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Micrel
Low-Noise Amplifier Input Matching
Capacitor C3 and inductor L2 form the “L” sh ape input
matching network. The capacitor provides additional
attenuation for low-frequency outside ba nd noise.
The inductor provides additional ESD pro tection for
the antenna pin. Two methods can be us ed to find
these values that best matched near 50 ? . One
method is done by calculating the values using the
equations below and the other is using a S mith chart
utility. The latter is made easier via a software plot
where components are added on. In this wa y, the user
can see the impedance moving directio n for best
values of C8 and L1 toward to central matc hing point,
like WinSmith by Noble Publishing.
To calculate the matching values, o ne needs to know
the input impedance of the device. Table 3 shows the
input im pe dance of the M IC RF219 and suggested
matching val ues for the most of ten use d frequencies.
These sugge ste d value s m ay be differen t if the layout
is not exactly the same as t he on e made here:
MICRF219
Doing the same calculation example with the Smith
Chart, would appear as follows,
First, one plots the input impedance of the device,
(Z = 18.6 – j174.2) ? @ 433.92MHz.(Figure 5):
F req (MHz)
315
390
418
C3 (pF)
1.8
1.5
1.5
L2(nH)
68
47
43
Z devic e ( ? )
33 - j2 35
23 – j199
21 – j186
433.92
1.5
39
19 – j174
Figure 5. Device’s Input Impedance, Z = 19 – j174 ?
Table 3. Matching Values for the Most Used
Frequencies
For the fr equency of 433.92MHz, the input impedance
is Z = 18.6 – j174.2 ? , then the matching components
are calculated by:
Equivalent par allel = B = 1/Z = 0.606 + j5 .68msiemens
Rp = 1 / Re (B); Xp = 1 / Im (B)
Rp = 1.6 5k ? ; Xp = 176.2 ?
Q = SQRT (Rp/50 + 1)
Q = 5.831
Xm = Rp / Q
Xm = 2 82.98 ?
Resonance Method for L-shape Matching Network
Second, one plots the shunt inductor (39nH) and the
series capacitor (1.5pF) for the desired input
impedance (Figure 6). One can then see the matching
leading to the center of the Smith Chart or close to
50 ? .
Lc = Xp / (2×Pi×f);
Lp = Xm / (2×Pi×f)
L2 = (Lc×Lp) / (Lc + Lp); C3 = 1 / (2×Pi×f×Xm)
L2 = 39.8nH
C3 = 1.3pF
June 2011
13
M9999-060811
(408) 944-0800
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