Band-pass filters
There are applications where a particular
band, or spread, or frequencies need to be filtered from a wider range
of mixed signals. Filter circuits can be designed to accomplish this
task by combining the properties of low-pass and high-pass into a single
filter. The result is called a band-pass filter. Creating a
bandpass filter from a low-pass and high-pass filter can be illustrated
using block diagrams:
What emerges from the series combination
of these two filter circuits is a circuit that will only allow passage
of those frequencies that are neither too high nor too low. Using real
components, here is what a typical schematic might look like:
capacitive bandpass filter
v1 1 0 ac 1 sin
r1 1 2 200
c1 2 0 2.5u
c2 2 3 1u
rload 3 0 1k
.ac lin 20 100 500
.plot ac v(3)
.end
freq v(3) 4.467E-01 5.012E-01 5.623E-01 6.310E-01
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
1.000E+02 4.703E-01 . * . . .
1.211E+02 5.155E-01 . . * . .
1.421E+02 5.469E-01 . . * . .
1.632E+02 5.676E-01 . . .* .
1.842E+02 5.801E-01 . . . * .
2.053E+02 5.865E-01 . . . * .
2.263E+02 5.882E-01 . . . * .
2.474E+02 5.864E-01 . . . * .
2.684E+02 5.820E-01 . . . * .
2.895E+02 5.755E-01 . . . * .
3.105E+02 5.676E-01 . . .* .
3.316E+02 5.585E-01 . . *. .
3.526E+02 5.487E-01 . . * . .
3.737E+02 5.384E-01 . . * . .
3.947E+02 5.277E-01 . . * . .
4.158E+02 5.169E-01 . . * . .
4.368E+02 5.060E-01 . .* . .
4.579E+02 4.951E-01 . *. . .
4.789E+02 4.843E-01 . * . . .
5.000E+02 4.736E-01 . * . . .
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
Load voltage peaks within narrow frequency range
Band-pass filters can also be constructed
using inductors, but as mentioned before, the reactive "purity" of
capacitors gives them a design advantage. If we were to design a
bandpass filter using inductors, it might look something like this:
The fact that the high-pass section comes
"first" in this design instead of the low-pass section makes no
difference in its overall operation. It will still filter out all
frequencies too high or too low.
While the general idea of combining
low-pass and high-pass filters together to make a bandpass filter is
sound, it is not without certain limitations. Because this type of
band-pass filter works by relying on either section to block
unwanted frequencies, it can be difficult to design such a filter to
allow unhindered passage within the desired frequency range. Both the
low-pass and high-pass sections will always be blocking signals to some
extent, and their combined effort makes for an attenuated (reduced
amplitude) signal at best, even at the peak of the "pass-band" frequency
range. Notice the curve peak on the previous SPICE analysis: the load
voltage of this filter never rises above 0.59 volts, although the source
voltage is a full volt. This signal attenuation becomes more pronounced
if the filter is designed to be more selective (steeper curve, narrower
band of passable frequencies).
There are other methods to achieve
band-pass operation without sacrificing signal strength within the
pass-band. We will discuss those methods a little later in this chapter.
- REVIEW:
- A band-pass filter works to
screen out frequencies that are too low or too high, giving easy
passage only to frequencies within a certain range.
- Band-pass filters can be made by
stacking a low-pass filter on the end of a high-pass filter, or
visa-versa.
- "Attenuate" means to reduce or
diminish in amplitude. When you turn down the volume control on your
stereo, you are "attenuating" the signal being sent to the speakers.
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