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Impedance-Matching Networks |
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Impedance-matching is important
in achieving maximum power transfer from one stage to another in a
transmitter system. Below are some examples of
impedance-matching networks commonly used to match the driving stage
impedance Zin to the driven stage impedance Zload. These circuits
fall under the category of L-Type impedance-matching networks, because
of the L-shaped configurations present within them.
Figure 1.
L-Type Low-Pass Impedance-Matching Network for ZLoad < Zin
The
low-pass LC impedance-matching network in Figure 1 makes Zload 'appear'
larger to match Zin. The values of L and C are chosen to resonate
at the transmitter frequency. At resonance,
the inductive and capacitive reactances are equal, and the impedance
exhibited by the parallel resonant LC network to Zin is very high.
Figure 2.
L-Type Low-Pass Impedance-Matching Network for ZLoad > Zin
The
low-pass LC impedance-matching network in Figure 2 reduces the load
impedance seen by Zin, so it is used when ZLoad>Zin. Here, C is in
parallel with ZLoad, and the LC network form a series resonant circuit
with ZLoad.
At
resonance, the impedance of the tuned network is very low, which is how
this impedance-matching network decreases the load impedance seen by Zin.
Figure 3.
L-Type High-Pass Impedance-Matching Network for ZLoad < Zin
The
high-pass LC impedance-matching network in Figure 3 increases the load
impedance seen by Zin, so it is used when ZLoad<Zin. As in Figure 1, the
LC network in Figure 3 is basically a parallel resonant circuit, and
therefore exhibits a very high impedance at resonance..
Figure 4.
L-Type High-Pass Impedance-Matching Network for ZLoad > Zin
The
high-pass LC impedance-matching network in Figure 4 reduces the load
impedance seen by Zin, so it is used when ZLoad>Zin.
As in
Figure 2, the LC network in Figure 4 is basically a series resonant
circuit, and therefore exhibits a very low impedance at resonance.
See
also: Impedance;
Impedance-Matching Networks;
Resonance
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