available. The MOSFET revolution is built upon its suitablility for high-density, simple to manufacture principles - areas which are not so important for power amplifer designs.

    This in part could explain the range and dsitribution of available BJT and MOSFET amplifer designs you are observing.

    Dave
     
  5. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
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    hi voduychau,

    this is my simple answer, bjt's sounds better. you may agree with it or you may not. i have designed power amps using fets but they never sound better or at par with a bjts.:) are you an audiophile?

    moz
     
  6. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    I think the gain-bandwidth product and linearity of BJTs is much better than that of MOSFETs. If you used MOSFETs large enough to get open-loop gain equivalent to that obtained with the BJTs, I think the open-loop (and closed-loop) bandwidth and slew rate would be much worse, and the linearity would be worse.
     
  7. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    Look at www.diyaudio.com , there you can discuss why mosfets are used mostly for output stage.
     
  8. dragan733

    Senior Member

    Dec 12, 2004
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    Hi voduychau

    About the difference between MOSFET and BJT. Know that MOSFET is a component that is drived by voltage and therefore it is used frequently at the output stages where the input voltages to the gate of the Mosfet are from 5V to 20V. Therefore Mosfet is drived from min voltage 5V. Under 5V it doesn't work, it is blocked, i.e it depends from the gate threshold voltage of the Fet. At some Fets this gate threshold voltage is 2V. The BJT is a component that is drived by current. Therefore the BJT can be drived from input voltage under 5V, or with signal mV, uV. The FET can't be drived with signals mV or uV
     
  9. voduychau

    Thread Starter Member

    Jan 3, 2007
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    somebody tell me FET/MosFET is good,and now i think they have something wrong in their benefit.

    with me,i like BJT,because it's easy for me.

    thanks for everybody!
     
  10. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
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    MOSFETs are good! As you are discovering, it is more a case of application specific requirements, where BJTs are suited to one application and FETs are suited to another. As an example of how "good" MOSFETs are, acquaint yourself with CMOS technology and its impact on the microelectronic world.

    Dave
     
  11. voduychau

    Thread Starter Member

    Jan 3, 2007
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    thank you very much,Dave!

    all best to you.
     
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