RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 331
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 9.43 shows the input and output stability circles for the 200 to 600 MHz frequency
range and show that the amplifier is unconditionally stable. The amplifier has a gain of
slightly more than 20 dB. The FET input impedance of 2.93 - j17.97 Ω is much smaller
than the impedance of the 50 Ω stability resistor (R1), so most of the current flowing
through the input matching network will flow into the FET. The S parameter file used
contains small signal parameters, but even if the amplifier has a 25 W output, then the
stability resistor will dissipate much less than 0.25W, which is negligible compared to the
output.
To lower production costs at UHF and microwave frequencies, the inductors in matching
networks like figure 9.30 can be replaced by Microstrip transmission lines, as shown in
figure 7.16. That will change the input and output match slightly, so that further
optimisation may be required. Figure 9.44 shows the schematic and figure 9.45 shows the
PCB layout for the MRFE6VS25N 512 MHz test circuit used by NXP [3]. To
accommodate slight FET variations, capacitor pads are normally included in the PCB
layout. That allows the capacitors C6 to C10 in figure 44 to be varied if needed to tune
the amplifier's performance.
Figure 9.44. Schematic of 512 MHz amplifier from data sheet [3].
Figure 9.45. Manufacturer PCB from data sheet [3].
RF Electronics: Design and Simulation
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