I have an RF amplifier (GHz), 50W. Power source 28V. Please tell me how do they calculate output power? I know only 1 method: V^2/r.

So, if I try 28^2/50 I got 15W. But its 50W. Another words, if I have 50W and 50 Ohm load, I need amplitude about 50V. But the power source only 28V. How is it possible?

  • \$\begingroup\$ It might have a transformer step-up output stage or use a power inverter inside the amplifier. Or. if it's a class C amplifier it can probably produce 50 volts p-p from the output transistor stage. \$\endgroup\$
    – Andy aka
    Dec 9, 2019 at 16:43
  • 1
    \$\begingroup\$ Link the datasheet for your amplifier. \$\endgroup\$ Dec 9, 2019 at 17:17

1 Answer 1


There is a matching network between the rf power device and the output which transforms the 20V or so of swing at the device drain to ~50V RMS at the output.

The output is 50R nominal, the drain isn't, and may well in fact be significantly reactive.

In the microwave bands this matching network is often a planar structure printed onto the PCB.

  • \$\begingroup\$ So, there is only 1 way to know - measure it. Right ? And if I do that I will get amplitude 50V ? So that V^2/R still work ? I will get 50V RMS ? \$\endgroup\$
    – Sid
    Dec 9, 2019 at 16:50
  • \$\begingroup\$ If the thing is delivering 50W into a correctly matched 50 ohm load, then by definition there will be 50V RMS across the load. Now measuring that accurately is a pain in the arse at microwave frequencies, even my very expensive HP RF power meter depends on the loads and cables being right and metrology grade RF connectors are Expensive. \$\endgroup\$
    – Dan Mills
    Dec 9, 2019 at 16:54
  • \$\begingroup\$ @is often a planar structure printed onto the PCB@ Would like to see that. \$\endgroup\$
    – Sid
    Dec 9, 2019 at 16:59

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