frequency response of bjt

Discussion in 'General Electronics Chat' started by bellamkonda revanth, Oct 2, 2010.

  1. bellamkonda revanth

    Thread Starter New Member

    Aug 17, 2010
    could anyone please clear my conceptional flaw please?.....:confused:
    if we analyze the frequency response of low frequency the external coupling capacitors are taken into account and at high frequency only the internal capacitors of the transistors are taken into account.....
    now my doubt is why we have to take in this way(low fre external cap and high fre internal cap of bjt)...why we hav to do it like this?...
    please clarify my doubt....:)
  2. Ghar

    Active Member

    Mar 8, 2010
    The coupling capacitors are set up as high pass filters (meant to block DC and unintentionally some low frequency AC) while the internal capacitances are unwanted low-pass filters which cut off the gain at high frequency.

    That means the coupling capacitors can be ignored at high frequency (you're well into the passband, they're not doing anything) while at low frequency the internal capacitances don't do anything (you're well below the cut off frequency).