Multistage using BJT

Discussion in 'The Projects Forum' started by Communications Student, May 7, 2007.

  1. Communications Student

    Thread Starter Member

    May 3, 2007
    10
    0
    Plz guys I need ur help, I have a project,I need to design a multistage amplifier using BJT whith a voltage gain 100, and Rload=1Kohm, and it will operates in the frequency range from o to 5Mhz

    Any ideas????
     
  2. 03GTS

    Member

    Jan 5, 2007
    12
    0
    i might have something that will help you but i will have to look for an old handout i have. i'll get back to you
     
  3. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    here's a simple schem of multi stge bjt amps. i have deliberately left out the values for you to find out. just use the appropiate formulas to arrive at the desired function.:)

    moz
     
  4. Communications Student

    Thread Starter Member

    May 3, 2007
    10
    0
    Thankss soo much 03GTS and Mozikluv, and I'll begin to work it out wiz the configuration tht u attached Moz, and I appreciate ur efforts o3GTS hoppin u find ur handout
     
  5. Communications Student

    Thread Starter Member

    May 3, 2007
    10
    0
    Hey guys nothing new?? the dead line for the project is 19/5 :(((((
     
  6. thingmaker3

    Retired Moderator

    May 16, 2005
    5,072
    6
    http://forum.allaboutcircuits.com/showthread.php?t=3000

    Please review rule number six.

    This forum is not here to do your schoolwork for you.

    If you have specific questions, we will be happy to help you work through problems. You must do the work, though.

    Do you have a specific question about how to compute the component values?
     
  7. Communications Student

    Thread Starter Member

    May 3, 2007
    10
    0
    Ok moderator I'm sorry but really I didn't mean to be tht lazy and tht dependent on others, so as it's my very first time to post in here, don't be too harsh on me ;)

    So guys plz I need ur help, those are the values I computed based upon the pdf file above attached by Mozikluv:

    R1=6oo ohm, R2=R3=50 kohm, R4=95kohm,R5=483.33 ohm,R7 =7.1 kohm
    Problem is I can't compute R6,plus in the second BJT I have very small base current : I base=1.5 micro A, I collector= I emitter = 0,45mA (as I took hFE=300) , and also I don't know the values of the capacitors:((((((((((((((((
     
  8. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    am attaching formulas to help you out.

    why is your R7 value higher than your load value (R8).

    Review your emitter bypass cap. lesson

    biasing your second Q (pnp) is practically the same as your first Q

    moz:)
     
  9. Communications Student

    Thread Starter Member

    May 3, 2007
    10
    0
    Thankss so much MOz for ur kindess and ur help, I'll work it out today with the help of ur formulas, plus I asked my teacher assistant today at college, he told me tht i have to buy the BJT first, adn measure its hFE using the Avo-meter and not just assume it any value in my design.
     
  10. Communications Student

    Thread Starter Member

    May 3, 2007
    10
    0
    in the formulas u sent me : Av = Vo/Vs = gm1 gm2(R4||rπ2 ) (R7||R8 ) ( R2/(R2 +R1))
    Input resistance = R1||R2||r pi 1

    how would i compute gm which is the differentiation of Id over Vgs ( JFET) while the required project and the configuration u attached is a BJT ??
    and can I use anozer BJT other than 2N3904 and 2N3906 ??
     
  11. Communications Student

    Thread Starter Member

    May 3, 2007
    10
    0
    Thankss in advance really u help me lots guyzz
     
  12. mozikluv

    AAC Fanatic!

    Jan 22, 2004
    1,437
    1
    1. gm = ICQ / VT

    2. yes you can sub that with other transistors

    moz
     
  13. mannooha

    New Member

    May 14, 2007
    1
    0
    for communication student:
    first: Dr\M.Mesel7y will be very sad
    second: gm=1/re
     
  14. Communications Student

    Thread Starter Member

    May 3, 2007
    10
    0
    Thankss Moz :)
     
  15. omar_megane

    New Member

    May 16, 2007
    1
    0
    The pdf attached contains a 2-stage CE BJTS with gain=10 and Rl=10k and a very good Q-point position.
    I have a question, What is the advantage of putting the so called "swamping resistance" in the emitter side of the CE BJT amplifiers???
     
  16. Communications Student

    Thread Starter Member

    May 3, 2007
    10
    0
    Omar megane, the pdf u sent me is MORE THAN PERFECT AND VERYY USEFUL, REALLY THANK U SOOO MUCH :)

    the advantage of putting the so called "swamping resistance" in the emitter side of the CE BJT amplifiers is tht in the DC analysis, it will help to get a good q point in the active region, and as we don't want this resistance in the AC analysis, as it will decrease the gain, we put a by pass capacitor in parallel wiz it which will act as short circuit in AC and open circuit in DC

    hope I can help as u did :)
     
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