BJT Frequency Response!

Discussion in 'Homework Help' started by zaman999, Nov 20, 2011.

  1. zaman999

    Thread Starter New Member

    Jan 29, 2011
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    0
    Hi! In the freq response of a bjt,we see that gain is low for lower values of frequency,at mid band freq gain remains constant and at high frequencies gain again decreases.

    My doubt is
    1.how the coupling capacitors lead to decrease in gain?? :eek:
    2.why the gain is constant at mid band freq?? :confused:




    Thank You.
     
  2. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,367
    You need to show your circuit. Series coupling capacitors constitute a high pass filter, cutting out low frequencies. Increase the value of the capacitor by x10 or x100 and repeat the frequency response test.
     
  3. zaman999

    Thread Starter New Member

    Jan 29, 2011
    26
    0
    Yes,i've included the circuit and the frequency response.
    Why is the gain low around 100Hz and beyond 20kHz??
    also in midband region gain seems to be constant?
     
  4. MrChips

    Moderator

    Oct 2, 2009
    12,449
    3,367
    So, increase CB and CC to 1uF and see what happens.
     
  5. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    I disagree.
    The transistor has its emitter resistor bypassed by CE. Therefore its input impedance is so low that the circuit will have -3dB response at about 2544Hz, not 100Hz because the value of the input coupling capacitor CB is much too low.

    The high frequency response is poor because the SL100 transistor is very old and has high capacitances.

    The value of CC also might be too low to pass low frequencies to a load.
     
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