CHAPTER 4 - Impedance Matching
176
Now we know that
and that ideally we would like to be equal to . If we
substitute these values into equation (4.3-1), we can then work out the value of
, which is the
reactance of the parallel capacitor C
P
(Figure 4.3-2). This works out to be
So,
in order to make the real part of the impedance seen by the generator equal to 100Ω, we must
use a shunt capacitor with a reactance
.
Now we can determine the reactance
of the series capacitor C
S
in our equivalent series R-C
network (Figure 4.3-3) by using the formulae in Table 4.2-2 yet again.
So we have determined the value of the reactance of the shunt capacitor (
) which gives us the
right value for the resistive part of the load impedance (
) but now we have also ended up with an
equivalent series reactance (
) which we need to get rid of to ensure maximum power transfer to
the load. This can be achieved by an equal and opposite series reactance i.e. by a series inductor
(Figure 4.3-4) with reactance
.
Figure 4.3-4 We can resonate out the apparent series capacitance C
S
with a series inductor L
S
At this stage we must make two important points. Firstly, although we have made use of the
equivalent series R-C for our parallel R-C network to make it easier to determine the value of L
S
, the
actual physical R-C network will still be a parallel R-C! This is shown in Figure 4.3-5.
Figure 4.3-5 L-section matching, actual circuit
C
S
V
SOURCE
R
SOURCE
R
LS
L
S
Equivalent series R-C
V
SOURCE
R
SOURCE
L
S
Actual Parallel R-C
C
P
R
LP
Conquer Radio Frequency
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