Wednesday, 22 February 2012

Band-Pass Filters

Discuss about band pass filters (BPF).

High-Pass Filters


  • Figure shows some common implementations of high-pass filters.  
  • Note that in each of the high-pass filters shown above, the inductors are in shunt with the input while the capacitors are in series with the input.  
  • This is because the reactance XL of an inductor increases with the signal frequency, i.e., XL = 2πfL, while the reactance XC of a capacitor decreases with the signal frequency, i.e., XC = 1 / 2πfC.  
  • Thus in these high-pass filters, the capacitors resist the passing of an ac signal as the frequency decreases, while the inductors shunt them towards the ground as the frequency decreases.  
  • Either way,  the effect is to attenuate the signal as frequency decreases.

The following equations apply to the high-pass filters in Figure above:
1)  L = Zo / 4πf 
2)  C = 1 / (4πf Zo) 
3)  Zo = sqrt(L/C)
4)  f = 1 / (4π sqrt(LC))
where Zo is the line impedance and f is the cut-off frequency of the filter.