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need help

  1. #1
    جديد

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    need help

    dear members:
    sorry for writting in english but really i need help for this question
    " what is thae charactaristics of the transistors that determine the value of ft"

    please help me if you can

    thank you
    fadii



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  2. #2
    مشرف متميز

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    Any transistor has Ft value but may be listed in better data books see
    [***]http://www.alldatasheet.com[/***]
    If you mean what sets this value in its fabrication, it is its stray capacitance specially between Collector and base as it appears multiplied by B in its output, also the junction storage time as it affects the mobility of the electron-hole inside


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  3. #3
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    thank you

    dear majed :
    thank you very much for your reply , but i still didnot understand your answer , actually i mean " what is the chractaristics of transistor that determine the value of ft " in fabrication
    so can your explain your answer , and if you can send any article aboput this issue

    thank you tooo much
    fadi


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    من مواضيع fd_shaheen :


  4. #4
    مشرف متميز

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    Dear
    When fabricating a transistor, 3 inter electrod capacitances are formed as a result of being 2 condutors with insulator between, They are input Cbe, Output Cce, and Feedback Ccb.
    Both input and output capacitors affect frequency responce hence Ft.

    Trans.png

    The most effective one is the feedback capacitance Ccb as circuit analysis shows that it appears in the output multiplied by the open loop gain = Hfe or B.
    To decrease Cbe, the area of collector-base should be decreased, also doping which affects both the junction thickness and B. This in turn will reduce Vce max and Ic max. that explains why most UHF and higher frequencies mostly run at small supply voltages.

    Oscillators have one more problem is the stored charge in the base-emitter junction as transistor switches on and off , much the same way and regular diode has which makes diodes can not swith above certain frequency.

    Microwave power transistors are not really a transistor, they are like a chip (IC) with several transistors added in a phisical arrangement so they add their output power in a phase sequence


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