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I've been casually aware that signal gain can be added in "stages" but never really considered how that might actually work in practice. And it seems that some RF amplifiers really prefer to only "sample" the input signal rather than use any of its incoming power, e.g. some QRP amps I've seen even attenuate the input signal before using it to drive their output signal.

But for example I happened to recently see a classified listing for a Mirage B-5016-G amplifier which is sold as taking "50 watts in, 160 watts out". Does an amp like that simply allow high powered input signals, but still just dissipate/attenuate them down to essentially nothing before using only the underlying signal to gate the output current?

Or are there transistors where the input power must be relatively high in order to control an even higher power output?

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Modern LDMOS FETs have a very high native gain — for example the MRFE6VP61K25 in its "standard test circuit" has a gain of 27dB at HF frequencies, and >20dB up to 350MHz. 27dB equals 1500W out for 3W in.

When used in an amp, they usually have attenuators on their inputs, for two reasons:

  1. To ease SWR-matching of the input. Any mismatch on the amplifier's input will be reduced by the attenuator value. To use completely made up values: if the amplifier section would have an input impedance ranging from 30-j20 to 70+j40 over some frequency range, the same amplifier behind an ideal 10dB pad would have an impedance ranging from 48-j2 to 52+j4 — what was once <2:1 is now <1.1:1.

  2. To comply with an FCC Part 97 rule that says that you can't sell an amplifier with more than 15dB of total gain. The same amplifier sold overseas might have a substantially higher gain, while the version sold in the US turns more of the input power into heat.

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  • $\begingroup$ Interesting! So, full disclosure I've already asked electronics.stackexchange.com/questions/640708/… which might cover some similar ground, but to be clear it almost seems that you're saying the opposite of the other answer here on this thread? Specifically it sounds like you're saying a modern amp would probably toss 50W signal and just attenuate that to a 3W sample to drive a FET? Whereas the other answer implies (iiuc) that the original 50W input is somehow combined with some additional power generated summing up to the total output. $\endgroup$
    – natevw - AF7TB
    Nov 1, 2022 at 4:24
  • $\begingroup$ @natevw-AF7TB pretty much yeah. Older devices had less gain and did need more drive, but either way, more or less none of the drive power actually "makes it" to the output. $\endgroup$
    – hobbs - KC2G
    Nov 1, 2022 at 6:28

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