RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 308
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Lowpass T Network
The Lowpass T network is very good for use as an output-matching network for a power
amplifier stage, where the amplifier is to be connected to an antenna. Having a series
inductor (L1) as the element connected to the device results in an open circuit to the
second harmonic currents, resulting in a reduced current flow through the transistor and
resulting in a higher efficiency. However, since the output voltage is more distorted a
higher inter-modulation distortion may result. For more severe filtering of harmonics, a
cascaded set of Lowpass matching networks is desirable.
Figure 9.2. T matching network.
The equations for the Lowpass T network are as follows:
D D L
X QR X
1
where R
D
+ jX
D
is the device impedance Eqn. 9.7
A R X
L L
2
Eqn. 9.8
A Q
B
X
C
1
Eqn. 9.9
where
1 1
) 1 (
2
L L
D
R
B
R
Q R
A
Eqn. 9.10
) 1 (
2
Q R B
D
Eqn. 9.11
Note this network clearly shows that the Q used in the matching network relates to the
impedance transformation ratio. The minimum Q occurs when A
1
=0, so that
1
min
D
L
R
R
Q
Eqn. 9.12
RF Electronics: Design and Simulation
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