Multisim common collector confusion

Discussion in 'General Electronics Chat' started by jaydnul, Nov 5, 2015.

  1. jaydnul

    Thread Starter Member

    Apr 2, 2015
    88
    0
    I am trying to do a basic current gain amplifier. Everything works until I place the capacitor in series with the 10 ohm load to remove the DC offset: [​IMG]
    I have no clue what's going on here. I tried playing with C1 value but it didn't do much. Shouldn't the output just be the input at the origin? What am I doing wrong?

    Thanks

    Edit: the transistor is an ideal NPN
     
    Last edited: Nov 5, 2015
  2. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,033
    The 10Ω load is far too large for the 100µF capacitor to make any difference. Try raising C1 and R5.
     
  3. jaydnul

    Thread Starter Member

    Apr 2, 2015
    88
    0
    So for this circuit to drive a 4 ohm speaker I would need a giant capacitor? Like .1 Farads?
     
  4. crutschow

    Expert

    Mar 14, 2008
    12,991
    3,227
    Guessing at part values does not generally work well for electronic design. :rolleyes:
    If you do the math for the -3dB point [C = 1/(2πRf)] it requires nearly a 2,000μF capacitor to give a -3dB rolloff into a 4 ohm load at f = 20Hz.
     
    Last edited: Nov 5, 2015
  5. wayneh

    Expert

    Sep 9, 2010
    12,093
    3,033
    Your guess is only 50x higher than crutschow's calculated value. I've seen worse. :D
     
  6. Picbuster

    Member

    Dec 2, 2013
    373
    50
    Well your problem is simple to explain.
    the current (Emit + Basis) goes via 10 ohm.
    open then sw then it goes via 1K so Ue goes up and when above 0,6V The xtor is switched off.
    adive: measure Ue when sw is open and alter the 1K resistor
     
  7. jaydnul

    Thread Starter Member

    Apr 2, 2015
    88
    0
    Haha ya, .1F was an exaggerated estimation. I'm still thinking about this problem though; if I have a DC biased audio signal and I want to send it into a 4 ohm speaker, is the only simple option a giant capacitor or is there some technique that I am missing?
     
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