RF Electronics Chapter 9: Impedance Matching of Power Amplifiers Page 335
2022, C. J. Kikkert, James Cook University, ISBN 978-0-6486803-9-0.
Figure 9.52. Stability of the transformer matched power amplifier, 10MHz - 2GHz.
The transformer matching is suitable for the first step in an amplifier design by using
simulation to obtain the desired performance by determining optimum biasing and
matching for the amplifier. For a practical amplifier at 1 GHz, it is difficult to realise
transformers with the required turns-ratio, power handling capability and low losses. A
50 Ω floating coaxial-line-balun, as shown in figure 4.55, can be used to produce
differential signals with a 25 Ω impedance. A Marchand Balun can also be used if desired.
Figure 4.2 shows that a two-line impedance transformation can produce a flat impedance
transformation over a wide bandwidth. Figure 9.52 shows how this is implemented in the
design for the amplifier. For better stability, the stability resistors (Rin) are moved close
to the Gate of the FET. That is a lower impedance, requiring Rin to be changed to 1 Ω.
Resistors Rin2 have been added to provide stability at frequencies <100 MHz The output
matching-network is the same but the resistors are not present.
Figure 9.53. Transmission line based, input matching network.
In figure 9.53, Fc is the centre frequency for all the transmission lines, corresponding to
the 90° electrical length. If needed, that electrical length of the individual lines in the
input or output matching networks can be tuned, to obtain a better performance. Using
the transformer-amplifier as a starting point, the input and output matching networks are
tuned to obtain a good PAE, bandwidth, stability and harmonic levels. To be able to get
RF Electronics: Design and Simulation
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