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RF Amplifier Impedance Matching

January 28th Fact-of-the-Day

It often has been written and most people seem to believe that the output impedance of an RF amplifier should match the load impedance; so that for example, if an amplifier is driving a 50 ohm load, the amplifier output impedance should be 50 ohms. That generally isn't true for the simple reason that contrary to popular belief, the efficiency would be poor. Consider a 1 KW amplifier with 50 ohms resistive source impedance driving a 50 ohm resistive load (both source and load tuned to resonance). The source and load impedances are in series, so 500 watts would be dissipated in waste heat within the amplifier and 500 watts would be delivered to the load. To have high efficiency the source impedance must be small compared to the load impedance. Note that high efficiency is not the same as maximum power transfer which is the reason for the common misunderstanding. ©2005 Martek International All rights reserved.

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