CHAPTER 4 - Impedance Matching
178
Figure 4.3-7 L-C (a) and C-L (b) L-section matching
These two networks will behave in a very similar fashion at 100 MHz but the difference between
them lies in their frequency response across a wider range of frequencies.
Figure 4.3-8 Ratio of Output and Input powers in dB for the circuits in Figure 4.3-7
Figure 4.3-8 shows that the circuit in Figure 4.3-7(a), has a low-pass response. This is quite
predictable since, as the frequency increases, the impedance of the parallel branch (capacitor)
diminishes thereby providing a low impedance path to ground and sinking current in there!
The circuit in Figure 4.3-7(b) instead exhibits a high-pass behaviour since, at low frequency, the
impedance of the parallel branch (inductor) is very low and sinks current into the ground but, at high
frequency, such an impedance is very high and hence current is allowed to flow into the load
thereby delivering power to it. A high-pass design is particularly useful when we need to block DC.
V
SOURCE
100 Ω
L
S
=477 nH
C
P
=4.8 pF
1000 Ω
(a)
V
SOURCE
100 Ω
C
S
=5.3 pF
L
P
=530 nH
1000 Ω
(b)
dB
L
S
& C
P
C
S
& L
P
Conquer Radio Frequency
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