RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 321
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
As an example, consider the impedance matching of the input of an MRFE6VS25N
LDMOSFET over a 250MHz to 500MHz range. The geometric mean frequency if that
range is 354 MHz. At 352 MHz the MRFE6VS25N datasheet gives a large signal source
impedance of 7.2 +j4.3 . The device impedance is the complex conjugate of that and is
thus 7.2 -j4.3 . That is equivalent to 9.77 in parallel with -j16.36 . The reactive
impedance corresponds to a capacitance of 27.57 pF at 353 MHz. An impedance
transformation from 50 to 7.2 is thus required. That can be achieved by cascading
three pi networks with impedance transformations of 1.908:1. The impedance levels are
thus 7.2 , 13.74 , 26.21 and 50 . The match is required for the 250MHz to
500MHz range and provide a return loss of less than 20 dB. Figure 9.25 shows the
equations used to calculate the initial element values. Figure 9.26 shows the circuit of the
resulting matching network.
Figure 9.26. Initial broadband match schematic.
For a wideband match, a low Q is used. Figure 9.25 shows that for the required X0=-4.3,
a minimum Q for the Pi section closest to the FET is 0.5972. A lower Q will result in a
negative Cp1 in figure 9.26. As a result Q = 0.4 is used in figures 9.25 to 9.28. That results
in practical components for the Pi networks. If X0 = 0, then Xp becomes infinite and the
expression for Xp must be disabled and the equation Xp = 1e9 must be enabled. The
equations in figure 9.25 can accommodate any impedance level. They can also be
extended to include more sections.
Figure 9.27. Starting broadband match S
21
and S
22
.
RF Electronics: Design and Simulation
321 www.cadence.com/go/awr
