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I'm considering using one of the following LDMOS FETs as an RF amplifier:

The datasheets provide \$Z_{source}\$ and \$Z_{load}\$ metrics for some frequencies (as below), but not for all frequencies they support and not for the frequency that I'm looking for (146MHz center)---though the FETs do support that frequency. If I can get the \$Z_{source}\$ and \$Z_{load}\$ at 146MHz, then I could use those numbers to create a matching network (L/Pi/Tee).

  • If you were to match this component to 50 Ohms, how would you do so?
  • What information from the datasheet do you use to calculate a matching network at 146MHz?
  • Is there an online calculator (or some formula) into which I would plug in that information?
  • If it helps you answer the question, the Icom IC-9700 uses the AFT05MP075NR1:
    • The service manual PDF is available here
    • Page 8 (2-2) has a photo of the components labeled "PA-A"
    • Page 87 (10-14) shows the TX amplifier pipeline schematic in orange (RX is green)

Here is NXP's test fixture from the AFM907N datasheet on Page 11:

Test Fixture

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2 Answers 2

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The manufacturer should provide linear models (S-parameters) in the form of something that can be consumed by an RF/Microwave CAD package which usually covers the designed-for range of operation with a fairly large number of points. Look for "touchstone" or "s2p" (two-port network).

The S-params file for AFT05MP075N is available from NXP here: https://www.nxp.com/part/AFT05MP075N#/ under software and tools. The range extends down to a point at 200 MHz (measured).

Given a known system impedance, one can convert S-parameter values into impedances (Z).

The CAD tool then usually interpolates to get values for the precise value you require for calculation. The tool can usually synthesise the matching network too, although, it would be much more valuable to understand the theory and check the result with the tool.

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  • \$\begingroup\$ I want to mark this as the best answer (thank you!) and also wish to provide some additional information provided by NXP in case someone else finds this useful. TL;DR: use the "RF High-power Models" from the link above in ADS or AWR to match for high-power FETs--not .s2p files because they may be for small-signals. NXP pointed me here nxp.com/docs/en/application-note/AN1526.pdf for power amp FET best practices (for NXP's or others' devices), notably, "Power amplifiers provide higher power gain and better efficiency at the rated output power if the output is purposely mismatched." \$\endgroup\$
    – KJ7LNW
    Aug 20, 2020 at 22:36
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Read about PI_network matching.

Also read on inline stubs and shunting stubs.

Check into using the Smith Chart for matching.

Surely the American Radio Relay League has literature on matching.

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  • \$\begingroup\$ I'm familiar with L, Tee, and Pi matching---but how can I use the smith chart if the smith and test fixture values only give values for 450-520MHz but I need 146MHz? \$\endgroup\$
    – KJ7LNW
    Aug 7, 2020 at 19:56

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