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I believe I understand how an excessively high impedance can damage a final output power MOSFET (voltage rises until avalanche breakdown). But how does an excessively low impedance do that? If the impedance is too low, won't the current simply go through the load, bypassing damage to the MOSFET?

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Step away from RF for the moment, and think about the basic operation of a single-supply transistor amplifier, or even a voltage regulator.

schematic

simulate this circuit – Schematic created using CircuitLab

Ignoring the details of how the gate is controlled by the input signal, the current flows in a single simple circuit: through the supply, the transistor, and the load. Therefore, however much current flows through the load, also flows through the transistor. If the load has a lower impedance than intended, then more current flows through the transistor as well as the load than intended, and therefore the transistor gets hotter. The above circuit is a DC circuit, but exactly the same principle applies to AC/RF circuits — scale up instantaneous currents and you scale up waste heat.

Thus, the transistor is still performing its function — no electronic failure has immediately occurred as a result of the load impedance — but the additional heat may raise its temperature to the failure point (depending, of course, on the transistor's limits and the surrounding heat-sink / cooling system).

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  • $\begingroup$ The way I typically see the load is in parallel with the PA transistor, not in series. How is the load in series with it? Is it under a certain portion of the AC cycle? $\endgroup$
    – Noji
    May 16 at 11:50
  • $\begingroup$ @Noji I admit I'm not especially familiar with RF amplifier design. But, the circuit has to deliver power to the load somehow, and the transistor must be involved or there's no point, and the circuit must be (mostly-)linear or it would distort (too much), so the current in the transistor must (roughly) scale with the current in the load. $\endgroup$
    – Kevin Reid AG6YO
    May 16 at 15:15

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