FET VS BJT

Discussion in 'Homework Help' started by shivi24, Mar 21, 2009.

  1. shivi24

    Thread Starter New Member

    Mar 21, 2009
    6
    0
    what are the limiting conditions where FET cannot replace BJT?
    need to submit an assignmnt in college:D
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
  3. Dave

    Retired Moderator

    Nov 17, 2003
    6,960
    144
    The question is a touch ambiguous, however BJTs excel where the tranconductance characteristics of the device are an important factor in an application.

    Dave
     
  4. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    I agree with Dave. The BJT has a much higher transconductance, gm, or Iout/Vin than does a FET. Mosfets have very low values, Jfets are better but not as high as BJT gms. Since the voltage ampification of a CE or CB amp is directly proportional to gm BJTs are best used for high voltage gains. For example a CE amp has a gain of -gmRc. The higher gm the higher the gain. On the other hand Mosfets and Jfets have high input impedences and so make for great voltage followers and Mosfets are excellent power devices.
     
  5. shivi24

    Thread Starter New Member

    Mar 21, 2009
    6
    0
    i still hvnt got the answer to my ques.. my professor is asking for the exact condition where FET cant replace BJT...
    and she said something like till a particulr frequeny FET works and beyond that BJT works..i have to write something related to frequenct and bandwidth.. plz sombody help me out :(:confused:
     
  6. PRS

    Well-Known Member

    Aug 24, 2008
    989
    35
    Shivi24, I'm stumped. Please let me know what she says.
     
  7. shivi24

    Thread Starter New Member

    Mar 21, 2009
    6
    0
    she said that in 99% cases an FET can replace a BJt.. but in 1% case it cannot be replaced. that case is related to frequency i.e. till a particular frequency both work/ fet works( i dnt remebr exatly). and beyond that BJt works. so i have to find information regarding that banwidth and frequeny. i gave her the comparison btwn the two but shes unhappy about it and demanding for a new assignmnt.
     
  8. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    The frequency does not need to be the limiting factor.
    There are fets that will work upto several GigaHz's.

    Greetings,
    Bertus
     
  9. shivi24

    Thread Starter New Member

    Mar 21, 2009
    6
    0
    can u throw some more light on that?
     
  10. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
  11. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    FETs have been in production almost as long as BJTs.

    BJT's have generally been preferred for two main reasons.

    Firstly price. Until recently FETs have been substantially more expensive per unit than BJTs. Now they are very similar and indeed in VLSI circuits they are cheaper.

    Secondly FETs show a very much larger spread of (important) parameters so that you cannot guarantee that another FET of the same type (even from the same batch) will actually work if plugged into a circuit. This is particularly true of analog circuits. This also makes manufacturing more difficult using FETS.

    However nowadays there are a good range of switching FETs cheaply available for digital switching circuits where the parameters are unimportant.

    I expect Bertus will come up with some very high voltage FETs, but I do believe that the BJT currently holds the high voltage crown in the kilvolt range.

    So the answer to your question would be that there are still some high voltages that FETs can't reach.
     
    Last edited: Apr 11, 2009
  12. bertus

    Administrator

    Apr 5, 2008
    15,648
    2,347
    Hello,

    I could find a FREDFET of 1KVolts 5.5 Amp.
    See datasheet BUZ380.

    The IGBT (wich is a kind of combination of fet and bjt) are available with much more power.
    See datasheet HGTG10N120.

    Greetings,
    Bertus
     
  13. shivi24

    Thread Starter New Member

    Mar 21, 2009
    6
    0
    Studiot sir, cn u b little more specific regardin the frequency. Cn u tel me the frequency at whch it stops working n the reason behind.
     
  14. dsp_redux

    Active Member

    Apr 11, 2009
    182
    5
    At the gate of a FET, it is always supposed that there is no current entering the gate. The thing is, ate the gate, it acts like all semi-conductors. You'll find, at very high frequencies the some current may pass through it. Look for the "Miller effect" on the internet or for the large-signal circuit of a FET or the frequencial small-signal of FETs. Sedra and Smith have a book on that too. Almost gave you the answer there... you'll learn a lot by searching for these words... *cough* Miller*cough* effect...
     
  15. studiot

    AAC Fanatic!

    Nov 9, 2007
    5,005
    513
    I don't believe I mentioned frequency.

    I did say

    By this I meant the pinchoff or cutoff voltage of a JFET. By comparison the Vbe voltage of a BJT is pretty well rock solid.
     
  16. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Is prof looking for something related to input impedance related to frequency?

    Interference is all I can think of.
     
  17. shivi24

    Thread Starter New Member

    Mar 21, 2009
    6
    0
    Thank you everyone. i have finally submitted my assignment putting in the points which u all suggesgtd. and no complaints till now!! thanx..
     
Loading...